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<h1>LLVM's Analysis and Transform Passes</h1>
<ol>
<li><a href="#intro">Introduction</a></li>
<li><a href="#analyses">Analysis Passes</a>
<li><a href="#transforms">Transform Passes</a></li>
<li><a href="#utilities">Utility Passes</a></li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
and Gordon Henriksen</p>
</div>
<!-- ======================================================================= -->
<h2><a name="intro">Introduction</a></h2>
<div>
<p>This document serves as a high level summary of the optimization features
that LLVM provides. Optimizations are implemented as Passes that traverse some
portion of a program to either collect information or transform the program.
The table below divides the passes that LLVM provides into three categories.
Analysis passes compute information that other passes can use or for debugging
or program visualization purposes. Transform passes can use (or invalidate)
the analysis passes. Transform passes all mutate the program in some way.
Utility passes provides some utility but don't otherwise fit categorization.
For example passes to extract functions to bitcode or write a module to
bitcode are neither analysis nor transform passes.
<p>The table below provides a quick summary of each pass and links to the more
complete pass description later in the document.</p>
<table>
<tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr>
<tr><th>Option</th><th>Name</th></tr>
<tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr>
<tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (stateless AA impl)</td></tr>
<tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
<tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
<tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
<tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
<tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
<tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
<tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
<tr><td><a href="#dot-cfg-only">-dot-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
<tr><td><a href="#dot-dom">-dot-dom</a></td><td>Print dominance tree of function to 'dot' file</td></tr>
<tr><td><a href="#dot-dom-only">-dot-dom-only</a></td><td>Print dominance tree of function to 'dot' file (with no function bodies)</td></tr>
<tr><td><a href="#dot-postdom">-dot-postdom</a></td><td>Print postdominance tree of function to 'dot' file</td></tr>
<tr><td><a href="#dot-postdom-only">-dot-postdom-only</a></td><td>Print postdominance tree of function to 'dot' file (with no function bodies)</td></tr>
<tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr>
<tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
<tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
<tr><td><a href="#iv-users">-iv-users</a></td><td>Induction Variable Users</td></tr>
<tr><td><a href="#lazy-value-info">-lazy-value-info</a></td><td>Lazy Value Information Analysis</td></tr>
<tr><td><a href="#lda">-lda</a></td><td>Loop Dependence Analysis</td></tr>
<tr><td><a href="#libcall-aa">-libcall-aa</a></td><td>LibCall Alias Analysis</td></tr>
<tr><td><a href="#lint">-lint</a></td><td>Statically lint-checks LLVM IR</td></tr>
<tr><td><a href="#loops">-loops</a></td><td>Natural Loop Information</td></tr>
<tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
<tr><td><a href="#module-debuginfo">-module-debuginfo</a></td><td>Decodes module-level debug info</td></tr>
<tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
<tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
<tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
<tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
<tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr>
<tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr>
<tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr>
<tr><td><a href="#print-dbginfo">-print-dbginfo</a></td><td>Print debug info in human readable form</td></tr>
<tr><td><a href="#print-dom-info">-print-dom-info</a></td><td>Dominator Info Printer</td></tr>
<tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
<tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr>
<tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr>
<tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr>
<tr><td><a href="#profile-estimator">-profile-estimator</a></td><td>Estimate profiling information</td></tr>
<tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
<tr><td><a href="#profile-verifier">-profile-verifier</a></td><td>Verify profiling information</td></tr>
<tr><td><a href="#regions">-regions</a></td><td>Detect single entry single exit regions</td></tr>
<tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
<tr><td><a href="#scev-aa">-scev-aa</a></td><td>ScalarEvolution-based Alias Analysis</td></tr>
<tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
<tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
<tr><th>Option</th><th>Name</th></tr>
<tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
<tr><td><a href="#always-inline">-always-inline</a></td><td>Inliner for always_inline functions</td></tr>
<tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr>
<tr><td><a href="#bb-vectorize">-bb-vectorize</a></td><td>Combine instructions to form vector instructions within basic blocks</td></tr>
<tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr>
<tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr>
<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr>
<tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr>
<tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr>
<tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr>
<tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr>
<tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr>
<tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr>
<tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr>
<tr><td><a href="#functionattrs">-functionattrs</a></td><td>Deduce function attributes</td></tr>
<tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr>
<tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr>
<tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr>
<tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr>
<tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr>
<tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr>
<tr><td><a href="#insert-optimal-edge-profiling">-insert-optimal-edge-profiling</a></td><td>Insert optimal instrumentation for edge profiling</td></tr>
<tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr>
<tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr>
<tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr>
<tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr>
<tr><td><a href="#jump-threading">-jump-threading</a></td><td>Jump Threading</td></tr>
<tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr>
<tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr>
<tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Delete dead loops</td></tr>
<tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr>
<tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr>
<tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr>
<tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr>
<tr><td><a href="#loop-simplify">-loop-simplify</a></td><td>Canonicalize natural loops</td></tr>
<tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr>
<tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr>
<tr><td><a href="#loweratomic">-loweratomic</a></td><td>Lower atomic intrinsics to non-atomic form</td></tr>
<tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr>
<tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
<tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
<tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>MemCpy Optimization</td></tr>
<tr><td><a href="#mergefunc">-mergefunc</a></td><td>Merge Functions</td></tr>
<tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
<tr><td><a href="#partial-inliner">-partial-inliner</a></td><td>Partial Inliner</td></tr>
<tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
<tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
<tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
<tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates (DT)</td></tr>
<tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
<tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
<tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
<tr><td><a href="#sink">-sink</a></td><td>Code sinking</td></tr>
<tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments to multiple ret values</td></tr>
<tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
<tr><td><a href="#strip-dead-debug-info">-strip-dead-debug-info</a></td><td>Strip debug info for unused symbols</td></tr>
<tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Strip Unused Function Prototypes</td></tr>
<tr><td><a href="#strip-debug-declare">-strip-debug-declare</a></td><td>Strip all llvm.dbg.declare intrinsics</td></tr>
<tr><td><a href="#strip-nondebug">-strip-nondebug</a></td><td>Strip all symbols, except dbg symbols, from a module</td></tr>
<tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
<tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
<tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
<tr><th>Option</th><th>Name</th></tr>
<tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
<tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
<tr><td><a href="#instnamer">-instnamer</a></td><td>Assign names to anonymous instructions</td></tr>
<tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
<tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
<tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
<tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
<tr><td><a href="#view-dom">-view-dom</a></td><td>View dominance tree of function</td></tr>
<tr><td><a href="#view-dom-only">-view-dom-only</a></td><td>View dominance tree of function (with no function bodies)</td></tr>
<tr><td><a href="#view-postdom">-view-postdom</a></td><td>View postdominance tree of function</td></tr>
<tr><td><a href="#view-postdom-only">-view-postdom-only</a></td><td>View postdominance tree of function (with no function bodies)</td></tr>
</table>
</div>
<!-- ======================================================================= -->
<h2><a name="analyses">Analysis Passes</a></h2>
<div>
<p>This section describes the LLVM Analysis Passes.</p>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="aa-eval">-aa-eval: Exhaustive Alias Analysis Precision Evaluator</a>
</h3>
<div>
<p>This is a simple N^2 alias analysis accuracy evaluator.
Basically, for each function in the program, it simply queries to see how the
alias analysis implementation answers alias queries between each pair of
pointers in the function.</p>
<p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
Spadini, and Wojciech Stryjewski.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="basicaa">-basicaa: Basic Alias Analysis (stateless AA impl)</a>
</h3>
<div>
<p>A basic alias analysis pass that implements identities (two different
globals cannot alias, etc), but does no stateful analysis.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="basiccg">-basiccg: Basic CallGraph Construction</a>
</h3>
<div>
<p>Yet to be written.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a>
</h3>
<div>
<p>
A pass which can be used to count how many alias queries
are being made and how the alias analysis implementation being used responds.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="debug-aa">-debug-aa: AA use debugger</a>
</h3>
<div>
<p>
This simple pass checks alias analysis users to ensure that if they
create a new value, they do not query AA without informing it of the value.
It acts as a shim over any other AA pass you want.
</p>
<p>
Yes keeping track of every value in the program is expensive, but this is
a debugging pass.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="domfrontier">-domfrontier: Dominance Frontier Construction</a>
</h3>
<div>
<p>
This pass is a simple dominator construction algorithm for finding forward
dominator frontiers.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="domtree">-domtree: Dominator Tree Construction</a>
</h3>
<div>
<p>
This pass is a simple dominator construction algorithm for finding forward
dominators.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the call graph into a
<code>.dot</code> graph. This graph can then be processed with the "dot" tool
to convert it to postscript or some other suitable format.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the control flow graph
into a <code>.dot</code> graph. This graph can then be processed with the
"dot" tool to convert it to postscript or some other suitable format.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the control flow graph
into a <code>.dot</code> graph, omitting the function bodies. This graph can
then be processed with the "dot" tool to convert it to postscript or some
other suitable format.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dot-dom">-dot-dom: Print dominance tree of function to 'dot' file</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the dominator tree
into a <code>.dot</code> graph. This graph can then be processed with the
"dot" tool to convert it to postscript or some other suitable format.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dot-dom-only">-dot-dom-only: Print dominance tree of function to 'dot' file (with no function bodies)</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the dominator tree
into a <code>.dot</code> graph, omitting the function bodies. This graph can
then be processed with the "dot" tool to convert it to postscript or some
other suitable format.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dot-postdom">-dot-postdom: Print postdominance tree of function to 'dot' file</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the post dominator tree
into a <code>.dot</code> graph. This graph can then be processed with the
"dot" tool to convert it to postscript or some other suitable format.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dot-postdom-only">-dot-postdom-only: Print postdominance tree of function to 'dot' file (with no function bodies)</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the post dominator tree
into a <code>.dot</code> graph, omitting the function bodies. This graph can
then be processed with the "dot" tool to convert it to postscript or some
other suitable format.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a>
</h3>
<div>
<p>
This simple pass provides alias and mod/ref information for global values
that do not have their address taken, and keeps track of whether functions
read or write memory (are "pure"). For this simple (but very common) case,
we can provide pretty accurate and useful information.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="instcount">-instcount: Counts the various types of Instructions</a>
</h3>
<div>
<p>
This pass collects the count of all instructions and reports them
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="intervals">-intervals: Interval Partition Construction</a>
</h3>
<div>
<p>
This analysis calculates and represents the interval partition of a function,
or a preexisting interval partition.
</p>
<p>
In this way, the interval partition may be used to reduce a flow graph down
to its degenerate single node interval partition (unless it is irreducible).
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="iv-users">-iv-users: Induction Variable Users</a>
</h3>
<div>
<p>Bookkeeping for "interesting" users of expressions computed from
induction variables.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a>
</h3>
<div>
<p>Interface for lazy computation of value constraint information.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="lda">-lda: Loop Dependence Analysis</a>
</h3>
<div>
<p>Loop dependence analysis framework, which is used to detect dependences in
memory accesses in loops.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a>
</h3>
<div>
<p>LibCall Alias Analysis.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="lint">-lint: Statically lint-checks LLVM IR</a>
</h3>
<div>
<p>This pass statically checks for common and easily-identified constructs
which produce undefined or likely unintended behavior in LLVM IR.</p>
<p>It is not a guarantee of correctness, in two ways. First, it isn't
comprehensive. There are checks which could be done statically which are
not yet implemented. Some of these are indicated by TODO comments, but
those aren't comprehensive either. Second, many conditions cannot be
checked statically. This pass does no dynamic instrumentation, so it
can't check for all possible problems.</p>
<p>Another limitation is that it assumes all code will be executed. A store
through a null pointer in a basic block which is never reached is harmless,
but this pass will warn about it anyway.</p>
<p>Optimization passes may make conditions that this pass checks for more or
less obvious. If an optimization pass appears to be introducing a warning,
it may be that the optimization pass is merely exposing an existing
condition in the code.</p>
<p>This code may be run before instcombine. In many cases, instcombine checks
for the same kinds of things and turns instructions with undefined behavior
into unreachable (or equivalent). Because of this, this pass makes some
effort to look through bitcasts and so on.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loops">-loops: Natural Loop Information</a>
</h3>
<div>
<p>
This analysis is used to identify natural loops and determine the loop depth
of various nodes of the CFG. Note that the loops identified may actually be
several natural loops that share the same header node... not just a single
natural loop.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="memdep">-memdep: Memory Dependence Analysis</a>
</h3>
<div>
<p>
An analysis that determines, for a given memory operation, what preceding
memory operations it depends on. It builds on alias analysis information, and
tries to provide a lazy, caching interface to a common kind of alias
information query.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="module-debuginfo">-module-debuginfo: Decodes module-level debug info</a>
</h3>
<div>
<p>This pass decodes the debug info metadata in a module and prints in a
(sufficiently-prepared-) human-readable form.
For example, run this pass from opt along with the -analyze option, and
it'll print to standard output.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a>
</h3>
<div>
<p>
This is the default implementation of the Alias Analysis interface. It always
returns "I don't know" for alias queries. NoAA is unlike other alias analysis
implementations, in that it does not chain to a previous analysis. As such it
doesn't follow many of the rules that other alias analyses must.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="no-profile">-no-profile: No Profile Information</a>
</h3>
<div>
<p>
The default "no profile" implementation of the abstract
<code>ProfileInfo</code> interface.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a>
</h3>
<div>
<p>
This pass is a simple post-dominator construction algorithm for finding
post-dominator frontiers.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a>
</h3>
<div>
<p>
This pass is a simple post-dominator construction algorithm for finding
post-dominators.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a>
</h3>
<div>
<p>Yet to be written.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-callgraph">-print-callgraph: Print a call graph</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the call graph to
standard error in a human-readable form.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the SCCs of the call
graph to standard error in a human-readable form.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints the SCCs of each
function CFG to standard error in a human-readable form.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a>
</h3>
<div>
<p>Pass that prints instructions, and associated debug info:</p>
<ul>
<li>source/line/col information</li>
<li>original variable name</li>
<li>original type name</li>
</ul>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-dom-info">-print-dom-info: Dominator Info Printer</a>
</h3>
<div>
<p>Dominator Info Printer.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a>
</h3>
<div>
<p>
This pass, only available in <code>opt</code>, prints out call sites to
external functions that are called with constant arguments. This can be
useful when looking for standard library functions we should constant fold
or handle in alias analyses.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-function">-print-function: Print function to stderr</a>
</h3>
<div>
<p>
The <code>PrintFunctionPass</code> class is designed to be pipelined with
other <code>FunctionPass</code>es, and prints out the functions of the module
as they are processed.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-module">-print-module: Print module to stderr</a>
</h3>
<div>
<p>
This pass simply prints out the entire module when it is executed.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="print-used-types">-print-used-types: Find Used Types</a>
</h3>
<div>
<p>
This pass is used to seek out all of the types in use by the program. Note
that this analysis explicitly does not include types only used by the symbol
table.
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="profile-estimator">-profile-estimator: Estimate profiling information</a>
</h3>
<div>
<p>Profiling information that estimates the profiling information
in a very crude and unimaginative way.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a>
</h3>
<div>
<p>
A concrete implementation of profiling information that loads the information
from a profile dump file.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="profile-verifier">-profile-verifier: Verify profiling information</a>
</h3>
<div>
<p>Pass that checks profiling information for plausibility.</p>
</div>
<h3>
<a name="regions">-regions: Detect single entry single exit regions</a>
</h3>
<div>
<p>
The <code>RegionInfo</code> pass detects single entry single exit regions in a
function, where a region is defined as any subgraph that is connected to the
remaining graph at only two spots. Furthermore, an hierarchical region tree is
built.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a>
</h3>
<div>
<p>
The <code>ScalarEvolution</code> analysis can be used to analyze and
catagorize scalar expressions in loops. It specializes in recognizing general
induction variables, representing them with the abstract and opaque
<code>SCEV</code> class. Given this analysis, trip counts of loops and other
important properties can be obtained.
</p>
<p>
This analysis is primarily useful for induction variable substitution and
strength reduction.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="scev-aa">-scev-aa: ScalarEvolution-based Alias Analysis</a>
</h3>
<div>
<p>Simple alias analysis implemented in terms of ScalarEvolution queries.
This differs from traditional loop dependence analysis in that it tests
for dependencies within a single iteration of a loop, rather than
dependencies between different iterations.
ScalarEvolution has a more complete understanding of pointer arithmetic
than BasicAliasAnalysis' collection of ad-hoc analyses.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="targetdata">-targetdata: Target Data Layout</a>
</h3>
<div>
<p>Provides other passes access to information on how the size and alignment
required by the target ABI for various data types.</p>
</div>
</div>
<!-- ======================================================================= -->
<h2><a name="transforms">Transform Passes</a></h2>
<div>
<p>This section describes the LLVM Transform Passes.</p>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="adce">-adce: Aggressive Dead Code Elimination</a>
</h3>
<div>
<p>ADCE aggressively tries to eliminate code. This pass is similar to
<a href="#dce">DCE</a> but it assumes that values are dead until proven
otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to
the liveness of values.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="always-inline">-always-inline: Inliner for always_inline functions</a>
</h3>
<div>
<p>A custom inliner that handles only functions that are marked as
"always inline".</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a>
</h3>
<div>
<p>
This pass promotes "by reference" arguments to be "by value" arguments. In
practice, this means looking for internal functions that have pointer
arguments. If it can prove, through the use of alias analysis, that an
argument is *only* loaded, then it can pass the value into the function
instead of the address of the value. This can cause recursive simplification
of code and lead to the elimination of allocas (especially in C++ template
code like the STL).
</p>
<p>
This pass also handles aggregate arguments that are passed into a function,
scalarizing them if the elements of the aggregate are only loaded. Note that
it refuses to scalarize aggregates which would require passing in more than
three operands to the function, because passing thousands of operands for a
large array or structure is unprofitable!
</p>
<p>
Note that this transformation could also be done for arguments that are only
stored to (returning the value instead), but does not currently. This case
would be best handled when and if LLVM starts supporting multiple return
values from functions.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="bb-vectorize">-bb-vectorize: Basic-Block Vectorization</a>
</h3>
<div>
<p>This pass combines instructions inside basic blocks to form vector
instructions. It iterates over each basic block, attempting to pair
compatible instructions, repeating this process until no additional
pairs are selected for vectorization. When the outputs of some pair
of compatible instructions are used as inputs by some other pair of
compatible instructions, those pairs are part of a potential
vectorization chain. Instruction pairs are only fused into vector
instructions when they are part of a chain longer than some
threshold length. Moreover, the pass attempts to find the best
possible chain for each pair of compatible instructions. These
heuristics are intended to prevent vectorization in cases where
it would not yield a performance increase of the resulting code.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a>
</h3>
<div>
<p>This pass is a very simple profile guided basic block placement algorithm.
The idea is to put frequently executed blocks together at the start of the
function and hopefully increase the number of fall-through conditional
branches. If there is no profile information for a particular function, this
pass basically orders blocks in depth-first order.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a>
</h3>
<div>
<p>
Break all of the critical edges in the CFG by inserting a dummy basic block.
It may be "required" by passes that cannot deal with critical edges. This
transformation obviously invalidates the CFG, but can update forward dominator
(set, immediate dominators, tree, and frontier) information.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="codegenprepare">-codegenprepare: Optimize for code generation</a>
</h3>
<div>
This pass munges the code in the input function to better prepare it for
SelectionDAG-based code generation. This works around limitations in it's
basic-block-at-a-time approach. It should eventually be removed.
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="constmerge">-constmerge: Merge Duplicate Global Constants</a>
</h3>
<div>
<p>
Merges duplicate global constants together into a single constant that is
shared. This is useful because some passes (ie TraceValues) insert a lot of
string constants into the program, regardless of whether or not an existing
string is available.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="constprop">-constprop: Simple constant propagation</a>
</h3>
<div>
<p>This file implements constant propagation and merging. It looks for
instructions involving only constant operands and replaces them with a
constant value instead of an instruction. For example:</p>
<blockquote><pre>add i32 1, 2</pre></blockquote>
<p>becomes</p>
<blockquote><pre>i32 3</pre></blockquote>
<p>NOTE: this pass has a habit of making definitions be dead. It is a good
idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass
sometime after running this pass.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dce">-dce: Dead Code Elimination</a>
</h3>
<div>
<p>
Dead code elimination is similar to <a href="#die">dead instruction
elimination</a>, but it rechecks instructions that were used by removed
instructions to see if they are newly dead.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="deadargelim">-deadargelim: Dead Argument Elimination</a>
</h3>
<div>
<p>
This pass deletes dead arguments from internal functions. Dead argument
elimination removes arguments which are directly dead, as well as arguments
only passed into function calls as dead arguments of other functions. This
pass also deletes dead arguments in a similar way.
</p>
<p>
This pass is often useful as a cleanup pass to run after aggressive
interprocedural passes, which add possibly-dead arguments.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a>
</h3>
<div>
<p>
This pass is used to cleanup the output of GCC. It eliminate names for types
that are unused in the entire translation unit, using the <a
href="#findusedtypes">find used types</a> pass.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="die">-die: Dead Instruction Elimination</a>
</h3>
<div>
<p>
Dead instruction elimination performs a single pass over the function,
removing instructions that are obviously dead.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="dse">-dse: Dead Store Elimination</a>
</h3>
<div>
<p>
A trivial dead store elimination that only considers basic-block local
redundant stores.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="functionattrs">-functionattrs: Deduce function attributes</a>
</h3>
<div>
<p>A simple interprocedural pass which walks the call-graph, looking for
functions which do not access or only read non-local memory, and marking them
readnone/readonly. In addition, it marks function arguments (of pointer type)
'nocapture' if a call to the function does not create any copies of the pointer
value that outlive the call. This more or less means that the pointer is only
dereferenced, and not returned from the function or stored in a global.
This pass is implemented as a bottom-up traversal of the call-graph.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="globaldce">-globaldce: Dead Global Elimination</a>
</h3>
<div>
<p>
This transform is designed to eliminate unreachable internal globals from the
program. It uses an aggressive algorithm, searching out globals that are
known to be alive. After it finds all of the globals which are needed, it
deletes whatever is left over. This allows it to delete recursive chunks of
the program which are unreachable.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="globalopt">-globalopt: Global Variable Optimizer</a>
</h3>
<div>
<p>
This pass transforms simple global variables that never have their address
taken. If obviously true, it marks read/write globals as constant, deletes
variables only stored to, etc.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="gvn">-gvn: Global Value Numbering</a>
</h3>
<div>
<p>
This pass performs global value numbering to eliminate fully and partially
redundant instructions. It also performs redundant load elimination.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="indvars">-indvars: Canonicalize Induction Variables</a>
</h3>
<div>
<p>
This transformation analyzes and transforms the induction variables (and
computations derived from them) into simpler forms suitable for subsequent
analysis and transformation.
</p>
<p>
This transformation makes the following changes to each loop with an
identifiable induction variable:
</p>
<ol>
<li>All loops are transformed to have a <em>single</em> canonical
induction variable which starts at zero and steps by one.</li>
<li>The canonical induction variable is guaranteed to be the first PHI node
in the loop header block.</li>
<li>Any pointer arithmetic recurrences are raised to use array
subscripts.</li>
</ol>
<p>
If the trip count of a loop is computable, this pass also makes the following
changes:
</p>
<ol>
<li>The exit condition for the loop is canonicalized to compare the
induction value against the exit value. This turns loops like:
<blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote>
into
<blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li>
<li>Any use outside of the loop of an expression derived from the indvar
is changed to compute the derived value outside of the loop, eliminating
the dependence on the exit value of the induction variable. If the only
purpose of the loop is to compute the exit value of some derived
expression, this transformation will make the loop dead.</li>
</ol>
<p>
This transformation should be followed by strength reduction after all of the
desired loop transformations have been performed. Additionally, on targets
where it is profitable, the loop could be transformed to count down to zero
(the "do loop" optimization).
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="inline">-inline: Function Integration/Inlining</a>
</h3>
<div>
<p>
Bottom-up inlining of functions into callees.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a>
</h3>
<div>
<p>
This pass instruments the specified program with counters for edge profiling.
Edge profiling can give a reasonable approximation of the hot paths through a
program, and is used for a wide variety of program transformations.
</p>
<p>
Note that this implementation is very naïve. It inserts a counter for
<em>every</em> edge in the program, instead of using control flow information
to prune the number of counters inserted.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a>
</h3>
<div>
<p>This pass instruments the specified program with counters for edge profiling.
Edge profiling can give a reasonable approximation of the hot paths through a
program, and is used for a wide variety of program transformations.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="instcombine">-instcombine: Combine redundant instructions</a>
</h3>
<div>
<p>
Combine instructions to form fewer, simple
instructions. This pass does not modify the CFG This pass is where algebraic
simplification happens.
</p>
<p>
This pass combines things like:
</p>
<blockquote><pre
>%Y = add i32 %X, 1
%Z = add i32 %Y, 1</pre></blockquote>
<p>
into:
</p>
<blockquote><pre
>%Z = add i32 %X, 2</pre></blockquote>
<p>
This is a simple worklist driven algorithm.
</p>
<p>
This pass guarantees that the following canonicalizations are performed on
the program:
</p>
<ul>
<li>If a binary operator has a constant operand, it is moved to the right-
hand side.</li>
<li>Bitwise operators with constant operands are always grouped so that
shifts are performed first, then <code>or</code>s, then
<code>and</code>s, then <code>xor</code>s.</li>
<li>Compare instructions are converted from <code><</code>,
<code>></code>, <code>≤</code>, or <code>≥</code> to
<code>=</code> or <code>≠</code> if possible.</li>
<li>All <code>cmp</code> instructions on boolean values are replaced with
logical operations.</li>
<li><code>add <var>X</var>, <var>X</var></code> is represented as
<code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li>
<li>Multiplies with a constant power-of-two argument are transformed into
shifts.</li>
<li>… etc.</li>
</ul>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="internalize">-internalize: Internalize Global Symbols</a>
</h3>
<div>
<p>
This pass loops over all of the functions in the input module, looking for a
main function. If a main function is found, all other functions and all
global variables with initializers are marked as internal.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a>
</h3>
<div>
<p>
This pass implements an <em>extremely</em> simple interprocedural constant
propagation pass. It could certainly be improved in many different ways,
like using a worklist. This pass makes arguments dead, but does not remove
them. The existing dead argument elimination pass should be run after this
to clean up the mess.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a>
</h3>
<div>
<p>
An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
Propagation</a>.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="jump-threading">-jump-threading: Jump Threading</a>
</h3>
<div>
<p>
Jump threading tries to find distinct threads of control flow running through
a basic block. This pass looks at blocks that have multiple predecessors and
multiple successors. If one or more of the predecessors of the block can be
proven to always cause a jump to one of the successors, we forward the edge
from the predecessor to the successor by duplicating the contents of this
block.
</p>
<p>
An example of when this can occur is code like this:
</p>
<pre
>if () { ...
X = 4;
}
if (X < 3) {</pre>
<p>
In this case, the unconditional branch at the end of the first if can be
revectored to the false side of the second if.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a>
</h3>
<div>
<p>
This pass transforms loops by placing phi nodes at the end of the loops for
all values that are live across the loop boundary. For example, it turns
the left into the right code:
</p>
<pre
>for (...) for (...)
if (c) if (c)
X1 = ... X1 = ...
else else
X2 = ... X2 = ...
X3 = phi(X1, X2) X3 = phi(X1, X2)
... = X3 + 4 X4 = phi(X3)
... = X4 + 4</pre>
<p>
This is still valid LLVM; the extra phi nodes are purely redundant, and will
be trivially eliminated by <code>InstCombine</code>. The major benefit of
this transformation is that it makes many other loop optimizations, such as
LoopUnswitching, simpler.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="licm">-licm: Loop Invariant Code Motion</a>
</h3>
<div>
<p>
This pass performs loop invariant code motion, attempting to remove as much
code from the body of a loop as possible. It does this by either hoisting
code into the preheader block, or by sinking code to the exit blocks if it is
safe. This pass also promotes must-aliased memory locations in the loop to
live in registers, thus hoisting and sinking "invariant" loads and stores.
</p>
<p>
This pass uses alias analysis for two purposes:
</p>
<ul>
<li>Moving loop invariant loads and calls out of loops. If we can determine
that a load or call inside of a loop never aliases anything stored to,
we can hoist it or sink it like any other instruction.</li>
<li>Scalar Promotion of Memory - If there is a store instruction inside of
the loop, we try to move the store to happen AFTER the loop instead of
inside of the loop. This can only happen if a few conditions are true:
<ul>
<li>The pointer stored through is loop invariant.</li>
<li>There are no stores or loads in the loop which <em>may</em> alias
the pointer. There are no calls in the loop which mod/ref the
pointer.</li>
</ul>
If these conditions are true, we can promote the loads and stores in the
loop of the pointer to use a temporary alloca'd variable. We then use
the mem2reg functionality to construct the appropriate SSA form for the
variable.</li>
</ul>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-deletion">-loop-deletion: Delete dead loops</a>
</h3>
<div>
<p>
This file implements the Dead Loop Deletion Pass. This pass is responsible
for eliminating loops with non-infinite computable trip counts that have no
side effects or volatile instructions, and do not contribute to the
computation of the function's return value.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-extract">-loop-extract: Extract loops into new functions</a>
</h3>
<div>
<p>
A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to
extract each top-level loop into its own new function. If the loop is the
<em>only</em> loop in a given function, it is not touched. This is a pass most
useful for debugging via bugpoint.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a>
</h3>
<div>
<p>
Similar to <a href="#loop-extract">Extract loops into new functions</a>,
this pass extracts one natural loop from the program into a function if it
can. This is used by bugpoint.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a>
</h3>
<div>
<p>
This pass performs a strength reduction on array references inside loops that
have as one or more of their components the loop induction variable. This is
accomplished by creating a new value to hold the initial value of the array
access for the first iteration, and then creating a new GEP instruction in
the loop to increment the value by the appropriate amount.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-rotate">-loop-rotate: Rotate Loops</a>
</h3>
<div>
<p>A simple loop rotation transformation.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-simplify">-loop-simplify: Canonicalize natural loops</a>
</h3>
<div>
<p>
This pass performs several transformations to transform natural loops into a
simpler form, which makes subsequent analyses and transformations simpler and
more effective.
</p>
<p>
Loop pre-header insertion guarantees that there is a single, non-critical
entry edge from outside of the loop to the loop header. This simplifies a
number of analyses and transformations, such as LICM.
</p>
<p>
Loop exit-block insertion guarantees that all exit blocks from the loop
(blocks which are outside of the loop that have predecessors inside of the
loop) only have predecessors from inside of the loop (and are thus dominated
by the loop header). This simplifies transformations such as store-sinking
that are built into LICM.
</p>
<p>
This pass also guarantees that loops will have exactly one backedge.
</p>
<p>
Note that the simplifycfg pass will clean up blocks which are split out but
end up being unnecessary, so usage of this pass should not pessimize
generated code.
</p>
<p>
This pass obviously modifies the CFG, but updates loop information and
dominator information.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-unroll">-loop-unroll: Unroll loops</a>
</h3>
<div>
<p>
This pass implements a simple loop unroller. It works best when loops have
been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass,
allowing it to determine the trip counts of loops easily.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loop-unswitch">-loop-unswitch: Unswitch loops</a>
</h3>
<div>
<p>
This pass transforms loops that contain branches on loop-invariant conditions
to have multiple loops. For example, it turns the left into the right code:
</p>
<pre
>for (...) if (lic)
A for (...)
if (lic) A; B; C
B else
C for (...)
A; C</pre>
<p>
This can increase the size of the code exponentially (doubling it every time
a loop is unswitched) so we only unswitch if the resultant code will be
smaller than a threshold.
</p>
<p>
This pass expects LICM to be run before it to hoist invariant conditions out
of the loop, to make the unswitching opportunity obvious.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="loweratomic">-loweratomic: Lower atomic intrinsics to non-atomic form</a>
</h3>
<div>
<p>
This pass lowers atomic intrinsics to non-atomic form for use in a known
non-preemptible environment.
</p>
<p>
The pass does not verify that the environment is non-preemptible (in
general this would require knowledge of the entire call graph of the
program including any libraries which may not be available in bitcode form);
it simply lowers every atomic intrinsic.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a>
</h3>
<div>
<p>
This transformation is designed for use by code generators which do not yet
support stack unwinding. This pass supports two models of exception handling
lowering, the 'cheap' support and the 'expensive' support.
</p>
<p>
'Cheap' exception handling support gives the program the ability to execute
any program which does not "throw an exception", by turning 'invoke'
instructions into calls and by turning 'unwind' instructions into calls to
abort(). If the program does dynamically use the unwind instruction, the
program will print a message then abort.
</p>
<p>
'Expensive' exception handling support gives the full exception handling
support to the program at the cost of making the 'invoke' instruction
really expensive. It basically inserts setjmp/longjmp calls to emulate the
exception handling as necessary.
</p>
<p>
Because the 'expensive' support slows down programs a lot, and EH is only
used for a subset of the programs, it must be specifically enabled by the
<tt>-enable-correct-eh-support</tt> option.
</p>
<p>
Note that after this pass runs the CFG is not entirely accurate (exceptional
control flow edges are not correct anymore) so only very simple things should
be done after the lowerinvoke pass has run (like generation of native code).
This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
support the invoke instruction yet" lowering pass.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a>
</h3>
<div>
<p>
Rewrites <tt>switch</tt> instructions with a sequence of branches, which
allows targets to get away with not implementing the switch instruction until
it is convenient.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="mem2reg">-mem2reg: Promote Memory to Register</a>
</h3>
<div>
<p>
This file promotes memory references to be register references. It promotes
<tt>alloca</tt> instructions which only have <tt>load</tt>s and
<tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator
frontiers to place <tt>phi</tt> nodes, then traversing the function in
depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as
appropriate. This is just the standard SSA construction algorithm to construct
"pruned" SSA form.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="memcpyopt">-memcpyopt: MemCpy Optimization</a>
</h3>
<div>
<p>
This pass performs various transformations related to eliminating memcpy
calls, or transforming sets of stores into memset's.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="mergefunc">-mergefunc: Merge Functions</a>
</h3>
<div>
<p>This pass looks for equivalent functions that are mergable and folds them.
A hash is computed from the function, based on its type and number of
basic blocks.
Once all hashes are computed, we perform an expensive equality comparison
on each function pair. This takes n^2/2 comparisons per bucket, so it's
important that the hash function be high quality. The equality comparison
iterates through each instruction in each basic block.
When a match is found the functions are folded. If both functions are
overridable, we move the functionality into a new internal function and
leave two overridable thunks to it.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="mergereturn">-mergereturn: Unify function exit nodes</a>
</h3>
<div>
<p>
Ensure that functions have at most one <tt>ret</tt> instruction in them.
Additionally, it keeps track of which node is the new exit node of the CFG.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="partial-inliner">-partial-inliner: Partial Inliner</a>
</h3>
<div>
<p>This pass performs partial inlining, typically by inlining an if
statement that surrounds the body of the function.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="prune-eh">-prune-eh: Remove unused exception handling info</a>
</h3>
<div>
<p>
This file implements a simple interprocedural pass which walks the call-graph,
turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and
only if the callee cannot throw an exception. It implements this as a
bottom-up traversal of the call-graph.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="reassociate">-reassociate: Reassociate expressions</a>
</h3>
<div>
<p>
This pass reassociates commutative expressions in an order that is designed
to promote better constant propagation, GCSE, LICM, PRE, etc.
</p>
<p>
For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5)
</p>
<p>
In the implementation of this algorithm, constants are assigned rank = 0,
function arguments are rank = 1, and other values are assigned ranks
corresponding to the reverse post order traversal of current function
(starting at 2), which effectively gives values in deep loops higher rank
than values not in loops.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="reg2mem">-reg2mem: Demote all values to stack slots</a>
</h3>
<div>
<p>
This file demotes all registers to memory references. It is intended to be
the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to
<tt>load</tt> instructions, the only values live across basic blocks are
<tt>alloca</tt> instructions and <tt>load</tt> instructions before
<tt>phi</tt> nodes. It is intended that this should make CFG hacking much
easier. To make later hacking easier, the entry block is split into two, such
that all introduced <tt>alloca</tt> instructions (and nothing else) are in the
entry block.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates (DT)</a>
</h3>
<div>
<p>
The well-known scalar replacement of aggregates transformation. This
transform breaks up <tt>alloca</tt> instructions of aggregate type (structure
or array) into individual <tt>alloca</tt> instructions for each member if
possible. Then, if possible, it transforms the individual <tt>alloca</tt>
instructions into nice clean scalar SSA form.
</p>
<p>
This combines a simple scalar replacement of aggregates algorithm with the <a
href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact,
especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>,
then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to
promote works well.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="sccp">-sccp: Sparse Conditional Constant Propagation</a>
</h3>
<div>
<p>
Sparse conditional constant propagation and merging, which can be summarized
as:
</p>
<ol>
<li>Assumes values are constant unless proven otherwise</li>
<li>Assumes BasicBlocks are dead unless proven otherwise</li>
<li>Proves values to be constant, and replaces them with constants</li>
<li>Proves conditional branches to be unconditional</li>
</ol>
<p>
Note that this pass has a habit of making definitions be dead. It is a good
idea to to run a DCE pass sometime after running this pass.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a>
</h3>
<div>
<p>
Applies a variety of small optimizations for calls to specific well-known
function calls (e.g. runtime library functions). For example, a call
<tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be
transformed into simply <tt>return 3</tt>.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="simplifycfg">-simplifycfg: Simplify the CFG</a>
</h3>
<div>
<p>
Performs dead code elimination and basic block merging. Specifically:
</p>
<ol>
<li>Removes basic blocks with no predecessors.</li>
<li>Merges a basic block into its predecessor if there is only one and the
predecessor only has one successor.</li>
<li>Eliminates PHI nodes for basic blocks with a single predecessor.</li>
<li>Eliminates a basic block that only contains an unconditional
branch.</li>
</ol>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="sink">-sink: Code sinking</a>
</h3>
<div>
<p>This pass moves instructions into successor blocks, when possible, so that
they aren't executed on paths where their results aren't needed.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="sretpromotion">-sretpromotion: Promote sret arguments to multiple ret values</a>
</h3>
<div>
<p>
This pass finds functions that return a struct (using a pointer to the struct
as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and
replaces them with a new function that simply returns each of the elements of
that struct (using multiple return values).
</p>
<p>
This pass works under a number of conditions:
</p>
<ul>
<li>The returned struct must not contain other structs</li>
<li>The returned struct must only be used to load values from</li>
<li>The placeholder struct passed in is the result of an <tt>alloca</tt></li>
</ul>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="strip">-strip: Strip all symbols from a module</a>
</h3>
<div>
<p>
performs code stripping. this transformation can delete:
</p>
<ol>
<li>names for virtual registers</li>
<li>symbols for internal globals and functions</li>
<li>debug information</li>
</ol>
<p>
note that this transformation makes code much less readable, so it should
only be used in situations where the <tt>strip</tt> utility would be used,
such as reducing code size or making it harder to reverse engineer code.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a>
</h3>
<div>
<p>
performs code stripping. this transformation can delete:
</p>
<ol>
<li>names for virtual registers</li>
<li>symbols for internal globals and functions</li>
<li>debug information</li>
</ol>
<p>
note that this transformation makes code much less readable, so it should
only be used in situations where the <tt>strip</tt> utility would be used,
such as reducing code size or making it harder to reverse engineer code.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="strip-dead-prototypes">-strip-dead-prototypes: Strip Unused Function Prototypes</a>
</h3>
<div>
<p>
This pass loops over all of the functions in the input module, looking for
dead declarations and removes them. Dead declarations are declarations of
functions for which no implementation is available (i.e., declarations for
unused library functions).
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</a>
</h3>
<div>
<p>This pass implements code stripping. Specifically, it can delete:</p>
<ul>
<li>names for virtual registers</li>
<li>symbols for internal globals and functions</li>
<li>debug information</li>
</ul>
<p>
Note that this transformation makes code much less readable, so it should
only be used in situations where the 'strip' utility would be used, such as
reducing code size or making it harder to reverse engineer code.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a>
</h3>
<div>
<p>This pass implements code stripping. Specifically, it can delete:</p>
<ul>
<li>names for virtual registers</li>
<li>symbols for internal globals and functions</li>
<li>debug information</li>
</ul>
<p>
Note that this transformation makes code much less readable, so it should
only be used in situations where the 'strip' utility would be used, such as
reducing code size or making it harder to reverse engineer code.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="tailcallelim">-tailcallelim: Tail Call Elimination</a>
</h3>
<div>
<p>
This file transforms calls of the current function (self recursion) followed
by a return instruction with a branch to the entry of the function, creating
a loop. This pass also implements the following extensions to the basic
algorithm:
</p>
<ul>
<li>Trivial instructions between the call and return do not prevent the
transformation from taking place, though currently the analysis cannot
support moving any really useful instructions (only dead ones).
<li>This pass transforms functions that are prevented from being tail
recursive by an associative expression to use an accumulator variable,
thus compiling the typical naive factorial or <tt>fib</tt> implementation
into efficient code.
<li>TRE is performed if the function returns void, if the return
returns the result returned by the call, or if the function returns a
run-time constant on all exits from the function. It is possible, though
unlikely, that the return returns something else (like constant 0), and
can still be TRE'd. It can be TRE'd if <em>all other</em> return
instructions in the function return the exact same value.
<li>If it can prove that callees do not access theier caller stack frame,
they are marked as eligible for tail call elimination (by the code
generator).
</ul>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="tailduplicate">-tailduplicate: Tail Duplication</a>
</h3>
<div>
<p>
This pass performs a limited form of tail duplication, intended to simplify
CFGs by removing some unconditional branches. This pass is necessary to
straighten out loops created by the C front-end, but also is capable of
making other code nicer. After this pass is run, the CFG simplify pass
should be run to clean up the mess.
</p>
</div>
</div>
<!-- ======================================================================= -->
<h2><a name="utilities">Utility Passes</a></h2>
<div>
<p>This section describes the LLVM Utility Passes.</p>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
</h3>
<div>
<p>
Same as dead argument elimination, but deletes arguments to functions which
are external. This is only for use by <a
href="Bugpoint.html">bugpoint</a>.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a>
</h3>
<div>
<p>
This pass is used by bugpoint to extract all blocks from the module into their
own functions.</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="instnamer">-instnamer: Assign names to anonymous instructions</a>
</h3>
<div>
<p>This is a little utility pass that gives instructions names, this is mostly
useful when diffing the effect of an optimization because deleting an
unnamed instruction can change all other instruction numbering, making the
diff very noisy.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="preverify">-preverify: Preliminary module verification</a>
</h3>
<div>
<p>
Ensures that the module is in the form required by the <a
href="#verifier">Module Verifier</a> pass.
</p>
<p>
Running the verifier runs this pass automatically, so there should be no need
to use it directly.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="verify">-verify: Module Verifier</a>
</h3>
<div>
<p>
Verifies an LLVM IR code. This is useful to run after an optimization which is
undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
emitting bitcode, and also that malformed bitcode is likely to make LLVM
crash. All language front-ends are therefore encouraged to verify their output
before performing optimizing transformations.
</p>
<ul>
<li>Both of a binary operator's parameters are of the same type.</li>
<li>Verify that the indices of mem access instructions match other
operands.</li>
<li>Verify that arithmetic and other things are only performed on
first-class types. Verify that shifts and logicals only happen on
integrals f.e.</li>
<li>All of the constants in a switch statement are of the correct type.</li>
<li>The code is in valid SSA form.</li>
<li>It is illegal to put a label into any other type (like a structure) or
to return one.</li>
<li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is
invalid.</li>
<li>PHI nodes must have an entry for each predecessor, with no extras.</li>
<li>PHI nodes must be the first thing in a basic block, all grouped
together.</li>
<li>PHI nodes must have at least one entry.</li>
<li>All basic blocks should only end with terminator insts, not contain
them.</li>
<li>The entry node to a function must not have predecessors.</li>
<li>All Instructions must be embedded into a basic block.</li>
<li>Functions cannot take a void-typed parameter.</li>
<li>Verify that a function's argument list agrees with its declared
type.</li>
<li>It is illegal to specify a name for a void value.</li>
<li>It is illegal to have an internal global value with no initializer.</li>
<li>It is illegal to have a ret instruction that returns a value that does
not agree with the function return value type.</li>
<li>Function call argument types match the function prototype.</li>
<li>All other things that are tested by asserts spread about the code.</li>
</ul>
<p>
Note that this does not provide full security verification (like Java), but
instead just tries to ensure that code is well-formed.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="view-cfg">-view-cfg: View CFG of function</a>
</h3>
<div>
<p>
Displays the control flow graph using the GraphViz tool.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a>
</h3>
<div>
<p>
Displays the control flow graph using the GraphViz tool, but omitting function
bodies.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="view-dom">-view-dom: View dominance tree of function</a>
</h3>
<div>
<p>
Displays the dominator tree using the GraphViz tool.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="view-dom-only">-view-dom-only: View dominance tree of function (with no function bodies)</a>
</h3>
<div>
<p>
Displays the dominator tree using the GraphViz tool, but omitting function
bodies.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="view-postdom">-view-postdom: View postdominance tree of function</a>
</h3>
<div>
<p>
Displays the post dominator tree using the GraphViz tool.
</p>
</div>
<!-------------------------------------------------------------------------- -->
<h3>
<a name="view-postdom-only">-view-postdom-only: View postdominance tree of function (with no function bodies)</a>
</h3>
<div>
<p>
Displays the post dominator tree using the GraphViz tool, but omitting
function bodies.
</p>
</div>
</div>
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