//===- AddDiscriminators.cpp - Insert DWARF path discriminators -----------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file adds DWARF discriminators to the IR. Path discriminators are
// used to decide what CFG path was taken inside sub-graphs whose instructions
// share the same line and column number information.
//
// The main user of this is the sample profiler. Instruction samples are
// mapped to line number information. Since a single line may be spread
// out over several basic blocks, discriminators add more precise location
// for the samples.
//
// For example,
//
// 1 #define ASSERT(P)
// 2 if (!(P))
// 3 abort()
// ...
// 100 while (true) {
// 101 ASSERT (sum < 0);
// 102 ...
// 130 }
//
// when converted to IR, this snippet looks something like:
//
// while.body: ; preds = %entry, %if.end
// %0 = load i32* %sum, align 4, !dbg !15
// %cmp = icmp slt i32 %0, 0, !dbg !15
// br i1 %cmp, label %if.end, label %if.then, !dbg !15
//
// if.then: ; preds = %while.body
// call void @abort(), !dbg !15
// br label %if.end, !dbg !15
//
// Notice that all the instructions in blocks 'while.body' and 'if.then'
// have exactly the same debug information. When this program is sampled
// at runtime, the profiler will assume that all these instructions are
// equally frequent. This, in turn, will consider the edge while.body->if.then
// to be frequently taken (which is incorrect).
//
// By adding a discriminator value to the instructions in block 'if.then',
// we can distinguish instructions at line 101 with discriminator 0 from
// the instructions at line 101 with discriminator 1.
//
// For more details about DWARF discriminators, please visit
// http://wiki.dwarfstd.org/index.php?title=Path_Discriminators
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/AddDiscriminators.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
using namespace llvm;
#define DEBUG_TYPE "add-discriminators"
namespace {
// The legacy pass of AddDiscriminators.
struct AddDiscriminatorsLegacyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
AddDiscriminatorsLegacyPass() : FunctionPass(ID) {
initializeAddDiscriminatorsLegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
};
} // end anonymous namespace
char AddDiscriminatorsLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(AddDiscriminatorsLegacyPass, "add-discriminators",
"Add DWARF path discriminators", false, false)
INITIALIZE_PASS_END(AddDiscriminatorsLegacyPass, "add-discriminators",
"Add DWARF path discriminators", false, false)
// Command line option to disable discriminator generation even in the
// presence of debug information. This is only needed when debugging
// debug info generation issues.
static cl::opt<bool> NoDiscriminators(
"no-discriminators", cl::init(false),
cl::desc("Disable generation of discriminator information."));
// Create the legacy AddDiscriminatorsPass.
FunctionPass *llvm::createAddDiscriminatorsPass() {
return new AddDiscriminatorsLegacyPass();
}
/// \brief Assign DWARF discriminators.
///
/// To assign discriminators, we examine the boundaries of every
/// basic block and its successors. Suppose there is a basic block B1
/// with successor B2. The last instruction I1 in B1 and the first
/// instruction I2 in B2 are located at the same file and line number.
/// This situation is illustrated in the following code snippet:
///
/// if (i < 10) x = i;
///
/// entry:
/// br i1 %cmp, label %if.then, label %if.end, !dbg !10
/// if.then:
/// %1 = load i32* %i.addr, align 4, !dbg !10
/// store i32 %1, i32* %x, align 4, !dbg !10
/// br label %if.end, !dbg !10
/// if.end:
/// ret void, !dbg !12
///
/// Notice how the branch instruction in block 'entry' and all the
/// instructions in block 'if.then' have the exact same debug location
/// information (!dbg !10).
///
/// To distinguish instructions in block 'entry' from instructions in
/// block 'if.then', we generate a new lexical block for all the
/// instruction in block 'if.then' that share the same file and line
/// location with the last instruction of block 'entry'.
///
/// This new lexical block will have the same location information as
/// the previous one, but with a new DWARF discriminator value.
///
/// One of the main uses of this discriminator value is in runtime
/// sample profilers. It allows the profiler to distinguish instructions
/// at location !dbg !10 that execute on different basic blocks. This is
/// important because while the predicate 'if (x < 10)' may have been
/// executed millions of times, the assignment 'x = i' may have only
/// executed a handful of times (meaning that the entry->if.then edge is
/// seldom taken).
///
/// If we did not have discriminator information, the profiler would
/// assign the same weight to both blocks 'entry' and 'if.then', which
/// in turn will make it conclude that the entry->if.then edge is very
/// hot.
///
/// To decide where to create new discriminator values, this function
/// traverses the CFG and examines instruction at basic block boundaries.
/// If the last instruction I1 of a block B1 is at the same file and line
/// location as instruction I2 of successor B2, then it creates a new
/// lexical block for I2 and all the instruction in B2 that share the same
/// file and line location as I2. This new lexical block will have a
/// different discriminator number than I1.
static bool addDiscriminators(Function &F) {
// If the function has debug information, but the user has disabled
// discriminators, do nothing.
// Simlarly, if the function has no debug info, do nothing.
// Finally, if this module is built with dwarf versions earlier than 4,
// do nothing (discriminator support is a DWARF 4 feature).
if (NoDiscriminators || !F.getSubprogram() ||
F.getParent()->getDwarfVersion() < 4)
return false;
bool Changed = false;
Module *M = F.getParent();
LLVMContext &Ctx = M->getContext();
DIBuilder Builder(*M, /*AllowUnresolved*/ false);
typedef std::pair<StringRef, unsigned> Location;
typedef DenseMap<const BasicBlock *, Metadata *> BBScopeMap;
typedef DenseMap<Location, BBScopeMap> LocationBBMap;
typedef DenseMap<Location, unsigned> LocationDiscriminatorMap;
typedef DenseSet<Location> LocationSet;
LocationBBMap LBM;
LocationDiscriminatorMap LDM;
// Traverse all instructions in the function. If the source line location
// of the instruction appears in other basic block, assign a new
// discriminator for this instruction.
for (BasicBlock &B : F) {
for (auto &I : B.getInstList()) {
if (isa<DbgInfoIntrinsic>(&I))
continue;
const DILocation *DIL = I.getDebugLoc();
if (!DIL)
continue;
Location L = std::make_pair(DIL->getFilename(), DIL->getLine());
auto &BBMap = LBM[L];
auto R = BBMap.insert(std::make_pair(&B, (Metadata *)nullptr));
if (BBMap.size() == 1)
continue;
bool InsertSuccess = R.second;
Metadata *&NewScope = R.first->second;
// If we could insert a different block in the same location, a
// discriminator is needed to distinguish both instructions.
if (InsertSuccess) {
auto *Scope = DIL->getScope();
auto *File =
Builder.createFile(DIL->getFilename(), Scope->getDirectory());
NewScope = Builder.createLexicalBlockFile(Scope, File, ++LDM[L]);
}
I.setDebugLoc(DILocation::get(Ctx, DIL->getLine(), DIL->getColumn(),
NewScope, DIL->getInlinedAt()));
DEBUG(dbgs() << DIL->getFilename() << ":" << DIL->getLine() << ":"
<< DIL->getColumn() << ":"
<< dyn_cast<DILexicalBlockFile>(NewScope)->getDiscriminator()
<< I << "\n");
Changed = true;
}
}
// Traverse all instructions and assign new discriminators to call
// instructions with the same lineno that are in the same basic block.
// Sample base profile needs to distinguish different function calls within
// a same source line for correct profile annotation.
for (BasicBlock &B : F) {
LocationSet CallLocations;
for (auto &I : B.getInstList()) {
CallInst *Current = dyn_cast<CallInst>(&I);
if (!Current || isa<DbgInfoIntrinsic>(&I))
continue;
DILocation *CurrentDIL = Current->getDebugLoc();
if (!CurrentDIL)
continue;
Location L =
std::make_pair(CurrentDIL->getFilename(), CurrentDIL->getLine());
if (!CallLocations.insert(L).second) {
auto *Scope = CurrentDIL->getScope();
auto *File = Builder.createFile(CurrentDIL->getFilename(),
Scope->getDirectory());
auto *NewScope = Builder.createLexicalBlockFile(Scope, File, ++LDM[L]);
Current->setDebugLoc(DILocation::get(Ctx, CurrentDIL->getLine(),
CurrentDIL->getColumn(), NewScope,
CurrentDIL->getInlinedAt()));
Changed = true;
}
}
}
return Changed;
}
bool AddDiscriminatorsLegacyPass::runOnFunction(Function &F) {
return addDiscriminators(F);
}
PreservedAnalyses AddDiscriminatorsPass::run(Function &F,
AnalysisManager<Function> &AM) {
if (!addDiscriminators(F))
return PreservedAnalyses::all();
// FIXME: should be all()
return PreservedAnalyses::none();
}