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//===- 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/Scalar.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/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"
using namespace llvm;
#define DEBUG_TYPE "add-discriminators"
namespace {
struct AddDiscriminators : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
AddDiscriminators() : FunctionPass(ID) {
initializeAddDiscriminatorsPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
};
}
char AddDiscriminators::ID = 0;
INITIALIZE_PASS_BEGIN(AddDiscriminators, "add-discriminators",
"Add DWARF path discriminators", false, false)
INITIALIZE_PASS_END(AddDiscriminators, "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."));
FunctionPass *llvm::createAddDiscriminatorsPass() {
return new AddDiscriminators();
}
static bool hasDebugInfo(const Function &F) {
NamedMDNode *CUNodes = F.getParent()->getNamedMetadata("llvm.dbg.cu");
return CUNodes != nullptr;
}
/// \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.
bool AddDiscriminators::runOnFunction(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 ||
!hasDebugInfo(F) ||
F.getParent()->getDwarfVersion() < 4)
return false;
bool Changed = false;
Module *M = F.getParent();
LLVMContext &Ctx = M->getContext();
DIBuilder Builder(*M, /*AllowUnresolved*/ false);
// Traverse all the blocks looking for instructions in different
// blocks that are at the same file:line location.
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
BasicBlock *B = I;
TerminatorInst *Last = B->getTerminator();
DebugLoc LastLoc = Last->getDebugLoc();
if (LastLoc.isUnknown()) continue;
DILocation LastDIL(LastLoc.getAsMDNode(Ctx));
for (unsigned I = 0; I < Last->getNumSuccessors(); ++I) {
BasicBlock *Succ = Last->getSuccessor(I);
Instruction *First = Succ->getFirstNonPHIOrDbgOrLifetime();
DebugLoc FirstLoc = First->getDebugLoc();
if (FirstLoc.isUnknown()) continue;
DILocation FirstDIL(FirstLoc.getAsMDNode(Ctx));
// If the first instruction (First) of Succ is at the same file
// location as B's last instruction (Last), add a new
// discriminator for First's location and all the instructions
// in Succ that share the same location with First.
if (FirstDIL.atSameLineAs(LastDIL)) {
// Create a new lexical scope and compute a new discriminator
// number for it.
StringRef Filename = FirstDIL.getFilename();
DIScope Scope = FirstDIL.getScope();
DIFile File = Builder.createFile(Filename, Scope.getDirectory());
unsigned Discriminator = FirstDIL.computeNewDiscriminator(Ctx);
DILexicalBlockFile NewScope =
Builder.createLexicalBlockFile(Scope, File, Discriminator);
DILocation NewDIL = FirstDIL.copyWithNewScope(Ctx, NewScope);
DebugLoc newDebugLoc = DebugLoc::getFromDILocation(NewDIL);
// Attach this new debug location to First and every
// instruction following First that shares the same location.
for (BasicBlock::iterator I1(*First), E1 = Succ->end(); I1 != E1;
++I1) {
if (I1->getDebugLoc() != FirstLoc) break;
I1->setDebugLoc(newDebugLoc);
DEBUG(dbgs() << NewDIL.getFilename() << ":" << NewDIL.getLineNumber()
<< ":" << NewDIL.getColumnNumber() << ":"
<< NewDIL.getDiscriminator() << *I1 << "\n");
}
DEBUG(dbgs() << "\n");
Changed = true;
}
}
}
return Changed;
}
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