AddDiscriminators.cpp File Reference

#include "llvm/Transforms/Utils/AddDiscriminators.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/SampleProfileLoaderBaseUtil.h"
#include <utility>

Go to the source code of this file.

Macros
#define	DEBUG_TYPE   "add-discriminators"

Functions
static bool	shouldHaveDiscriminator (const Instruction *I)
static bool	addDiscriminators (Function &F)
	Assign DWARF discriminators.

Variables
static cl::opt< bool >	NoDiscriminators ("no-discriminators", cl::init(false), cl::desc("Disable generation of discriminator information."))

Macro Definition Documentation

DEBUG_TYPE

#define DEBUG_TYPE   "add-discriminators"

Definition at line 76 of file AddDiscriminators.cpp.

Function Documentation

addDiscriminators()

static bool addDiscriminators ( Function &  F )

static

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.

Definition at line 140 of file AddDiscriminators.cpp.

References B, llvm::DILocation::cloneWithBaseDiscriminator(), llvm::sampleprofutil::createFSDiscriminatorVariable(), llvm::dbgs(), llvm::EnableFSDiscriminator, F, I, LLVM_DEBUG, NoDiscriminators, and shouldHaveDiscriminator().

Referenced by llvm::AddDiscriminatorsPass::run().

shouldHaveDiscriminator()

static bool shouldHaveDiscriminator ( const Instruction *  I )

static

Definition at line 85 of file AddDiscriminators.cpp.

References I.

Referenced by addDiscriminators().

Variable Documentation

NoDiscriminators

cl::opt< bool > NoDiscriminators("no-discriminators", cl::init(false), cl::desc("Disable generation of discriminator information.")) ( "no-discriminators"  , cl::init(false)  , cl::desc("Disable generation of discriminator information.")    )

static

Referenced by addDiscriminators().