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<?xml version="1.0"?> <!-- -*- sgml -*- -->
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<chapter id="mc-tech-docs"
xreflabel="The design and implementation of Valgrind">
<title>The Design and Implementation of Valgrind</title>
<subtitle>Detailed technical notes for hackers, maintainers and
the overly-curious</subtitle>
<sect1 id="mc-tech-docs.intro" xreflabel="Introduction">
<title>Introduction</title>
<para>This document contains a detailed, highly-technical description of
the internals of Valgrind. This is not the user manual; if you are an
end-user of Valgrind, you do not want to read this. Conversely, if you
really are a hacker-type and want to know how it works, I assume that
you have read the user manual thoroughly.</para>
<para>You may need to read this document several times, and carefully.
Some important things, I only say once.</para>
<para>[Note: this document is now very old, and a lot of its contents
are out of date, and misleading.]</para>
<sect2 id="mc-tech-docs.history" xreflabel="History">
<title>History</title>
<para>Valgrind came into public view in late Feb 2002. However, it has
been under contemplation for a very long time, perhaps seriously for
about five years. Somewhat over two years ago, I started working on the
x86 code generator for the Glasgow Haskell Compiler
(http://www.haskell.org/ghc), gaining familiarity with x86 internals on
the way. I then did Cacheprof, gaining further x86 experience. Some
time around Feb 2000 I started experimenting with a user-space x86
interpreter for x86-Linux. This worked, but it was clear that a
JIT-based scheme would be necessary to give reasonable performance for
Valgrind. Design work for the JITter started in earnest in Oct 2000,
and by early 2001 I had an x86-to-x86 dynamic translator which could run
quite large programs. This translator was in a sense pointless, since
it did not do any instrumentation or checking.</para>
<para>Most of the rest of 2001 was taken up designing and implementing
the instrumentation scheme. The main difficulty, which consumed a lot
of effort, was to design a scheme which did not generate large numbers
of false uninitialised-value warnings. By late 2001 a satisfactory
scheme had been arrived at, and I started to test it on ever-larger
programs, with an eventual eye to making it work well enough so that it
was helpful to folks debugging the upcoming version 3 of KDE. I've used
KDE since before version 1.0, and wanted to Valgrind to be an indirect
contribution to the KDE 3 development effort. At the start of Feb 02
the kde-core-devel crew started using it, and gave a huge amount of
helpful feedback and patches in the space of three weeks. Snapshot
20020306 is the result.</para>
<para>In the best Unix tradition, or perhaps in the spirit of Fred
Brooks' depressing-but-completely-accurate epitaph "build one to throw
away; you will anyway", much of Valgrind is a second or third rendition
of the initial idea. The instrumentation machinery
(<filename>vg_translate.c</filename>, <filename>vg_memory.c</filename>)
and core CPU simulation (<filename>vg_to_ucode.c</filename>,
<filename>vg_from_ucode.c</filename>) have had three redesigns and
rewrites; the register allocator, low-level memory manager
(<filename>vg_malloc2.c</filename>) and symbol table reader
(<filename>vg_symtab2.c</filename>) are on the second rewrite. In a
sense, this document serves to record some of the knowledge gained as a
result.</para>
</sect2>
<sect2 id="mc-tech-docs.overview" xreflabel="Design overview">
<title>Design overview</title>
<para>Valgrind is compiled into a Linux shared object,
<filename>valgrind.so</filename>, and also a dummy one,
<filename>valgrinq.so</filename>, of which more later. The
<filename>valgrind</filename> shell script adds
<filename>valgrind.so</filename> to the
<computeroutput>LD_PRELOAD</computeroutput> list of extra libraries to
be loaded with any dynamically linked library. This is a standard
trick, one which I assume the
<computeroutput>LD_PRELOAD</computeroutput> mechanism was developed to
support.</para>
<para><filename>valgrind.so</filename> is linked with the
<option>-z initfirst</option> flag, which
requests that its initialisation code is run before that of any
other object in the executable image. When this happens,
valgrind gains control. The real CPU becomes "trapped" in
<filename>valgrind.so</filename> and the translations it
generates. The synthetic CPU provided by Valgrind does, however,
return from this initialisation function. So the normal startup
actions, orchestrated by the dynamic linker
<filename>ld.so</filename>, continue as usual, except on the
synthetic CPU, not the real one. Eventually
<function>main</function> is run and returns, and
then the finalisation code of the shared objects is run,
presumably in inverse order to which they were initialised.
Remember, this is still all happening on the simulated CPU.
Eventually <filename>valgrind.so</filename>'s own finalisation
code is called. It spots this event, shuts down the simulated
CPU, prints any error summaries and/or does leak detection, and
returns from the initialisation code on the real CPU. At this
point, in effect the real and synthetic CPUs have merged back
into one, Valgrind has lost control of the program, and the
program finally <function>exit()s</function> back to
the kernel in the usual way.</para>
<para>The normal course of activity, once Valgrind has started
up, is as follows. Valgrind never runs any part of your program
(usually referred to as the "client"), not a single byte of it,
directly. Instead it uses function
<function>VG_(translate)</function> to translate
basic blocks (BBs, straight-line sequences of code) into
instrumented translations, and those are run instead. The
translations are stored in the translation cache (TC),
<computeroutput>vg_tc</computeroutput>, with the translation
table (TT), <computeroutput>vg_tt</computeroutput> supplying the
original-to-translation code address mapping. Auxiliary array
<computeroutput>VG_(tt_fast)</computeroutput> is used as a
direct-map cache for fast lookups in TT; it usually achieves a
hit rate of around 98% and facilitates an orig-to-trans lookup in
4 x86 insns, which is not bad.</para>
<para>Function <function>VG_(dispatch)</function> in
<filename>vg_dispatch.S</filename> is the heart of the JIT
dispatcher. Once a translated code address has been found, it is
executed simply by an x86 <computeroutput>call</computeroutput>
to the translation. At the end of the translation, the next
original code addr is loaded into
<computeroutput>%eax</computeroutput>, and the translation then
does a <computeroutput>ret</computeroutput>, taking it back to
the dispatch loop, with, interestingly, zero branch
mispredictions. The address requested in
<computeroutput>%eax</computeroutput> is looked up first in
<function>VG_(tt_fast)</function>, and, if not found,
by calling C helper
<function>VG_(search_transtab)</function>. If there
is still no translation available,
<function>VG_(dispatch)</function> exits back to the
top-level C dispatcher
<function>VG_(toploop)</function>, which arranges for
<function>VG_(translate)</function> to make a new
translation. All fairly unsurprising, really. There are various
complexities described below.</para>
<para>The translator, orchestrated by
<function>VG_(translate)</function>, is complicated
but entirely self-contained. It is described in great detail in
subsequent sections. Translations are stored in TC, with TT
tracking administrative information. The translations are
subject to an approximate LRU-based management scheme. With the
current settings, the TC can hold at most about 15MB of
translations, and LRU passes prune it to about 13.5MB. Given
that the orig-to-translation expansion ratio is about 13:1 to
14:1, this means TC holds translations for more or less a
megabyte of original code, which generally comes to about 70000
basic blocks for C++ compiled with optimisation on. Generating
new translations is expensive, so it is worth having a large TC
to minimise the (capacity) miss rate.</para>
<para>The dispatcher,
<function>VG_(dispatch)</function>, receives hints
from the translations which allow it to cheaply spot all control
transfers corresponding to x86
<computeroutput>call</computeroutput> and
<computeroutput>ret</computeroutput> instructions. It has to do
this in order to spot some special events:</para>
<itemizedlist>
<listitem>
<para>Calls to
<function>VG_(shutdown)</function>. This is
Valgrind's cue to exit. NOTE: actually this is done a
different way; it should be cleaned up.</para>
</listitem>
<listitem>
<para>Returns of system call handlers, to the return address
<function>VG_(signalreturn_bogusRA)</function>.
The signal simulator needs to know when a signal handler is
returning, so we spot jumps (returns) to this address.</para>
</listitem>
<listitem>
<para>Calls to <function>vg_trap_here</function>.
All <function>malloc</function>,
<function>free</function>, etc calls that the
client program makes are eventually routed to a call to
<function>vg_trap_here</function>, and Valgrind
does its own special thing with these calls. In effect this
provides a trapdoor, by which Valgrind can intercept certain
calls on the simulated CPU, run the call as it sees fit
itself (on the real CPU), and return the result to the
simulated CPU, quite transparently to the client
program.</para>
</listitem>
</itemizedlist>
<para>Valgrind intercepts the client's
<function>malloc</function>,
<function>free</function>, etc, calls, so that it can
store additional information. Each block
<function>malloc</function>'d by the client gives
rise to a shadow block in which Valgrind stores the call stack at
the time of the <function>malloc</function> call.
When the client calls <function>free</function>,
Valgrind tries to find the shadow block corresponding to the
address passed to <function>free</function>, and
emits an error message if none can be found. If it is found, the
block is placed on the freed blocks queue
<computeroutput>vg_freed_list</computeroutput>, it is marked as
inaccessible, and its shadow block now records the call stack at
the time of the <function>free</function> call.
Keeping <computeroutput>free</computeroutput>'d blocks in this
queue allows Valgrind to spot all (presumably invalid) accesses
to them. However, once the volume of blocks in the free queue
exceeds <function>VG_(clo_freelist_vol)</function>,
blocks are finally removed from the queue.</para>
<para>Keeping track of <literal>A</literal> and
<literal>V</literal> bits (note: if you don't know what these
are, you haven't read the user guide carefully enough) for memory
is done in <filename>vg_memory.c</filename>. This implements a
sparse array structure which covers the entire 4G address space
in a way which is reasonably fast and reasonably space efficient.
The 4G address space is divided up into 64K sections, each
covering 64Kb of address space. Given a 32-bit address, the top
16 bits are used to select one of the 65536 entries in
<function>VG_(primary_map)</function>. The resulting
"secondary" (<computeroutput>SecMap</computeroutput>) holds A and
V bits for the 64k of address space chunk corresponding to the
lower 16 bits of the address.</para>
</sect2>
<sect2 id="mc-tech-docs.design" xreflabel="Design decisions">
<title>Design decisions</title>
<para>Some design decisions were motivated by the need to make
Valgrind debuggable. Imagine you are writing a CPU simulator.
It works fairly well. However, you run some large program, like
Netscape, and after tens of millions of instructions, it crashes.
How can you figure out where in your simulator the bug is?</para>
<para>Valgrind's answer is: cheat. Valgrind is designed so that
it is possible to switch back to running the client program on
the real CPU at any point. Using the
<option>--stop-after= </option> flag, you can ask
Valgrind to run just some number of basic blocks, and then run
the rest of the way on the real CPU. If you are searching for a
bug in the simulated CPU, you can use this to do a binary search,
which quickly leads you to the specific basic block which is
causing the problem.</para>
<para>This is all very handy. It does constrain the design in
certain unimportant ways. Firstly, the layout of memory, when
viewed from the client's point of view, must be identical
regardless of whether it is running on the real or simulated CPU.
This means that Valgrind can't do pointer swizzling -- well, no
great loss -- and it can't run on the same stack as the client --
again, no great loss. Valgrind operates on its own stack,
<function>VG_(stack)</function>, which it switches to
at startup, temporarily switching back to the client's stack when
doing system calls for the client.</para>
<para>Valgrind also receives signals on its own stack,
<computeroutput>VG_(sigstack)</computeroutput>, but for different
gruesome reasons discussed below.</para>
<para>This nice clean
switch-back-to-the-real-CPU-whenever-you-like story is muddied by
signals. Problem is that signals arrive at arbitrary times and
tend to slightly perturb the basic block count, with the result
that you can get close to the basic block causing a problem but
can't home in on it exactly. My kludgey hack is to define
<computeroutput>SIGNAL_SIMULATION</computeroutput> to 1 towards
the bottom of <filename>vg_syscall_mem.c</filename>, so that
signal handlers are run on the real CPU and don't change the BB
counts.</para>
<para>A second hole in the switch-back-to-real-CPU story is that
Valgrind's way of delivering signals to the client is different
from that of the kernel. Specifically, the layout of the signal
delivery frame, and the mechanism used to detect a sighandler
returning, are different. So you can't expect to make the
transition inside a sighandler and still have things working, but
in practice that's not much of a restriction.</para>
<para>Valgrind's implementation of
<function>malloc</function>,
<function>free</function>, etc, (in
<filename>vg_clientmalloc.c</filename>, not the low-level stuff
in <filename>vg_malloc2.c</filename>) is somewhat complicated by
the need to handle switching back at arbitrary points. It does
work tho.</para>
</sect2>
<sect2 id="mc-tech-docs.correctness" xreflabel="Correctness">
<title>Correctness</title>
<para>There's only one of me, and I have a Real Life (tm) as well
as hacking Valgrind [allegedly :-]. That means I don't have time
to waste chasing endless bugs in Valgrind. My emphasis is
therefore on doing everything as simply as possible, with
correctness, stability and robustness being the number one
priority, more important than performance or functionality. As a
result:</para>
<itemizedlist>
<listitem>
<para>The code is absolutely loaded with assertions, and
these are <command>permanently enabled.</command> I have no
plan to remove or disable them later. Over the past couple
of months, as valgrind has become more widely used, they have
shown their worth, pulling up various bugs which would
otherwise have appeared as hard-to-find segmentation
faults.</para>
<para>I am of the view that it's acceptable to spend 5% of
the total running time of your valgrindified program doing
assertion checks and other internal sanity checks.</para>
</listitem>
<listitem>
<para>Aside from the assertions, valgrind contains various
sets of internal sanity checks, which get run at varying
frequencies during normal operation.
<function>VG_(do_sanity_checks)</function> runs
every 1000 basic blocks, which means 500 to 2000 times/second
for typical machines at present. It checks that Valgrind
hasn't overrun its private stack, and does some simple checks
on the memory permissions maps. Once every 25 calls it does
some more extensive checks on those maps. Etc, etc.</para>
<para>The following components also have sanity check code,
which can be enabled to aid debugging:</para>
<itemizedlist>
<listitem><para>The low-level memory-manager
(<computeroutput>VG_(mallocSanityCheckArena)</computeroutput>).
This does a complete check of all blocks and chains in an
arena, which is very slow. Is not engaged by default.</para>
</listitem>
<listitem>
<para>The symbol table reader(s): various checks to
ensure uniqueness of mappings; see
<function>VG_(read_symbols)</function> for a
start. Is permanently engaged.</para>
</listitem>
<listitem>
<para>The A and V bit tracking stuff in
<filename>vg_memory.c</filename>. This can be compiled
with cpp symbol
<computeroutput>VG_DEBUG_MEMORY</computeroutput> defined,
which removes all the fast, optimised cases, and uses
simple-but-slow fallbacks instead. Not engaged by
default.</para>
</listitem>
<listitem>
<para>Ditto
<computeroutput>VG_DEBUG_LEAKCHECK</computeroutput>.</para>
</listitem>
<listitem>
<para>The JITter parses x86 basic blocks into sequences
of UCode instructions. It then sanity checks each one
with <function>VG_(saneUInstr)</function> and
sanity checks the sequence as a whole with
<function>VG_(saneUCodeBlock)</function>.
This stuff is engaged by default, and has caught some
way-obscure bugs in the simulated CPU machinery in its
time.</para>
</listitem>
<listitem>
<para>The system call wrapper does
<function>VG_(first_and_last_secondaries_look_plausible)</function>
after every syscall; this is known to pick up bugs in the
syscall wrappers. Engaged by default.</para>
</listitem>
<listitem>
<para>The main dispatch loop, in
<function>VG_(dispatch)</function>, checks
that translations do not set
<computeroutput>%ebp</computeroutput> to any value
different from
<computeroutput>VG_EBP_DISPATCH_CHECKED</computeroutput>
or <computeroutput>& VG_(baseBlock)</computeroutput>.
In effect this test is free, and is permanently
engaged.</para>
</listitem>
<listitem>
<para>There are a couple of ifdefed-out consistency
checks I inserted whilst debugging the new register
allocater,
<computeroutput>vg_do_register_allocation</computeroutput>.</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>I try to avoid techniques, algorithms, mechanisms, etc,
for which I can supply neither a convincing argument that
they are correct, nor sanity-check code which might pick up
bugs in my implementation. I don't always succeed in this,
but I try. Basically the idea is: avoid techniques which
are, in practice, unverifiable, in some sense. When doing
anything, always have in mind: "how can I verify that this is
correct?"</para>
</listitem>
</itemizedlist>
<para>Some more specific things are:</para>
<itemizedlist>
<listitem>
<para>Valgrind runs in the same namespace as the client, at
least from <filename>ld.so</filename>'s point of view, and it
therefore absolutely had better not export any symbol with a
name which could clash with that of the client or any of its
libraries. Therefore, all globally visible symbols exported
from <filename>valgrind.so</filename> are defined using the
<computeroutput>VG_</computeroutput> CPP macro. As you'll
see from <filename>vg_constants.h</filename>, this appends
some arbitrary prefix to the symbol, in order that it be, we
hope, globally unique. Currently the prefix is
<computeroutput>vgPlain_</computeroutput>. For convenience
there are also <computeroutput>VGM_</computeroutput>,
<computeroutput>VGP_</computeroutput> and
<computeroutput>VGOFF_</computeroutput>. All locally defined
symbols are declared <computeroutput>static</computeroutput>
and do not appear in the final shared object.</para>
<para>To check this, I periodically do <computeroutput>nm
valgrind.so | grep " T "</computeroutput>, which shows you
all the globally exported text symbols. They should all have
an approved prefix, except for those like
<function>malloc</function>,
<function>free</function>, etc, which we
deliberately want to shadow and take precedence over the same
names exported from <filename>glibc.so</filename>, so that
valgrind can intercept those calls easily. Similarly,
<computeroutput>nm valgrind.so | grep " D "</computeroutput>
allows you to find any rogue data-segment symbol
names.</para>
</listitem>
<listitem>
<para>Valgrind tries, and almost succeeds, in being
completely independent of all other shared objects, in
particular of <filename>glibc.so</filename>. For example, we
have our own low-level memory manager in
<filename>vg_malloc2.c</filename>, which is a fairly standard
malloc/free scheme augmented with arenas, and
<filename>vg_mylibc.c</filename> exports reimplementations of
various bits and pieces you'd normally get from the C
library.</para>
<para>Why all the hassle? Because imagine the potential
chaos of both the simulated and real CPUs executing in
<filename>glibc.so</filename>. It just seems simpler and
cleaner to be completely self-contained, so that only the
simulated CPU visits <filename>glibc.so</filename>. In
practice it's not much hassle anyway. Also, valgrind starts
up before glibc has a chance to initialise itself, and who
knows what difficulties that could lead to. Finally, glibc
has definitions for some types, specifically
<computeroutput>sigset_t</computeroutput>, which conflict
(are different from) the Linux kernel's idea of same. When
Valgrind wants to fiddle around with signal stuff, it wants
to use the kernel's definitions, not glibc's definitions. So
it's simplest just to keep glibc out of the picture
entirely.</para>
<para>To find out which glibc symbols are used by Valgrind,
reinstate the link flags <option>-nostdlib
-Wl,-no-undefined</option>. This causes linking to
fail, but will tell you what you depend on. I have mostly,
but not entirely, got rid of the glibc dependencies; what
remains is, IMO, fairly harmless. AFAIK the current
dependencies are: <computeroutput>memset</computeroutput>,
<computeroutput>memcmp</computeroutput>,
<computeroutput>stat</computeroutput>,
<computeroutput>system</computeroutput>,
<computeroutput>sbrk</computeroutput>,
<computeroutput>setjmp</computeroutput> and
<computeroutput>longjmp</computeroutput>.</para>
</listitem>
<listitem>
<para>Similarly, valgrind should not really import any
headers other than the Linux kernel headers, since it knows
of no API other than the kernel interface to talk to. At the
moment this is really not in a good state, and
<computeroutput>vg_syscall_mem</computeroutput> imports, via
<filename>vg_unsafe.h</filename>, a significant number of
C-library headers so as to know the sizes of various structs
passed across the kernel boundary. This is of course
completely bogus, since there is no guarantee that the C
library's definitions of these structs matches those of the
kernel. I have started to sort this out using
<filename>vg_kerneliface.h</filename>, into which I had
intended to copy all kernel definitions which valgrind could
need, but this has not gotten very far. At the moment it
mostly contains definitions for
<computeroutput>sigset_t</computeroutput> and
<computeroutput>struct sigaction</computeroutput>, since the
kernel's definition for these really does clash with glibc's.
I plan to use a <computeroutput>vki_</computeroutput> prefix
on all these types and constants, to denote the fact that
they pertain to <command>V</command>algrind's
<command>K</command>ernel
<command>I</command>nterface.</para>
<para>Another advantage of having a
<filename>vg_kerneliface.h</filename> file is that it makes
it simpler to interface to a different kernel. Once can, for
example, easily imagine writing a new
<filename>vg_kerneliface.h</filename> for FreeBSD, or x86
NetBSD.</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="mc-tech-docs.limits" xreflabel="Current limitations">
<title>Current limitations</title>
<para>Support for weird (non-POSIX) signal stuff is patchy. Does
anybody care?</para>
</sect2>
</sect1>
<sect1 id="mc-tech-docs.jitter" xreflabel="The instrumenting JITter">
<title>The instrumenting JITter</title>
<para>This really is the heart of the matter. We begin with
various side issues.</para>
<sect2 id="mc-tech-docs.storage"
xreflabel="Run-time storage, and the use of host registers">
<title>Run-time storage, and the use of host registers</title>
<para>Valgrind translates client (original) basic blocks into
instrumented basic blocks, which live in the translation cache
TC, until either the client finishes or the translations are
ejected from TC to make room for newer ones.</para>
<para>Since it generates x86 code in memory, Valgrind has
complete control of the use of registers in the translations.
Now pay attention. I shall say this only once, and it is
important you understand this. In what follows I will refer to
registers in the host (real) cpu using their standard names,
<computeroutput>%eax</computeroutput>,
<computeroutput>%edi</computeroutput>, etc. I refer to registers
in the simulated CPU by capitalising them:
<computeroutput>%EAX</computeroutput>,
<computeroutput>%EDI</computeroutput>, etc. These two sets of
registers usually bear no direct relationship to each other;
there is no fixed mapping between them. This naming scheme is
used fairly consistently in the comments in the sources.</para>
<para>Host registers, once things are up and running, are used as
follows:</para>
<itemizedlist>
<listitem>
<para><computeroutput>%esp</computeroutput>, the real stack
pointer, points somewhere in Valgrind's private stack area,
<computeroutput>VG_(stack)</computeroutput> or, transiently,
into its signal delivery stack,
<computeroutput>VG_(sigstack)</computeroutput>.</para>
</listitem>
<listitem>
<para><computeroutput>%edi</computeroutput> is used as a
temporary in code generation; it is almost always dead,
except when used for the
<computeroutput>Left</computeroutput> value-tag operations.</para>
</listitem>
<listitem>
<para><computeroutput>%eax</computeroutput>,
<computeroutput>%ebx</computeroutput>,
<computeroutput>%ecx</computeroutput>,
<computeroutput>%edx</computeroutput> and
<computeroutput>%esi</computeroutput> are available to
Valgrind's register allocator. They are dead (carry
unimportant values) in between translations, and are live
only in translations. The one exception to this is
<computeroutput>%eax</computeroutput>, which, as mentioned
far above, has a special significance to the dispatch loop
<computeroutput>VG_(dispatch)</computeroutput>: when a
translation returns to the dispatch loop,
<computeroutput>%eax</computeroutput> is expected to contain
the original-code-address of the next translation to run.
The register allocator is so good at minimising spill code
that using five regs and not having to save/restore
<computeroutput>%edi</computeroutput> actually gives better
code than allocating to <computeroutput>%edi</computeroutput>
as well, but then having to push/pop it around special
uses.</para>
</listitem>
<listitem>
<para><computeroutput>%ebp</computeroutput> points
permanently at
<computeroutput>VG_(baseBlock)</computeroutput>. Valgrind's
translations are position-independent, partly because this is
convenient, but also because translations get moved around in
TC as part of the LRUing activity. <command>All</command>
static entities which need to be referred to from generated
code, whether data or helper functions, are stored starting
at <computeroutput>VG_(baseBlock)</computeroutput> and are
therefore reached by indexing from
<computeroutput>%ebp</computeroutput>. There is but one
exception, which is that by placing the value
<computeroutput>VG_EBP_DISPATCH_CHECKED</computeroutput> in
<computeroutput>%ebp</computeroutput> just before a return to
the dispatcher, the dispatcher is informed that the next
address to run, in <computeroutput>%eax</computeroutput>,
requires special treatment.</para>
</listitem>
<listitem>
<para>The real machine's FPU state is pretty much
unimportant, for reasons which will become obvious. Ditto
its <computeroutput>%eflags</computeroutput> register.</para>
</listitem>
</itemizedlist>
<para>The state of the simulated CPU is stored in memory, in
<computeroutput>VG_(baseBlock)</computeroutput>, which is a block
of 200 words IIRC. Recall that
<computeroutput>%ebp</computeroutput> points permanently at the
start of this block. Function
<computeroutput>vg_init_baseBlock</computeroutput> decides what
the offsets of various entities in
<computeroutput>VG_(baseBlock)</computeroutput> are to be, and
allocates word offsets for them. The code generator then emits
<computeroutput>%ebp</computeroutput> relative addresses to get
at those things. The sequence in which entities are allocated
has been carefully chosen so that the 32 most popular entities
come first, because this means 8-bit offsets can be used in the
generated code.</para>
<para>If I was clever, I could make
<computeroutput>%ebp</computeroutput> point 32 words along
<computeroutput>VG_(baseBlock)</computeroutput>, so that I'd have
another 32 words of short-form offsets available, but that's just
complicated, and it's not important -- the first 32 words take
99% (or whatever) of the traffic.</para>
<para>Currently, the sequence of stuff in
<computeroutput>VG_(baseBlock)</computeroutput> is as
follows:</para>
<itemizedlist>
<listitem>
<para>9 words, holding the simulated integer registers,
<computeroutput>%EAX</computeroutput>
.. <computeroutput>%EDI</computeroutput>, and the simulated
flags, <computeroutput>%EFLAGS</computeroutput>.</para>
</listitem>
<listitem>
<para>Another 9 words, holding the V bit "shadows" for the
above 9 regs.</para>
</listitem>
<listitem>
<para>The <command>addresses</command> of various helper
routines called from generated code:
<computeroutput>VG_(helper_value_check4_fail)</computeroutput>,
<computeroutput>VG_(helper_value_check0_fail)</computeroutput>,
which register V-check failures,
<computeroutput>VG_(helperc_STOREV4)</computeroutput>,
<computeroutput>VG_(helperc_STOREV1)</computeroutput>,
<computeroutput>VG_(helperc_LOADV4)</computeroutput>,
<computeroutput>VG_(helperc_LOADV1)</computeroutput>, which
do stores and loads of V bits to/from the sparse array which
keeps track of V bits in memory, and
<computeroutput>VGM_(handle_esp_assignment)</computeroutput>,
which messes with memory addressability resulting from
changes in <computeroutput>%ESP</computeroutput>.</para>
</listitem>
<listitem>
<para>The simulated <computeroutput>%EIP</computeroutput>.</para>
</listitem>
<listitem>
<para>24 spill words, for when the register allocator can't
make it work with 5 measly registers.</para>
</listitem>
<listitem>
<para>Addresses of helpers
<computeroutput>VG_(helperc_STOREV2)</computeroutput>,
<computeroutput>VG_(helperc_LOADV2)</computeroutput>. These
are here because 2-byte loads and stores are relatively rare,
so are placed above the magic 32-word offset boundary.</para>
</listitem>
<listitem>
<para>For similar reasons, addresses of helper functions
<computeroutput>VGM_(fpu_write_check)</computeroutput> and
<computeroutput>VGM_(fpu_read_check)</computeroutput>, which
handle the A/V maps testing and changes required by FPU
writes/reads.</para>
</listitem>
<listitem>
<para>Some other boring helper addresses:
<computeroutput>VG_(helper_value_check2_fail)</computeroutput>
and
<computeroutput>VG_(helper_value_check1_fail)</computeroutput>.
These are probably never emitted now, and should be
removed.</para>
</listitem>
<listitem>
<para>The entire state of the simulated FPU, which I believe
to be 108 bytes long.</para>
</listitem>
<listitem>
<para>Finally, the addresses of various other helper
functions in <filename>vg_helpers.S</filename>, which deal
with rare situations which are tedious or difficult to
generate code in-line for.</para>
</listitem>
</itemizedlist>
<para>As a general rule, the simulated machine's state lives
permanently in memory at
<computeroutput>VG_(baseBlock)</computeroutput>. However, the
JITter does some optimisations which allow the simulated integer
registers to be cached in real registers over multiple simulated
instructions within the same basic block. These are always
flushed back into memory at the end of every basic block, so that
the in-memory state is up-to-date between basic blocks. (This
flushing is implied by the statement above that the real
machine's allocatable registers are dead in between simulated
blocks).</para>
</sect2>
<sect2 id="mc-tech-docs.startup"
xreflabel="Startup, shutdown, and system calls">
<title>Startup, shutdown, and system calls</title>
<para>Getting into of Valgrind
(<computeroutput>VG_(startup)</computeroutput>, called from
<filename>valgrind.so</filename>'s initialisation section),
really means copying the real CPU's state into
<computeroutput>VG_(baseBlock)</computeroutput>, and then
installing our own stack pointer, etc, into the real CPU, and
then starting up the JITter. Exiting valgrind involves copying
the simulated state back to the real state.</para>
<para>Unfortunately, there's a complication at startup time.
Problem is that at the point where we need to take a snapshot of
the real CPU's state, the offsets in
<computeroutput>VG_(baseBlock)</computeroutput> are not set up
yet, because to do so would involve disrupting the real machine's
state significantly. The way round this is to dump the real
machine's state into a temporary, static block of memory,
<computeroutput>VG_(m_state_static)</computeroutput>. We can
then set up the <computeroutput>VG_(baseBlock)</computeroutput>
offsets at our leisure, and copy into it from
<computeroutput>VG_(m_state_static)</computeroutput> at some
convenient later time. This copying is done by
<computeroutput>VG_(copy_m_state_static_to_baseBlock)</computeroutput>.</para>
<para>On exit, the inverse transformation is (rather
unnecessarily) used: stuff in
<computeroutput>VG_(baseBlock)</computeroutput> is copied to
<computeroutput>VG_(m_state_static)</computeroutput>, and the
assembly stub then copies from
<computeroutput>VG_(m_state_static)</computeroutput> into the
real machine registers.</para>
<para>Doing system calls on behalf of the client
(<filename>vg_syscall.S</filename>) is something of a half-way
house. We have to make the world look sufficiently like that
which the client would normally have to make the syscall actually
work properly, but we can't afford to lose control. So the trick
is to copy all of the client's state, <command>except its program
counter</command>, into the real CPU, do the system call, and
copy the state back out. Note that the client's state includes
its stack pointer register, so one effect of this partial
restoration is to cause the system call to be run on the client's
stack, as it should be.</para>
<para>As ever there are complications. We have to save some of
our own state somewhere when restoring the client's state into
the CPU, so that we can keep going sensibly afterwards. In fact
the only thing which is important is our own stack pointer, but
for paranoia reasons I save and restore our own FPU state as
well, even though that's probably pointless.</para>
<para>The complication on the above complication is, that for
horrible reasons to do with signals, we may have to handle a
second client system call whilst the client is blocked inside
some other system call (unbelievable!). That means there's two
sets of places to dump Valgrind's stack pointer and FPU state
across the syscall, and we decide which to use by consulting
<computeroutput>VG_(syscall_depth)</computeroutput>, which is in
turn maintained by
<computeroutput>VG_(wrap_syscall)</computeroutput>.</para>
</sect2>
<sect2 id="mc-tech-docs.ucode" xreflabel="Introduction to UCode">
<title>Introduction to UCode</title>
<para>UCode lies at the heart of the x86-to-x86 JITter. The
basic premise is that dealing with the x86 instruction set head-on
is just too darn complicated, so we do the traditional
compiler-writer's trick and translate it into a simpler,
easier-to-deal-with form.</para>
<para>In normal operation, translation proceeds through six
stages, coordinated by
<computeroutput>VG_(translate)</computeroutput>:</para>
<orderedlist>
<listitem>
<para>Parsing of an x86 basic block into a sequence of UCode
instructions (<computeroutput>VG_(disBB)</computeroutput>).</para>
</listitem>
<listitem>
<para>UCode optimisation
(<computeroutput>vg_improve</computeroutput>), with the aim
of caching simulated registers in real registers over
multiple simulated instructions, and removing redundant
simulated <computeroutput>%EFLAGS</computeroutput>
saving/restoring.</para>
</listitem>
<listitem>
<para>UCode instrumentation
(<computeroutput>vg_instrument</computeroutput>), which adds
value and address checking code.</para>
</listitem>
<listitem>
<para>Post-instrumentation cleanup
(<computeroutput>vg_cleanup</computeroutput>), removing
redundant value-check computations.</para>
</listitem>
<listitem>
<para>Register allocation
(<computeroutput>vg_do_register_allocation</computeroutput>),
which, note, is done on UCode.</para>
</listitem>
<listitem>
<para>Emission of final instrumented x86 code
(<computeroutput>VG_(emit_code)</computeroutput>).</para>
</listitem>
</orderedlist>
<para>Notice how steps 2, 3, 4 and 5 are simple UCode-to-UCode
transformation passes, all on straight-line blocks of UCode (type
<computeroutput>UCodeBlock</computeroutput>). Steps 2 and 4 are
optimisation passes and can be disabled for debugging purposes,
with <option>--optimise=no</option> and
<option>--cleanup=no</option> respectively.</para>
<para>Valgrind can also run in a no-instrumentation mode, given
<option>--instrument=no</option>. This is useful
for debugging the JITter quickly without having to deal with the
complexity of the instrumentation mechanism too. In this mode,
steps 3 and 4 are omitted.</para>
<para>These flags combine, so that
<option>--instrument=no</option> together with
<option>--optimise=no</option> means only steps
1, 5 and 6 are used.
<option>--single-step=yes</option> causes each
x86 instruction to be treated as a single basic block. The
translations are terrible but this is sometimes instructive.</para>
<para>The <option>--stop-after=N</option> flag
switches back to the real CPU after
<computeroutput>N</computeroutput> basic blocks. It also re-JITs
the final basic block executed and prints the debugging info
resulting, so this gives you a way to get a quick snapshot of how
a basic block looks as it passes through the six stages mentioned
above. If you want to see full information for every block
translated (probably not, but still ...) find, in
<computeroutput>VG_(translate)</computeroutput>, the lines</para>
<programlisting><![CDATA[
dis = True;
dis = debugging_translation;]]></programlisting>
<para>and comment out the second line. This will spew out
debugging junk faster than you can possibly imagine.</para>
</sect2>
<sect2 id="mc-tech-docs.tags" xreflabel="UCode operand tags: type 'Tag'">
<title>UCode operand tags: type <computeroutput>Tag</computeroutput></title>
<para>UCode is, more or less, a simple two-address RISC-like
code. In keeping with the x86 AT&T assembly syntax,
generally speaking the first operand is the source operand, and
the second is the destination operand, which is modified when the
uinstr is notionally executed.</para>
<para>UCode instructions have up to three operand fields, each of
which has a corresponding <computeroutput>Tag</computeroutput>
describing it. Possible values for the tag are:</para>
<itemizedlist>
<listitem>
<para><computeroutput>NoValue</computeroutput>: indicates
that the field is not in use.</para>
</listitem>
<listitem>
<para><computeroutput>Lit16</computeroutput>: the field
contains a 16-bit literal.</para>
</listitem>
<listitem>
<para><computeroutput>Literal</computeroutput>: the field
denotes a 32-bit literal, whose value is stored in the
<computeroutput>lit32</computeroutput> field of the uinstr
itself. Since there is only one
<computeroutput>lit32</computeroutput> for the whole uinstr,
only one operand field may contain this tag.</para>
</listitem>
<listitem>
<para><computeroutput>SpillNo</computeroutput>: the field
contains a spill slot number, in the range 0 to 23 inclusive,
denoting one of the spill slots contained inside
<computeroutput>VG_(baseBlock)</computeroutput>. Such tags
only exist after register allocation.</para>
</listitem>
<listitem>
<para><computeroutput>RealReg</computeroutput>: the field
contains a number in the range 0 to 7 denoting an integer x86
("real") register on the host. The number is the Intel
encoding for integer registers. Such tags only exist after
register allocation.</para>
</listitem>
<listitem>
<para><computeroutput>ArchReg</computeroutput>: the field
contains a number in the range 0 to 7 denoting an integer x86
register on the simulated CPU. In reality this means a
reference to one of the first 8 words of
<computeroutput>VG_(baseBlock)</computeroutput>. Such tags
can exist at any point in the translation process.</para>
</listitem>
<listitem>
<para>Last, but not least,
<computeroutput>TempReg</computeroutput>. The field contains
the number of one of an infinite set of virtual (integer)
registers. <computeroutput>TempReg</computeroutput>s are used
everywhere throughout the translation process; you can have
as many as you want. The register allocator maps as many as
it can into <computeroutput>RealReg</computeroutput>s and
turns the rest into
<computeroutput>SpillNo</computeroutput>s, so
<computeroutput>TempReg</computeroutput>s should not exist
after the register allocation phase.</para>
<para><computeroutput>TempReg</computeroutput>s are always 32
bits long, even if the data they hold is logically shorter.
In that case the upper unused bits are required, and, I
think, generally assumed, to be zero.
<computeroutput>TempReg</computeroutput>s holding V bits for
quantities shorter than 32 bits are expected to have ones in
the unused places, since a one denotes "undefined".</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="mc-tech-docs.uinstr"
xreflabel="UCode instructions: type 'UInstr'">
<title>UCode instructions: type <computeroutput>UInstr</computeroutput></title>
<para>UCode was carefully designed to make it possible to do
register allocation on UCode and then translate the result into
x86 code without needing any extra registers ... well, that was
the original plan, anyway. Things have gotten a little more
complicated since then. In what follows, UCode instructions are
referred to as uinstrs, to distinguish them from x86
instructions. Uinstrs of course have uopcodes which are
(naturally) different from x86 opcodes.</para>
<para>A uinstr (type <computeroutput>UInstr</computeroutput>)
contains various fields, not all of which are used by any one
uopcode:</para>
<itemizedlist>
<listitem>
<para>Three 16-bit operand fields,
<computeroutput>val1</computeroutput>,
<computeroutput>val2</computeroutput> and
<computeroutput>val3</computeroutput>.</para>
</listitem>
<listitem>
<para>Three tag fields,
<computeroutput>tag1</computeroutput>,
<computeroutput>tag2</computeroutput> and
<computeroutput>tag3</computeroutput>. Each of these has a
value of type <computeroutput>Tag</computeroutput>, and they
describe what the <computeroutput>val1</computeroutput>,
<computeroutput>val2</computeroutput> and
<computeroutput>val3</computeroutput> fields contain.</para>
</listitem>
<listitem>
<para>A 32-bit literal field.</para>
</listitem>
<listitem>
<para>Two <computeroutput>FlagSet</computeroutput>s,
specifying which x86 condition codes are read and written by
the uinstr.</para>
</listitem>
<listitem>
<para>An opcode byte, containing a value of type
<computeroutput>Opcode</computeroutput>.</para>
</listitem>
<listitem>
<para>A size field, indicating the data transfer size
(1/2/4/8/10) in cases where this makes sense, or zero
otherwise.</para>
</listitem>
<listitem>
<para>A condition-code field, which, for jumps, holds a value
of type <computeroutput>Condcode</computeroutput>, indicating
the condition which applies. The encoding is as it is in the
x86 insn stream, except we add a 17th value
<computeroutput>CondAlways</computeroutput> to indicate an
unconditional transfer.</para>
</listitem>
<listitem>
<para>Various 1-bit flags, indicating whether this insn
pertains to an x86 CALL or RET instruction, whether a
widening is signed or not, etc.</para>
</listitem>
</itemizedlist>
<para>UOpcodes (type <computeroutput>Opcode</computeroutput>) are
divided into two groups: those necessary merely to express the
functionality of the x86 code, and extra uopcodes needed to
express the instrumentation. The former group contains:</para>
<itemizedlist>
<listitem>
<para><computeroutput>GET</computeroutput> and
<computeroutput>PUT</computeroutput>, which move values from
the simulated CPU's integer registers
(<computeroutput>ArchReg</computeroutput>s) into
<computeroutput>TempReg</computeroutput>s, and back.
<computeroutput>GETF</computeroutput> and
<computeroutput>PUTF</computeroutput> do the corresponding
thing for the simulated
<computeroutput>%EFLAGS</computeroutput>. There are no
corresponding insns for the FPU register stack, since we
don't explicitly simulate its registers.</para>
</listitem>
<listitem>
<para><computeroutput>LOAD</computeroutput> and
<computeroutput>STORE</computeroutput>, which, in RISC-like
fashion, are the only uinstrs able to interact with
memory.</para>
</listitem>
<listitem>
<para><computeroutput>MOV</computeroutput> and
<computeroutput>CMOV</computeroutput> allow unconditional and
conditional moves of values between
<computeroutput>TempReg</computeroutput>s.</para>
</listitem>
<listitem>
<para>ALU operations. Again in RISC-like fashion, these only
operate on <computeroutput>TempReg</computeroutput>s (before
reg-alloc) or <computeroutput>RealReg</computeroutput>s
(after reg-alloc). These are:
<computeroutput>ADD</computeroutput>,
<computeroutput>ADC</computeroutput>,
<computeroutput>AND</computeroutput>,
<computeroutput>OR</computeroutput>,
<computeroutput>XOR</computeroutput>,
<computeroutput>SUB</computeroutput>,
<computeroutput>SBB</computeroutput>,
<computeroutput>SHL</computeroutput>,
<computeroutput>SHR</computeroutput>,
<computeroutput>SAR</computeroutput>,
<computeroutput>ROL</computeroutput>,
<computeroutput>ROR</computeroutput>,
<computeroutput>RCL</computeroutput>,
<computeroutput>RCR</computeroutput>,
<computeroutput>NOT</computeroutput>,
<computeroutput>NEG</computeroutput>,
<computeroutput>INC</computeroutput>,
<computeroutput>DEC</computeroutput>,
<computeroutput>BSWAP</computeroutput>,
<computeroutput>CC2VAL</computeroutput> and
<computeroutput>WIDEN</computeroutput>.
<computeroutput>WIDEN</computeroutput> does signed or
unsigned value widening.
<computeroutput>CC2VAL</computeroutput> is used to convert
condition codes into a value, zero or one. The rest are
obvious.</para>
<para>To allow for more efficient code generation, we bend
slightly the restriction at the start of the previous para:
for <computeroutput>ADD</computeroutput>,
<computeroutput>ADC</computeroutput>,
<computeroutput>XOR</computeroutput>,
<computeroutput>SUB</computeroutput> and
<computeroutput>SBB</computeroutput>, we allow the first
(source) operand to also be an
<computeroutput>ArchReg</computeroutput>, that is, one of the
simulated machine's registers. Also, many of these ALU ops
allow the source operand to be a literal. See
<computeroutput>VG_(saneUInstr)</computeroutput> for the
final word on the allowable forms of uinstrs.</para>
</listitem>
<listitem>
<para><computeroutput>LEA1</computeroutput> and
<computeroutput>LEA2</computeroutput> are not strictly
necessary, but facilitate better translations. They
record the fancy x86 addressing modes in a direct way, which
allows those amodes to be emitted back into the final
instruction stream more or less verbatim.</para>
</listitem>
<listitem>
<para><computeroutput>CALLM</computeroutput> calls a
machine-code helper, one of the methods whose address is
stored at some
<computeroutput>VG_(baseBlock)</computeroutput> offset.
<computeroutput>PUSH</computeroutput> and
<computeroutput>POP</computeroutput> move values to/from
<computeroutput>TempReg</computeroutput> to the real
(Valgrind's) stack, and
<computeroutput>CLEAR</computeroutput> removes values from
the stack. <computeroutput>CALLM_S</computeroutput> and
<computeroutput>CALLM_E</computeroutput> delimit the
boundaries of call setups and clearings, for the benefit of
the instrumentation passes. Getting this right is critical,
and so <computeroutput>VG_(saneUCodeBlock)</computeroutput>
makes various checks on the use of these uopcodes.</para>
<para>It is important to understand that these uopcodes have
nothing to do with the x86
<computeroutput>call</computeroutput>,
<computeroutput>return,</computeroutput>
<computeroutput>push</computeroutput> or
<computeroutput>pop</computeroutput> instructions, and are
not used to implement them. Those guys turn into
combinations of <computeroutput>GET</computeroutput>,
<computeroutput>PUT</computeroutput>,
<computeroutput>LOAD</computeroutput>,
<computeroutput>STORE</computeroutput>,
<computeroutput>ADD</computeroutput>,
<computeroutput>SUB</computeroutput>, and
<computeroutput>JMP</computeroutput>. What these uopcodes
support is calling of helper functions such as
<computeroutput>VG_(helper_imul_32_64)</computeroutput>,
which do stuff which is too difficult or tedious to emit
inline.</para>
</listitem>
<listitem>
<para><computeroutput>FPU</computeroutput>,
<computeroutput>FPU_R</computeroutput> and
<computeroutput>FPU_W</computeroutput>. Valgrind doesn't
attempt to simulate the internal state of the FPU at all.
Consequently it only needs to be able to distinguish FPU ops
which read and write memory from those that don't, and for
those which do, it needs to know the effective address and
data transfer size. This is made easier because the x86 FP
instruction encoding is very regular, basically consisting of
16 bits for a non-memory FPU insn and 11 (IIRC) bits + an
address mode for a memory FPU insn. So our
<computeroutput>FPU</computeroutput> uinstr carries the 16
bits in its <computeroutput>val1</computeroutput> field. And
<computeroutput>FPU_R</computeroutput> and
<computeroutput>FPU_W</computeroutput> carry 11 bits in that
field, together with the identity of a
<computeroutput>TempReg</computeroutput> or (later)
<computeroutput>RealReg</computeroutput> which contains the
address.</para>
</listitem>
<listitem>
<para><computeroutput>JIFZ</computeroutput> is unique, in
that it allows a control-flow transfer which is not deemed to
end a basic block. It causes a jump to a literal (original)
address if the specified argument is zero.</para>
</listitem>
<listitem>
<para>Finally, <computeroutput>INCEIP</computeroutput>
advances the simulated <computeroutput>%EIP</computeroutput>
by the specified literal amount. This supports lazy
<computeroutput>%EIP</computeroutput> updating, as described
below.</para>
</listitem>
</itemizedlist>
<para>Stages 1 and 2 of the 6-stage translation process mentioned
above deal purely with these uopcodes, and no others. They are
sufficient to express pretty much all the x86 32-bit
protected-mode instruction set, at least everything understood by
a pre-MMX original Pentium (P54C).</para>
<para>Stages 3, 4, 5 and 6 also deal with the following extra
"instrumentation" uopcodes. They are used to express all the
definedness-tracking and -checking machinery which valgrind does.
In later sections we show how to create checking code for each of
the uopcodes above. Note that these instrumentation uopcodes,
although some appearing complicated, have been carefully chosen
so that efficient x86 code can be generated for them. GNU
superopt v2.5 did a great job helping out here. Anyways, the
uopcodes are as follows:</para>
<itemizedlist>
<listitem>
<para><computeroutput>GETV</computeroutput> and
<computeroutput>PUTV</computeroutput> are analogues to
<computeroutput>GET</computeroutput> and
<computeroutput>PUT</computeroutput> above. They are
identical except that they move the V bits for the specified
values back and forth to
<computeroutput>TempRegs</computeroutput>, rather than moving
the values themselves.</para>
</listitem>
<listitem>
<para>Similarly, <computeroutput>LOADV</computeroutput> and
<computeroutput>STOREV</computeroutput> read and write V bits
from the synthesised shadow memory that Valgrind maintains.
In fact they do more than that, since they also do
address-validity checks, and emit complaints if the
read/written addresses are unaddressable.</para>
</listitem>
<listitem>
<para><computeroutput>TESTV</computeroutput>, whose
parameters are a <computeroutput>TempReg</computeroutput> and
a size, tests the V bits in the
<computeroutput>TempReg</computeroutput>, at the specified
operation size (0/1/2/4 byte) and emits an error if any of
them indicate undefinedness. This is the only uopcode
capable of doing such tests.</para>
</listitem>
<listitem>
<para><computeroutput>SETV</computeroutput>, whose parameters
are also <computeroutput>TempReg</computeroutput> and a size,
makes the V bits in the
<computeroutput>TempReg</computeroutput> indicated
definedness, at the specified operation size. This is
usually used to generate the correct V bits for a literal
value, which is of course fully defined.</para>
</listitem>
<listitem>
<para><computeroutput>GETVF</computeroutput> and
<computeroutput>PUTVF</computeroutput> are analogues to
<computeroutput>GETF</computeroutput> and
<computeroutput>PUTF</computeroutput>. They move the single
V bit used to model definedness of
<computeroutput>%EFLAGS</computeroutput> between its home in
<computeroutput>VG_(baseBlock)</computeroutput> and the
specified <computeroutput>TempReg</computeroutput>.</para>
</listitem>
<listitem>
<para><computeroutput>TAG1</computeroutput> denotes one of a
family of unary operations on
<computeroutput>TempReg</computeroutput>s containing V bits.
Similarly, <computeroutput>TAG2</computeroutput> denotes one
in a family of binary operations on V bits.</para>
</listitem>
</itemizedlist>
<para>These 10 uopcodes are sufficient to express Valgrind's
entire definedness-checking semantics. In fact most of the
interesting magic is done by the
<computeroutput>TAG1</computeroutput> and
<computeroutput>TAG2</computeroutput> suboperations.</para>
<para>First, however, I need to explain about V-vector operation
sizes. There are 4 sizes: 1, 2 and 4, which operate on groups of
8, 16 and 32 V bits at a time, supporting the usual 1, 2 and 4
byte x86 operations. However there is also the mysterious size
0, which really means a single V bit. Single V bits are used in
various circumstances; in particular, the definedness of
<computeroutput>%EFLAGS</computeroutput> is modelled with a
single V bit. Now might be a good time to also point out that
for V bits, 1 means "undefined" and 0 means "defined".
Similarly, for A bits, 1 means "invalid address" and 0 means
"valid address". This seems counterintuitive (and so it is), but
testing against zero on x86s saves instructions compared to
testing against all 1s, because many ALU operations set the Z
flag for free, so to speak.</para>
<para>With that in mind, the tag ops are:</para>
<itemizedlist>
<listitem>
<formalpara>
<title>(UNARY) Pessimising casts:</title>
<para><computeroutput>VgT_PCast40</computeroutput>,
<computeroutput>VgT_PCast20</computeroutput>,
<computeroutput>VgT_PCast10</computeroutput>,
<computeroutput>VgT_PCast01</computeroutput>,
<computeroutput>VgT_PCast02</computeroutput> and
<computeroutput>VgT_PCast04</computeroutput>. A "pessimising
cast" takes a V-bit vector at one size, and creates a new one
at another size, pessimised in the sense that if any of the
bits in the source vector indicate undefinedness, then all
the bits in the result indicate undefinedness. In this case
the casts are all to or from a single V bit, so for example
<computeroutput>VgT_PCast40</computeroutput> is a pessimising
cast from 32 bits to 1, whereas
<computeroutput>VgT_PCast04</computeroutput> simply copies
the single source V bit into all 32 bit positions in the
result. Surprisingly, these ops can all be implemented very
efficiently.</para>
</formalpara>
<para>There are also the pessimising casts
<computeroutput>VgT_PCast14</computeroutput>, from 8 bits to
32, <computeroutput>VgT_PCast12</computeroutput>, from 8 bits
to 16, and <computeroutput>VgT_PCast11</computeroutput>, from
8 bits to 8. This last one seems nonsensical, but in fact it
isn't a no-op because, as mentioned above, any undefined (1)
bits in the source infect the entire result.</para>
</listitem>
<listitem>
<formalpara>
<title>(UNARY) Propagating undefinedness upwards in a
word:</title>
<para><computeroutput>VgT_Left4</computeroutput>,
<computeroutput>VgT_Left2</computeroutput> and
<computeroutput>VgT_Left1</computeroutput>. These are used
to simulate the worst-case effects of carry propagation in
adds and subtracts. They return a V vector identical to the
original, except that if the original contained any undefined
bits, then it and all bits above it are marked as undefined
too. Hence the Left bit in the names.</para></formalpara>
</listitem>
<listitem>
<formalpara>
<title>(UNARY) Signed and unsigned value widening:</title>
<para><computeroutput>VgT_SWiden14</computeroutput>,
<computeroutput>VgT_SWiden24</computeroutput>,
<computeroutput>VgT_SWiden12</computeroutput>,
<computeroutput>VgT_ZWiden14</computeroutput>,
<computeroutput>VgT_ZWiden24</computeroutput> and
<computeroutput>VgT_ZWiden12</computeroutput>. These mimic
the definedness effects of standard signed and unsigned
integer widening. Unsigned widening creates zero bits in the
new positions, so
<computeroutput>VgT_ZWiden*</computeroutput> accordingly park
mark those parts of their argument as defined. Signed
widening copies the sign bit into the new positions, so
<computeroutput>VgT_SWiden*</computeroutput> copies the
definedness of the sign bit into the new positions. Because
1 means undefined and 0 means defined, these operations can
(fascinatingly) be done by the same operations which they
mimic. Go figure.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>(BINARY) Undefined-if-either-Undefined,
Defined-if-either-Defined:</title>
<para><computeroutput>VgT_UifU4</computeroutput>,
<computeroutput>VgT_UifU2</computeroutput>,
<computeroutput>VgT_UifU1</computeroutput>,
<computeroutput>VgT_UifU0</computeroutput>,
<computeroutput>VgT_DifD4</computeroutput>,
<computeroutput>VgT_DifD2</computeroutput>,
<computeroutput>VgT_DifD1</computeroutput>. These do simple
bitwise operations on pairs of V-bit vectors, with
<computeroutput>UifU</computeroutput> giving undefined if
either arg bit is undefined, and
<computeroutput>DifD</computeroutput> giving defined if
either arg bit is defined. Abstract interpretation junkies,
if any make it this far, may like to think of them as meets
and joins (or is it joins and meets) in the definedness
lattices.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>(BINARY; one value, one V bits) Generate argument
improvement terms for AND and OR</title>
<para><computeroutput>VgT_ImproveAND4_TQ</computeroutput>,
<computeroutput>VgT_ImproveAND2_TQ</computeroutput>,
<computeroutput>VgT_ImproveAND1_TQ</computeroutput>,
<computeroutput>VgT_ImproveOR4_TQ</computeroutput>,
<computeroutput>VgT_ImproveOR2_TQ</computeroutput>,
<computeroutput>VgT_ImproveOR1_TQ</computeroutput>. These
help out with AND and OR operations. AND and OR have the
inconvenient property that the definedness of the result
depends on the actual values of the arguments as well as
their definedness. At the bit level:</para></formalpara>
<programlisting><![CDATA[
1 AND undefined = undefined, but
0 AND undefined = 0, and
similarly
0 OR undefined = undefined, but
1 OR undefined = 1.]]></programlisting>
<para>It turns out that gcc (quite legitimately) generates
code which relies on this fact, so we have to model it
properly in order to avoid flooding users with spurious value
errors. The ultimate definedness result of AND and OR is
calculated using <computeroutput>UifU</computeroutput> on the
definedness of the arguments, but we also
<computeroutput>DifD</computeroutput> in some "improvement"
terms which take into account the above phenomena.</para>
<para><computeroutput>ImproveAND</computeroutput> takes as
its first argument the actual value of an argument to AND
(the T) and the definedness of that argument (the Q), and
returns a V-bit vector which is defined (0) for bits which
have value 0 and are defined; this, when
<computeroutput>DifD</computeroutput> into the final result
causes those bits to be defined even if the corresponding bit
in the other argument is undefined.</para>
<para>The <computeroutput>ImproveOR</computeroutput> ops do
the dual thing for OR arguments. Note that XOR does not have
this property that one argument can make the other
irrelevant, so there is no need for such complexity for
XOR.</para>
</listitem>
</itemizedlist>
<para>That's all the tag ops. If you stare at this long enough,
and then run Valgrind and stare at the pre- and post-instrumented
ucode, it should be fairly obvious how the instrumentation
machinery hangs together.</para>
<para>One point, if you do this: in order to make it easy to
differentiate <computeroutput>TempReg</computeroutput>s carrying
values from <computeroutput>TempReg</computeroutput>s carrying V
bit vectors, Valgrind prints the former as (for example)
<computeroutput>t28</computeroutput> and the latter as
<computeroutput>q28</computeroutput>; the fact that they carry
the same number serves to indicate their relationship. This is
purely for the convenience of the human reader; the register
allocator and code generator don't regard them as
different.</para>
</sect2>
<sect2 id="mc-tech-docs.trans" xreflabel="Translation into UCode">
<title>Translation into UCode</title>
<para><computeroutput>VG_(disBB)</computeroutput> allocates a new
<computeroutput>UCodeBlock</computeroutput> and then uses
<computeroutput>disInstr</computeroutput> to translate x86
instructions one at a time into UCode, dumping the result in the
<computeroutput>UCodeBlock</computeroutput>. This goes on until
a control-flow transfer instruction is encountered.</para>
<para>Despite the large size of
<filename>vg_to_ucode.c</filename>, this translation is really
very simple. Each x86 instruction is translated entirely
independently of its neighbours, merrily allocating new
<computeroutput>TempReg</computeroutput>s as it goes. The idea
is to have a simple translator -- in reality, no more than a
macro-expander -- and the -- resulting bad UCode translation is
cleaned up by the UCode optimisation phase which follows. To
give you an idea of some x86 instructions and their translations
(this is a complete basic block, as Valgrind sees it):</para>
<programlisting><![CDATA[
0x40435A50: incl %edx
0: GETL %EDX, t0
1: INCL t0 (-wOSZAP)
2: PUTL t0, %EDX
0x40435A51: movsbl (%edx),%eax
3: GETL %EDX, t2
4: LDB (t2), t2
5: WIDENL_Bs t2
6: PUTL t2, %EAX
0x40435A54: testb $0x20, 1(%ecx,%eax,2)
7: GETL %EAX, t6
8: GETL %ECX, t8
9: LEA2L 1(t8,t6,2), t4
10: LDB (t4), t10
11: MOVB $0x20, t12
12: ANDB t12, t10 (-wOSZACP)
13: INCEIPo $9
0x40435A59: jnz-8 0x40435A50
14: Jnzo $0x40435A50 (-rOSZACP)
15: JMPo $0x40435A5B]]></programlisting>
<para>Notice how the block always ends with an unconditional jump
to the next block. This is a bit unnecessary, but makes many
things simpler.</para>
<para>Most x86 instructions turn into sequences of
<computeroutput>GET</computeroutput>,
<computeroutput>PUT</computeroutput>,
<computeroutput>LEA1</computeroutput>,
<computeroutput>LEA2</computeroutput>,
<computeroutput>LOAD</computeroutput> and
<computeroutput>STORE</computeroutput>. Some complicated ones
however rely on calling helper bits of code in
<filename>vg_helpers.S</filename>. The ucode instructions
<computeroutput>PUSH</computeroutput>,
<computeroutput>POP</computeroutput>,
<computeroutput>CALL</computeroutput>,
<computeroutput>CALLM_S</computeroutput> and
<computeroutput>CALLM_E</computeroutput> support this. The
calling convention is somewhat ad-hoc and is not the C calling
convention. The helper routines must save all integer registers,
and the flags, that they use. Args are passed on the stack
underneath the return address, as usual, and if result(s) are to
be returned, it (they) are either placed in dummy arg slots
created by the ucode <computeroutput>PUSH</computeroutput>
sequence, or just overwrite the incoming args.</para>
<para>In order that the instrumentation mechanism can handle
calls to these helpers,
<computeroutput>VG_(saneUCodeBlock)</computeroutput> enforces the
following restrictions on calls to helpers:</para>
<itemizedlist>
<listitem>
<para>Each <computeroutput>CALL</computeroutput> uinstr must
be bracketed by a preceding
<computeroutput>CALLM_S</computeroutput> marker (dummy
uinstr) and a trailing
<computeroutput>CALLM_E</computeroutput> marker. These
markers are used by the instrumentation mechanism later to
establish the boundaries of the
<computeroutput>PUSH</computeroutput>,
<computeroutput>POP</computeroutput> and
<computeroutput>CLEAR</computeroutput> sequences for the
call.</para>
</listitem>
<listitem>
<para><computeroutput>PUSH</computeroutput>,
<computeroutput>POP</computeroutput> and
<computeroutput>CLEAR</computeroutput> may only appear inside
sections bracketed by
<computeroutput>CALLM_S</computeroutput> and
<computeroutput>CALLM_E</computeroutput>, and nowhere else.</para>
</listitem>
<listitem>
<para>In any such bracketed section, no two
<computeroutput>PUSH</computeroutput> insns may push the same
<computeroutput>TempReg</computeroutput>. Dually, no two two
<computeroutput>POP</computeroutput>s may pop the same
<computeroutput>TempReg</computeroutput>.</para>
</listitem>
<listitem>
<para>Finally, although this is not checked, args should be
removed from the stack with
<computeroutput>CLEAR</computeroutput>, rather than
<computeroutput>POP</computeroutput>s into a
<computeroutput>TempReg</computeroutput> which is not
subsequently used. This is because the instrumentation
mechanism assumes that all values
<computeroutput>POP</computeroutput>ped from the stack are
actually used.</para>
</listitem>
</itemizedlist>
<para>Some of the translations may appear to have redundant
<computeroutput>TempReg</computeroutput>-to-<computeroutput>TempReg</computeroutput>
moves. This helps the next phase, UCode optimisation, to
generate better code.</para>
</sect2>
<sect2 id="mc-tech-docs.optim" xreflabel="UCode optimisation">
<title>UCode optimisation</title>
<para>UCode is then subjected to an improvement pass
(<computeroutput>vg_improve()</computeroutput>), which blurs the
boundaries between the translations of the original x86
instructions. It's pretty straightforward. Three
transformations are done:</para>
<itemizedlist>
<listitem>
<para>Redundant <computeroutput>GET</computeroutput>
elimination. Actually, more general than that -- eliminates
redundant fetches of ArchRegs. In our running example,
uinstr 3 <computeroutput>GET</computeroutput>s
<computeroutput>%EDX</computeroutput> into
<computeroutput>t2</computeroutput> despite the fact that, by
looking at the previous uinstr, it is already in
<computeroutput>t0</computeroutput>. The
<computeroutput>GET</computeroutput> is therefore removed,
and <computeroutput>t2</computeroutput> renamed to
<computeroutput>t0</computeroutput>. Assuming
<computeroutput>t0</computeroutput> is allocated to a host
register, it means the simulated
<computeroutput>%EDX</computeroutput> will exist in a host
CPU register for more than one simulated x86 instruction,
which seems to me to be a highly desirable property.</para>
<para>There is some mucking around to do with subregisters;
<computeroutput>%AL</computeroutput> vs
<computeroutput>%AH</computeroutput>
<computeroutput>%AX</computeroutput> vs
<computeroutput>%EAX</computeroutput> etc. I can't remember
how it works, but in general we are very conservative, and
these tend to invalidate the caching.</para>
</listitem>
<listitem>
<para>Redundant <computeroutput>PUT</computeroutput>
elimination. This annuls
<computeroutput>PUT</computeroutput>s of values back to
simulated CPU registers if a later
<computeroutput>PUT</computeroutput> would overwrite the
earlier <computeroutput>PUT</computeroutput> value, and there
is no intervening reads of the simulated register
(<computeroutput>ArchReg</computeroutput>).</para>
<para>As before, we are paranoid when faced with subregister
references. Also, <computeroutput>PUT</computeroutput>s of
<computeroutput>%ESP</computeroutput> are never annulled,
because it is vital the instrumenter always has an up-to-date
<computeroutput>%ESP</computeroutput> value available,
<computeroutput>%ESP</computeroutput> changes affect
addressability of the memory around the simulated stack
pointer.</para>
<para>The implication of the above paragraph is that the
simulated machine's registers are only lazily updated once
the above two optimisation phases have run, with the
exception of <computeroutput>%ESP</computeroutput>.
<computeroutput>TempReg</computeroutput>s go dead at the end
of every basic block, from which is is inferrable that any
<computeroutput>TempReg</computeroutput> caching a simulated
CPU reg is flushed (back into the relevant
<computeroutput>VG_(baseBlock)</computeroutput> slot) at the
end of every basic block. The further implication is that
the simulated registers are only up-to-date at in between
basic blocks, and not at arbitrary points inside basic
blocks. And the consequence of that is that we can only
deliver signals to the client in between basic blocks. None
of this seems any problem in practice.</para>
</listitem>
<listitem>
<para>Finally there is a simple def-use thing for condition
codes. If an earlier uinstr writes the condition codes, and
the next uinsn along which actually cares about the condition
codes writes the same or larger set of them, but does not
read any, the earlier uinsn is marked as not writing any
condition codes. This saves a lot of redundant cond-code
saving and restoring.</para>
</listitem>
</itemizedlist>
<para>The effect of these transformations on our short block is
rather unexciting, and shown below. On longer basic blocks they
can dramatically improve code quality.</para>
<programlisting><![CDATA[
at 3: delete GET, rename t2 to t0 in (4 .. 6)
at 7: delete GET, rename t6 to t0 in (8 .. 9)
at 1: annul flag write OSZAP due to later OSZACP
Improved code:
0: GETL %EDX, t0
1: INCL t0
2: PUTL t0, %EDX
4: LDB (t0), t0
5: WIDENL_Bs t0
6: PUTL t0, %EAX
8: GETL %ECX, t8
9: LEA2L 1(t8,t0,2), t4
10: LDB (t4), t10
11: MOVB $0x20, t12
12: ANDB t12, t10 (-wOSZACP)
13: INCEIPo $9
14: Jnzo $0x40435A50 (-rOSZACP)
15: JMPo $0x40435A5B]]></programlisting>
</sect2>
<sect2 id="mc-tech-docs.instrum" xreflabel="UCode instrumentation">
<title>UCode instrumentation</title>
<para>Once you understand the meaning of the instrumentation
uinstrs, discussed in detail above, the instrumentation scheme is
fairly straightforward. Each uinstr is instrumented in
isolation, and the instrumentation uinstrs are placed before the
original uinstr. Our running example continues below. I have
placed a blank line after every original ucode, to make it easier
to see which instrumentation uinstrs correspond to which
originals.</para>
<para>As mentioned somewhere above,
<computeroutput>TempReg</computeroutput>s carrying values have
names like <computeroutput>t28</computeroutput>, and each one has
a shadow carrying its V bits, with names like
<computeroutput>q28</computeroutput>. This pairing aids in
reading instrumented ucode.</para>
<para>One decision about all this is where to have "observation
points", that is, where to check that V bits are valid. I use a
minimalistic scheme, only checking where a failure of validity
could cause the original program to (seg)fault. So the use of
values as memory addresses causes a check, as do conditional
jumps (these cause a check on the definedness of the condition
codes). And arguments <computeroutput>PUSH</computeroutput>ed
for helper calls are checked, hence the weird restrictions on
help call preambles described above.</para>
<para>Another decision is that once a value is tested, it is
thereafter regarded as defined, so that we do not emit multiple
undefined-value errors for the same undefined value. That means
that <computeroutput>TESTV</computeroutput> uinstrs are always
followed by <computeroutput>SETV</computeroutput> on the same
(shadow) <computeroutput>TempReg</computeroutput>s. Most of
these <computeroutput>SETV</computeroutput>s are redundant and
are removed by the post-instrumentation cleanup phase.</para>
<para>The instrumentation for calling helper functions deserves
further comment. The definedness of results from a helper is
modelled using just one V bit. So, in short, we do pessimising
casts of the definedness of all the args, down to a single bit,
and then <computeroutput>UifU</computeroutput> these bits
together. So this single V bit will say "undefined" if any part
of any arg is undefined. This V bit is then pessimally cast back
up to the result(s) sizes, as needed. If, by seeing that all the
args are got rid of with <computeroutput>CLEAR</computeroutput>
and none with <computeroutput>POP</computeroutput>, Valgrind sees
that the result of the call is not actually used, it immediately
examines the result V bit with a
<computeroutput>TESTV</computeroutput> --
<computeroutput>SETV</computeroutput> pair. If it did not do
this, there would be no observation point to detect that the some
of the args to the helper were undefined. Of course, if the
helper's results are indeed used, we don't do this, since the
result usage will presumably cause the result definedness to be
checked at some suitable future point.</para>
<para>In general Valgrind tries to track definedness on a
bit-for-bit basis, but as the above para shows, for calls to
helpers we throw in the towel and approximate down to a single
bit. This is because it's too complex and difficult to track
bit-level definedness through complex ops such as integer
multiply and divide, and in any case there is no reasonable code
fragments which attempt to (eg) multiply two partially-defined
values and end up with something meaningful, so there seems
little point in modelling multiplies, divides, etc, in that level
of detail.</para>
<para>Integer loads and stores are instrumented with firstly a
test of the definedness of the address, followed by a
<computeroutput>LOADV</computeroutput> or
<computeroutput>STOREV</computeroutput> respectively. These turn
into calls to (for example)
<computeroutput>VG_(helperc_LOADV4)</computeroutput>. These
helpers do two things: they perform an address-valid check, and
they load or store V bits from/to the relevant address in the
(simulated V-bit) memory.</para>
<para>FPU loads and stores are different. As above the
definedness of the address is first tested. However, the helper
routine for FPU loads
(<computeroutput>VGM_(fpu_read_check)</computeroutput>) emits an
error if either the address is invalid or the referenced area
contains undefined values. It has to do this because we do not
simulate the FPU at all, and so cannot track definedness of
values loaded into it from memory, so we have to check them as
soon as they are loaded into the FPU, ie, at this point. We
notionally assume that everything in the FPU is defined.</para>
<para>It follows therefore that FPU writes first check the
definedness of the address, then the validity of the address, and
finally mark the written bytes as well-defined.</para>
<para>If anyone is inspired to extend Valgrind to MMX/SSE insns,
I suggest you use the same trick. It works provided that the
FPU/MMX unit is not used to merely as a conduit to copy partially
undefined data from one place in memory to another.
Unfortunately the integer CPU is used like that (when copying C
structs with holes, for example) and this is the cause of much of
the elaborateness of the instrumentation here described.</para>
<para><computeroutput>vg_instrument()</computeroutput> in
<filename>vg_translate.c</filename> actually does the
instrumentation. There are comments explaining how each uinstr
is handled, so we do not repeat that here. As explained already,
it is bit-accurate, except for calls to helper functions.
Unfortunately the x86 insns
<computeroutput>bt/bts/btc/btr</computeroutput> are done by
helper fns, so bit-level accuracy is lost there. This should be
fixed by doing them inline; it will probably require adding a
couple new uinstrs. Also, left and right rotates through the
carry flag (x86 <computeroutput>rcl</computeroutput> and
<computeroutput>rcr</computeroutput>) are approximated via a
single V bit; so far this has not caused anyone to complain. The
non-carry rotates, <computeroutput>rol</computeroutput> and
<computeroutput>ror</computeroutput>, are much more common and
are done exactly. Re-visiting the instrumentation for AND and
OR, they seem rather verbose, and I wonder if it could be done
more concisely now.</para>
<para>The lowercase <computeroutput>o</computeroutput> on many of
the uopcodes in the running example indicates that the size field
is zero, usually meaning a single-bit operation.</para>
<para>Anyroads, the post-instrumented version of our running
example looks like this:</para>
<programlisting><![CDATA[
Instrumented code:
0: GETVL %EDX, q0
1: GETL %EDX, t0
2: TAG1o q0 = Left4 ( q0 )
3: INCL t0
4: PUTVL q0, %EDX
5: PUTL t0, %EDX
6: TESTVL q0
7: SETVL q0
8: LOADVB (t0), q0
9: LDB (t0), t0
10: TAG1o q0 = SWiden14 ( q0 )
11: WIDENL_Bs t0
12: PUTVL q0, %EAX
13: PUTL t0, %EAX
14: GETVL %ECX, q8
15: GETL %ECX, t8
16: MOVL q0, q4
17: SHLL $0x1, q4
18: TAG2o q4 = UifU4 ( q8, q4 )
19: TAG1o q4 = Left4 ( q4 )
20: LEA2L 1(t8,t0,2), t4
21: TESTVL q4
22: SETVL q4
23: LOADVB (t4), q10
24: LDB (t4), t10
25: SETVB q12
26: MOVB $0x20, t12
27: MOVL q10, q14
28: TAG2o q14 = ImproveAND1_TQ ( t10, q14 )
29: TAG2o q10 = UifU1 ( q12, q10 )
30: TAG2o q10 = DifD1 ( q14, q10 )
31: MOVL q12, q14
32: TAG2o q14 = ImproveAND1_TQ ( t12, q14 )
33: TAG2o q10 = DifD1 ( q14, q10 )
34: MOVL q10, q16
35: TAG1o q16 = PCast10 ( q16 )
36: PUTVFo q16
37: ANDB t12, t10 (-wOSZACP)
38: INCEIPo $9
39: GETVFo q18
40: TESTVo q18
41: SETVo q18
42: Jnzo $0x40435A50 (-rOSZACP)
43: JMPo $0x40435A5B]]></programlisting>
</sect2>
<sect2 id="mc-tech-docs.cleanup"
xreflabel="UCode post-instrumentation cleanup">
<title>UCode post-instrumentation cleanup</title>
<para>This pass, coordinated by
<computeroutput>vg_cleanup()</computeroutput>, removes redundant
definedness computation created by the simplistic instrumentation
pass. It consists of two passes,
<computeroutput>vg_propagate_definedness()</computeroutput>
followed by
<computeroutput>vg_delete_redundant_SETVs</computeroutput>.</para>
<para><computeroutput>vg_propagate_definedness()</computeroutput>
is a simple constant-propagation and constant-folding pass. It
tries to determine which
<computeroutput>TempReg</computeroutput>s containing V bits will
always indicate "fully defined", and it propagates this
information as far as it can, and folds out as many operations as
possible. For example, the instrumentation for an ADD of a
literal to a variable quantity will be reduced down so that the
definedness of the result is simply the definedness of the
variable quantity, since the literal is by definition fully
defined.</para>
<para><computeroutput>vg_delete_redundant_SETVs</computeroutput>
removes <computeroutput>SETV</computeroutput>s on shadow
<computeroutput>TempReg</computeroutput>s for which the next
action is a write. I don't think there's anything else worth
saying about this; it is simple. Read the sources for
details.</para>
<para>So the cleaned-up running example looks like this. As
above, I have inserted line breaks after every original
(non-instrumentation) uinstr to aid readability. As with
straightforward ucode optimisation, the results in this block are
undramatic because it is so short; longer blocks benefit more
because they have more redundancy which gets eliminated.</para>
<programlisting><![CDATA[
at 29: delete UifU1 due to defd arg1
at 32: change ImproveAND1_TQ to MOV due to defd arg2
at 41: delete SETV
at 31: delete MOV
at 25: delete SETV
at 22: delete SETV
at 7: delete SETV
0: GETVL %EDX, q0
1: GETL %EDX, t0
2: TAG1o q0 = Left4 ( q0 )
3: INCL t0
4: PUTVL q0, %EDX
5: PUTL t0, %EDX
6: TESTVL q0
8: LOADVB (t0), q0
9: LDB (t0), t0
10: TAG1o q0 = SWiden14 ( q0 )
11: WIDENL_Bs t0
12: PUTVL q0, %EAX
13: PUTL t0, %EAX
14: GETVL %ECX, q8
15: GETL %ECX, t8
16: MOVL q0, q4
17: SHLL $0x1, q4
18: TAG2o q4 = UifU4 ( q8, q4 )
19: TAG1o q4 = Left4 ( q4 )
20: LEA2L 1(t8,t0,2), t4
21: TESTVL q4
23: LOADVB (t4), q10
24: LDB (t4), t10
26: MOVB $0x20, t12
27: MOVL q10, q14
28: TAG2o q14 = ImproveAND1_TQ ( t10, q14 )
30: TAG2o q10 = DifD1 ( q14, q10 )
32: MOVL t12, q14
33: TAG2o q10 = DifD1 ( q14, q10 )
34: MOVL q10, q16
35: TAG1o q16 = PCast10 ( q16 )
36: PUTVFo q16
37: ANDB t12, t10 (-wOSZACP)
38: INCEIPo $9
39: GETVFo q18
40: TESTVo q18
42: Jnzo $0x40435A50 (-rOSZACP)
43: JMPo $0x40435A5B]]></programlisting>
</sect2>
<sect2 id="mc-tech-docs.transfrom" xreflabel="Translation from UCode">
<title>Translation from UCode</title>
<para>This is all very simple, even though
<filename>vg_from_ucode.c</filename> is a big file.
Position-independent x86 code is generated into a dynamically
allocated array <computeroutput>emitted_code</computeroutput>;
this is doubled in size when it overflows. Eventually the array
is handed back to the caller of
<computeroutput>VG_(translate)</computeroutput>, who must copy
the result into TC and TT, and free the array.</para>
<para>This file is structured into four layers of abstraction,
which, thankfully, are glued back together with extensive
<computeroutput>__inline__</computeroutput> directives. From the
bottom upwards:</para>
<itemizedlist>
<listitem>
<para>Address-mode emitters,
<computeroutput>emit_amode_regmem_reg</computeroutput> et
al.</para>
</listitem>
<listitem>
<para>Emitters for specific x86 instructions. There are
quite a lot of these, with names such as
<computeroutput>emit_movv_offregmem_reg</computeroutput>.
The <computeroutput>v</computeroutput> suffix is Intel
parlance for a 16/32 bit insn; there are also
<computeroutput>b</computeroutput> suffixes for 8 bit
insns.</para>
</listitem>
<listitem>
<para>The next level up are the
<computeroutput>synth_*</computeroutput> functions, which
synthesise possibly a sequence of raw x86 instructions to do
some simple task. Some of these are quite complex because
they have to work around Intel's silly restrictions on
subregister naming. See
<computeroutput>synth_nonshiftop_reg_reg</computeroutput> for
example.</para>
</listitem>
<listitem>
<para>Finally, at the top of the heap, we have
<computeroutput>emitUInstr()</computeroutput>, which emits
code for a single uinstr.</para>
</listitem>
</itemizedlist>
<para>Some comments:</para>
<itemizedlist>
<listitem>
<para>The hack for FPU instructions becomes apparent here.
To do a <computeroutput>FPU</computeroutput> ucode
instruction, we load the simulated FPU's state into from its
<computeroutput>VG_(baseBlock)</computeroutput> into the real
FPU using an x86 <computeroutput>frstor</computeroutput>
insn, do the ucode <computeroutput>FPU</computeroutput> insn
on the real CPU, and write the updated FPU state back into
<computeroutput>VG_(baseBlock)</computeroutput> using an
<computeroutput>fnsave</computeroutput> instruction. This is
pretty brutal, but is simple and it works, and even seems
tolerably efficient. There is no attempt to cache the
simulated FPU state in the real FPU over multiple
back-to-back ucode FPU instructions.</para>
<para><computeroutput>FPU_R</computeroutput> and
<computeroutput>FPU_W</computeroutput> are also done this
way, with the minor complication that we need to patch in
some addressing mode bits so the resulting insn knows the
effective address to use. This is easy because of the
regularity of the x86 FPU instruction encodings.</para>
</listitem>
<listitem>
<para>An analogous trick is done with ucode insns which
claim, in their <computeroutput>flags_r</computeroutput> and
<computeroutput>flags_w</computeroutput> fields, that they
read or write the simulated
<computeroutput>%EFLAGS</computeroutput>. For such cases we
first copy the simulated
<computeroutput>%EFLAGS</computeroutput> into the real
<computeroutput>%eflags</computeroutput>, then do the insn,
then, if the insn says it writes the flags, copy back to
<computeroutput>%EFLAGS</computeroutput>. This is a bit
expensive, which is why the ucode optimisation pass goes to
some effort to remove redundant flag-update annotations.</para>
</listitem>
</itemizedlist>
<para>And so ... that's the end of the documentation for the
instrumentating translator! It's really not that complex,
because it's composed as a sequence of simple(ish) self-contained
transformations on straight-line blocks of code.</para>
</sect2>
<sect2 id="mc-tech-docs.dispatch" xreflabel="Top-level dispatch loop">
<title>Top-level dispatch loop</title>
<para>Urk. In <computeroutput>VG_(toploop)</computeroutput>.
This is basically boring and unsurprising, not to mention fiddly
and fragile. It needs to be cleaned up.</para>
<para>The only perhaps surprise is that the whole thing is run on
top of a <computeroutput>setjmp</computeroutput>-installed
exception handler, because, supposing a translation got a
segfault, we have to bail out of the Valgrind-supplied exception
handler <computeroutput>VG_(oursignalhandler)</computeroutput>
and immediately start running the client's segfault handler, if
it has one. In particular we can't finish the current basic
block and then deliver the signal at some convenient future
point, because signals like SIGILL, SIGSEGV and SIGBUS mean that
the faulting insn should not simply be re-tried. (I'm sure there
is a clearer way to explain this).</para>
</sect2>
<sect2 id="mc-tech-docs.lazy"
xreflabel="Lazy updates of the simulated program counter">
<title>Lazy updates of the simulated program counter</title>
<para>Simulated <computeroutput>%EIP</computeroutput> is not
updated after every simulated x86 insn as this was regarded as
too expensive. Instead ucode
<computeroutput>INCEIP</computeroutput> insns move it along as
and when necessary. Currently we don't allow it to fall more
than 4 bytes behind reality (see
<computeroutput>VG_(disBB)</computeroutput> for the way this
works).</para>
<para>Note that <computeroutput>%EIP</computeroutput> is always
brought up to date by the inner dispatch loop in
<computeroutput>VG_(dispatch)</computeroutput>, so that if the
client takes a fault we know at least which basic block this
happened in.</para>
</sect2>
<sect2 id="mc-tech-docs.signals" xreflabel="Signals">
<title>Signals</title>
<para>Horrible, horrible. <filename>vg_signals.c</filename>.
Basically, since we have to intercept all system calls anyway, we
can see when the client tries to install a signal handler. If it
does so, we make a note of what the client asked to happen, and
ask the kernel to route the signal to our own signal handler,
<computeroutput>VG_(oursignalhandler)</computeroutput>. This
simply notes the delivery of signals, and returns.</para>
<para>Every 1000 basic blocks, we see if more signals have
arrived. If so,
<computeroutput>VG_(deliver_signals)</computeroutput> builds
signal delivery frames on the client's stack, and allows their
handlers to be run. Valgrind places in these signal delivery
frames a bogus return address,
<computeroutput>VG_(signalreturn_bogusRA)</computeroutput>, and
checks all jumps to see if any jump to it. If so, this is a sign
that a signal handler is returning, and if so Valgrind removes
the relevant signal frame from the client's stack, restores the
from the signal frame the simulated state before the signal was
delivered, and allows the client to run onwards. We have to do
it this way because some signal handlers never return, they just
<computeroutput>longjmp()</computeroutput>, which nukes the
signal delivery frame.</para>
<para>The Linux kernel has a different but equally horrible hack
for detecting signal handler returns. Discovering it is left as
an exercise for the reader.</para>
</sect2>
<sect2 id="mc-tech-docs.todo">
<title>To be written</title>
<para>The following is a list of as-yet-not-written stuff. Apologies.</para>
<orderedlist>
<listitem>
<para>The translation cache and translation table</para>
</listitem>
<listitem>
<para>Exceptions, creating new translations</para>
</listitem>
<listitem>
<para>Self-modifying code</para>
</listitem>
<listitem>
<para>Errors, error contexts, error reporting, suppressions</para>
</listitem>
<listitem>
<para>Client malloc/free</para>
</listitem>
<listitem>
<para>Low-level memory management</para>
</listitem>
<listitem>
<para>A and V bitmaps</para>
</listitem>
<listitem>
<para>Symbol table management</para>
</listitem>
<listitem>
<para>Dealing with system calls</para>
</listitem>
<listitem>
<para>Namespace management</para>
</listitem>
<listitem>
<para>GDB attaching</para>
</listitem>
<listitem>
<para>Non-dependence on glibc or anything else</para>
</listitem>
<listitem>
<para>The leak detector</para>
</listitem>
<listitem>
<para>Performance problems</para>
</listitem>
<listitem>
<para>Continuous sanity checking</para>
</listitem>
<listitem>
<para>Tracing, or not tracing, child processes</para>
</listitem>
<listitem>
<para>Assembly glue for syscalls</para>
</listitem>
</orderedlist>
</sect2>
</sect1>
<sect1 id="mc-tech-docs.extensions" xreflabel="Extensions">
<title>Extensions</title>
<para>Some comments about Stuff To Do.</para>
<sect2 id="mc-tech-docs.bugs" xreflabel="Bugs">
<title>Bugs</title>
<para>Stephan Kulow and Marc Mutz report problems with kmail in
KDE 3 CVS (RC2 ish) when run on Valgrind. Stephan has it
deadlocking; Marc has it looping at startup. I can't repro
either behaviour. Needs repro-ing and fixing.</para>
</sect2>
<sect2 id="mc-tech-docs.threads" xreflabel="Threads">
<title>Threads</title>
<para>Doing a good job of thread support strikes me as almost a
research-level problem. The central issues are how to do fast
cheap locking of the
<computeroutput>VG_(primary_map)</computeroutput> structure,
whether or not accesses to the individual secondary maps need
locking, what race-condition issues result, and whether the
already-nasty mess that is the signal simulator needs further
hackery.</para>
<para>I realise that threads are the most-frequently-requested
feature, and I am thinking about it all. If you have guru-level
understanding of fast mutual exclusion mechanisms and race
conditions, I would be interested in hearing from you.</para>
</sect2>
<sect2 id="mc-tech-docs.verify" xreflabel="Verification suite">
<title>Verification suite</title>
<para>Directory <computeroutput>tests/</computeroutput> contains
various ad-hoc tests for Valgrind. However, there is no
systematic verification or regression suite, that, for example,
exercises all the stuff in <filename>vg_memory.c</filename>, to
ensure that illegal memory accesses and undefined value uses are
detected as they should be. It would be good to have such a
suite.</para>
</sect2>
<sect2 id="mc-tech-docs.porting" xreflabel="Porting to other platforms">
<title>Porting to other platforms</title>
<para>It would be great if Valgrind was ported to FreeBSD and x86
NetBSD, and to x86 OpenBSD, if it's possible (doesn't OpenBSD use
a.out-style executables, not ELF ?)</para>
<para>The main difficulties, for an x86-ELF platform, seem to
be:</para>
<itemizedlist>
<listitem>
<para>You'd need to rewrite the
<computeroutput>/proc/self/maps</computeroutput> parser
(<filename>vg_procselfmaps.c</filename>). Easy.</para>
</listitem>
<listitem>
<para>You'd need to rewrite
<filename>vg_syscall_mem.c</filename>, or, more specifically,
provide one for your OS. This is tedious, but you can
implement syscalls on demand, and the Linux kernel interface
is, for the most part, going to look very similar to the *BSD
interfaces, so it's really a copy-paste-and-modify-on-demand
job. As part of this, you'd need to supply a new
<filename>vg_kerneliface.h</filename> file.</para>
</listitem>
<listitem>
<para>You'd also need to change the syscall wrappers for
Valgrind's internal use, in
<filename>vg_mylibc.c</filename>.</para>
</listitem>
</itemizedlist>
<para>All in all, I think a port to x86-ELF *BSDs is not really
very difficult, and in some ways I would like to see it happen,
because that would force a more clear factoring of Valgrind into
platform dependent and independent pieces. Not to mention, *BSD
folks also deserve to use Valgrind just as much as the Linux crew
do.</para>
</sect2>
</sect1>
<sect1 id="mc-tech-docs.easystuff"
xreflabel="Easy stuff which ought to be done">
<title>Easy stuff which ought to be done</title>
<sect2 id="mc-tech-docs.mmx" xreflabel="MMX Instructions">
<title>MMX Instructions</title>
<para>MMX insns should be supported, using the same trick as for
FPU insns. If the MMX registers are not used to copy
uninitialised junk from one place to another in memory, this
means we don't have to actually simulate the internal MMX unit
state, so the FPU hack applies. This should be fairly
easy.</para>
</sect2>
<sect2 id="mc-tech-docs.fixstabs" xreflabel="Fix stabs-info Reader">
<title>Fix stabs-info reader</title>
<para>The machinery in <filename>vg_symtab2.c</filename> which
reads "stabs" style debugging info is pretty weak. It usually
correctly translates simulated program counter values into line
numbers and procedure names, but the file name is often
completely wrong. I think the logic used to parse "stabs"
entries is weak. It should be fixed. The simplest solution,
IMO, is to copy either the logic or simply the code out of GNU
binutils which does this; since GDB can clearly get it right,
binutils (or GDB?) must have code to do this somewhere.</para>
</sect2>
<sect2 id="mc-tech-docs.x86instr" xreflabel="BT/BTC/BTS/BTR">
<title>BT/BTC/BTS/BTR</title>
<para>These are x86 instructions which test, complement, set, or
reset, a single bit in a word. At the moment they are both
incorrectly implemented and incorrectly instrumented.</para>
<para>The incorrect instrumentation is due to use of helper
functions. This means we lose bit-level definedness tracking,
which could wind up giving spurious uninitialised-value use
errors. The Right Thing to do is to invent a couple of new
UOpcodes, I think <computeroutput>GET_BIT</computeroutput> and
<computeroutput>SET_BIT</computeroutput>, which can be used to
implement all 4 x86 insns, get rid of the helpers, and give
bit-accurate instrumentation rules for the two new
UOpcodes.</para>
<para>I realised the other day that they are mis-implemented too.
The x86 insns take a bit-index and a register or memory location
to access. For registers the bit index clearly can only be in
the range zero to register-width minus 1, and I assumed the same
applied to memory locations too. But evidently not; for memory
locations the index can be arbitrary, and the processor will
index arbitrarily into memory as a result. This too should be
fixed. Sigh. Presumably indexing outside the immediate word is
not actually used by any programs yet tested on Valgrind, for
otherwise they (presumably) would simply not work at all. If you
plan to hack on this, first check the Intel docs to make sure my
understanding is really correct.</para>
</sect2>
<sect2 id="mc-tech-docs.prefetch" xreflabel="Using PREFETCH Instructions">
<title>Using PREFETCH Instructions</title>
<para>Here's a small but potentially interesting project for
performance junkies. Experiments with valgrind's code generator
and optimiser(s) suggest that reducing the number of instructions
executed in the translations and mem-check helpers gives
disappointingly small performance improvements. Perhaps this is
because performance of Valgrindified code is limited by cache
misses. After all, each read in the original program now gives
rise to at least three reads, one for the
<computeroutput>VG_(primary_map)</computeroutput>, one of the
resulting secondary, and the original. Not to mention, the
instrumented translations are 13 to 14 times larger than the
originals. All in all one would expect the memory system to be
hammered to hell and then some.</para>
<para>So here's an idea. An x86 insn involving a read from
memory, after instrumentation, will turn into ucode of the
following form:</para>
<programlisting><![CDATA[
... calculate effective addr, into ta and qa ...
TESTVL qa -- is the addr defined?
LOADV (ta), qloaded -- fetch V bits for the addr
LOAD (ta), tloaded -- do the original load]]></programlisting>
<para>At the point where the
<computeroutput>LOADV</computeroutput> is done, we know the
actual address (<computeroutput>ta</computeroutput>) from which
the real <computeroutput>LOAD</computeroutput> will be done. We
also know that the <computeroutput>LOADV</computeroutput> will
take around 20 x86 insns to do. So it seems plausible that doing
a prefetch of <computeroutput>ta</computeroutput> just before the
<computeroutput>LOADV</computeroutput> might just avoid a miss at
the <computeroutput>LOAD</computeroutput> point, and that might
be a significant performance win.</para>
<para>Prefetch insns are notoriously tempermental, more often
than not making things worse rather than better, so this would
require considerable fiddling around. It's complicated because
Intels and AMDs have different prefetch insns with different
semantics, so that too needs to be taken into account. As a
general rule, even placing the prefetches before the
<computeroutput>LOADV</computeroutput> insn is too near the
<computeroutput>LOAD</computeroutput>; the ideal distance is
apparently circa 200 CPU cycles. So it might be worth having
another analysis/transformation pass which pushes prefetches as
far back as possible, hopefully immediately after the effective
address becomes available.</para>
<para>Doing too many prefetches is also bad because they soak up
bus bandwidth / cpu resources, so some cleverness in deciding
which loads to prefetch and which to not might be helpful. One
can imagine not prefetching client-stack-relative
(<computeroutput>%EBP</computeroutput> or
<computeroutput>%ESP</computeroutput>) accesses, since the stack
in general tends to show good locality anyway.</para>
<para>There's quite a lot of experimentation to do here, but I
think it might make an interesting week's work for
someone.</para>
<para>As of 15-ish March 2002, I've started to experiment with
this, using the AMD
<computeroutput>prefetch/prefetchw</computeroutput> insns.</para>
</sect2>
<sect2 id="mc-tech-docs.pranges" xreflabel="User-defined Permission Ranges">
<title>User-defined Permission Ranges</title>
<para>This is quite a large project -- perhaps a month's hacking
for a capable hacker to do a good job -- but it's potentially
very interesting. The outcome would be that Valgrind could
detect a whole class of bugs which it currently cannot.</para>
<para>The presentation falls into two pieces.</para>
<sect3 id="mc-tech-docs.psetting"
xreflabel="Part 1: User-defined Address-range Permission Setting">
<title>Part 1: User-defined Address-range Permission Setting</title>
<para>Valgrind intercepts the client's
<computeroutput>malloc</computeroutput>,
<computeroutput>free</computeroutput>, etc calls, watches system
calls, and watches the stack pointer move. This is currently the
only way it knows about which addresses are valid and which not.
Sometimes the client program knows extra information about its
memory areas. For example, the client could at some point know
that all elements of an array are out-of-date. We would like to
be able to convey to Valgrind this information that the array is
now addressable-but-uninitialised, so that Valgrind can then warn
if elements are used before they get new values.</para>
<para>What I would like are some macros like this:</para>
<programlisting><![CDATA[
VALGRIND_MAKE_NOACCESS(addr, len)
VALGRIND_MAKE_WRITABLE(addr, len)
VALGRIND_MAKE_READABLE(addr, len)]]></programlisting>
<para>and also, to check that memory is
addressable/initialised,</para>
<programlisting><![CDATA[
VALGRIND_CHECK_ADDRESSABLE(addr, len)
VALGRIND_CHECK_INITIALISED(addr, len)]]></programlisting>
<para>I then include in my sources a header defining these
macros, rebuild my app, run under Valgrind, and get user-defined
checks.</para>
<para>Now here's a neat trick. It's a nuisance to have to
re-link the app with some new library which implements the above
macros. So the idea is to define the macros so that the
resulting executable is still completely stand-alone, and can be
run without Valgrind, in which case the macros do nothing, but
when run on Valgrind, the Right Thing happens. How to do this?
The idea is for these macros to turn into a piece of inline
assembly code, which (1) has no effect when run on the real CPU,
(2) is easily spotted by Valgrind's JITter, and (3) no sane
person would ever write, which is important for avoiding false
matches in (2). So here's a suggestion:</para>
<programlisting><![CDATA[
VALGRIND_MAKE_NOACCESS(addr, len)]]></programlisting>
<para>becomes (roughly speaking)</para>
<programlisting><![CDATA[
movl addr, %eax
movl len, %ebx
movl $1, %ecx -- 1 describes the action; MAKE_WRITABLE might be
-- 2, etc
rorl $13, %ecx
rorl $19, %ecx
rorl $11, %eax
rorl $21, %eax]]></programlisting>
<para>The rotate sequences have no effect, and it's unlikely they
would appear for any other reason, but they define a unique
byte-sequence which the JITter can easily spot. Using the
operand constraints section at the end of a gcc inline-assembly
statement, we can tell gcc that the assembly fragment kills
<computeroutput>%eax</computeroutput>,
<computeroutput>%ebx</computeroutput>,
<computeroutput>%ecx</computeroutput> and the condition codes, so
this fragment is made harmless when not running on Valgrind, runs
quickly when not on Valgrind, and does not require any other
library support.</para>
</sect3>
<sect3 id="mc-tech-docs.prange-detect"
xreflabel="Part 2: Using it to detect Interference between Stack
Variables">
<title>Part 2: Using it to detect Interference between Stack
Variables</title>
<para>Currently Valgrind cannot detect errors of the following
form:</para>
<programlisting><![CDATA[
void fooble ( void )
{
int a[10];
int b[10];
a[10] = 99;
}]]></programlisting>
<para>Now imagine rewriting this as</para>
<programlisting><![CDATA[
void fooble ( void )
{
int spacer0;
int a[10];
int spacer1;
int b[10];
int spacer2;
VALGRIND_MAKE_NOACCESS(&spacer0, sizeof(int));
VALGRIND_MAKE_NOACCESS(&spacer1, sizeof(int));
VALGRIND_MAKE_NOACCESS(&spacer2, sizeof(int));
a[10] = 99;
}]]></programlisting>
<para>Now the invalid write is certain to hit
<computeroutput>spacer0</computeroutput> or
<computeroutput>spacer1</computeroutput>, so Valgrind will spot
the error.</para>
<para>There are two complications.</para>
<orderedlist>
<listitem>
<para>The first is that we don't want to annotate sources by
hand, so the Right Thing to do is to write a C/C++ parser,
annotator, prettyprinter which does this automatically, and
run it on post-CPP'd C/C++ source. The parser/prettyprinter
is probably not as hard as it sounds; I would write it in Haskell,
a powerful functional language well suited to doing symbolic
computation, with which I am intimately familiar. There is
already a C parser written in Haskell by someone in the
Haskell community, and that would probably be a good starting
point.</para>
</listitem>
<listitem>
<para>The second complication is how to get rid of these
<computeroutput>NOACCESS</computeroutput> records inside
Valgrind when the instrumented function exits; after all,
these refer to stack addresses and will make no sense
whatever when some other function happens to re-use the same
stack address range, probably shortly afterwards. I think I
would be inclined to define a special stack-specific
macro:</para>
<programlisting><![CDATA[
VALGRIND_MAKE_NOACCESS_STACK(addr, len)]]></programlisting>
<para>which causes Valgrind to record the client's
<computeroutput>%ESP</computeroutput> at the time it is
executed. Valgrind will then watch for changes in
<computeroutput>%ESP</computeroutput> and discard such
records as soon as the protected area is uncovered by an
increase in <computeroutput>%ESP</computeroutput>. I
hesitate with this scheme only because it is potentially
expensive, if there are hundreds of such records, and
considering that changes in
<computeroutput>%ESP</computeroutput> already require
expensive messing with stack access permissions.</para>
</listitem>
</orderedlist>
<para>This is probably easier and more robust than for the
instrumenter program to try and spot all exit points for the
procedure and place suitable deallocation annotations there.
Plus C++ procedures can bomb out at any point if they get an
exception, so spotting return points at the source level just
won't work at all.</para>
<para>Although some work, it's all eminently doable, and it would
make Valgrind into an even-more-useful tool.</para>
</sect3>
</sect2>
</sect1>
</chapter>