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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207196 91177308-0d34-0410-b5e6-96231b3b80d8
223 lines
9.0 KiB
C++
223 lines
9.0 KiB
C++
//===- AddDiscriminators.cpp - Insert DWARF path discriminators -----------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file adds DWARF discriminators to the IR. Path discriminators are
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// used to decide what CFG path was taken inside sub-graphs whose instructions
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// share the same line and column number information.
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//
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// The main user of this is the sample profiler. Instruction samples are
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// mapped to line number information. Since a single line may be spread
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// out over several basic blocks, discriminators add more precise location
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// for the samples.
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//
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// For example,
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//
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// 1 #define ASSERT(P)
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// 2 if (!(P))
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// 3 abort()
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// ...
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// 100 while (true) {
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// 101 ASSERT (sum < 0);
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// 102 ...
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// 130 }
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//
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// when converted to IR, this snippet looks something like:
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//
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// while.body: ; preds = %entry, %if.end
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// %0 = load i32* %sum, align 4, !dbg !15
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// %cmp = icmp slt i32 %0, 0, !dbg !15
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// br i1 %cmp, label %if.end, label %if.then, !dbg !15
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//
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// if.then: ; preds = %while.body
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// call void @abort(), !dbg !15
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// br label %if.end, !dbg !15
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//
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// Notice that all the instructions in blocks 'while.body' and 'if.then'
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// have exactly the same debug information. When this program is sampled
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// at runtime, the profiler will assume that all these instructions are
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// equally frequent. This, in turn, will consider the edge while.body->if.then
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// to be frequently taken (which is incorrect).
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//
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// By adding a discriminator value to the instructions in block 'if.then',
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// we can distinguish instructions at line 101 with discriminator 0 from
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// the instructions at line 101 with discriminator 1.
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//
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// For more details about DWARF discriminators, please visit
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// http://wiki.dwarfstd.org/index.php?title=Path_Discriminators
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DIBuilder.h"
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#include "llvm/IR/DebugInfo.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace llvm;
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#define DEBUG_TYPE "add-discriminators"
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namespace {
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struct AddDiscriminators : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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AddDiscriminators() : FunctionPass(ID) {
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initializeAddDiscriminatorsPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override;
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};
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}
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char AddDiscriminators::ID = 0;
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INITIALIZE_PASS_BEGIN(AddDiscriminators, "add-discriminators",
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"Add DWARF path discriminators", false, false)
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INITIALIZE_PASS_END(AddDiscriminators, "add-discriminators",
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"Add DWARF path discriminators", false, false)
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// Command line option to disable discriminator generation even in the
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// presence of debug information. This is only needed when debugging
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// debug info generation issues.
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static cl::opt<bool>
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NoDiscriminators("no-discriminators", cl::init(false),
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cl::desc("Disable generation of discriminator information."));
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FunctionPass *llvm::createAddDiscriminatorsPass() {
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return new AddDiscriminators();
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}
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static bool hasDebugInfo(const Function &F) {
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NamedMDNode *CUNodes = F.getParent()->getNamedMetadata("llvm.dbg.cu");
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return CUNodes != nullptr;
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}
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/// \brief Assign DWARF discriminators.
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///
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/// To assign discriminators, we examine the boundaries of every
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/// basic block and its successors. Suppose there is a basic block B1
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/// with successor B2. The last instruction I1 in B1 and the first
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/// instruction I2 in B2 are located at the same file and line number.
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/// This situation is illustrated in the following code snippet:
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///
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/// if (i < 10) x = i;
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///
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/// entry:
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/// br i1 %cmp, label %if.then, label %if.end, !dbg !10
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/// if.then:
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/// %1 = load i32* %i.addr, align 4, !dbg !10
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/// store i32 %1, i32* %x, align 4, !dbg !10
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/// br label %if.end, !dbg !10
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/// if.end:
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/// ret void, !dbg !12
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///
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/// Notice how the branch instruction in block 'entry' and all the
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/// instructions in block 'if.then' have the exact same debug location
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/// information (!dbg !10).
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///
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/// To distinguish instructions in block 'entry' from instructions in
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/// block 'if.then', we generate a new lexical block for all the
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/// instruction in block 'if.then' that share the same file and line
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/// location with the last instruction of block 'entry'.
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///
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/// This new lexical block will have the same location information as
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/// the previous one, but with a new DWARF discriminator value.
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///
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/// One of the main uses of this discriminator value is in runtime
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/// sample profilers. It allows the profiler to distinguish instructions
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/// at location !dbg !10 that execute on different basic blocks. This is
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/// important because while the predicate 'if (x < 10)' may have been
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/// executed millions of times, the assignment 'x = i' may have only
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/// executed a handful of times (meaning that the entry->if.then edge is
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/// seldom taken).
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///
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/// If we did not have discriminator information, the profiler would
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/// assign the same weight to both blocks 'entry' and 'if.then', which
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/// in turn will make it conclude that the entry->if.then edge is very
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/// hot.
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///
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/// To decide where to create new discriminator values, this function
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/// traverses the CFG and examines instruction at basic block boundaries.
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/// If the last instruction I1 of a block B1 is at the same file and line
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/// location as instruction I2 of successor B2, then it creates a new
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/// lexical block for I2 and all the instruction in B2 that share the same
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/// file and line location as I2. This new lexical block will have a
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/// different discriminator number than I1.
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bool AddDiscriminators::runOnFunction(Function &F) {
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// If the function has debug information, but the user has disabled
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// discriminators, do nothing.
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// Simlarly, if the function has no debug info, do nothing.
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// Finally, if this module is built with dwarf versions earlier than 4,
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// do nothing (discriminator support is a DWARF 4 feature).
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if (NoDiscriminators ||
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!hasDebugInfo(F) ||
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F.getParent()->getDwarfVersion() < 4)
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return false;
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bool Changed = false;
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Module *M = F.getParent();
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LLVMContext &Ctx = M->getContext();
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DIBuilder Builder(*M);
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// Traverse all the blocks looking for instructions in different
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// blocks that are at the same file:line location.
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
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BasicBlock *B = I;
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TerminatorInst *Last = B->getTerminator();
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DebugLoc LastLoc = Last->getDebugLoc();
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if (LastLoc.isUnknown()) continue;
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DILocation LastDIL(LastLoc.getAsMDNode(Ctx));
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for (unsigned I = 0; I < Last->getNumSuccessors(); ++I) {
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BasicBlock *Succ = Last->getSuccessor(I);
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Instruction *First = Succ->getFirstNonPHIOrDbgOrLifetime();
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DebugLoc FirstLoc = First->getDebugLoc();
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if (FirstLoc.isUnknown()) continue;
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DILocation FirstDIL(FirstLoc.getAsMDNode(Ctx));
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// If the first instruction (First) of Succ is at the same file
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// location as B's last instruction (Last), add a new
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// discriminator for First's location and all the instructions
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// in Succ that share the same location with First.
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if (FirstDIL.atSameLineAs(LastDIL)) {
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// Create a new lexical scope and compute a new discriminator
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// number for it.
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StringRef Filename = FirstDIL.getFilename();
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unsigned LineNumber = FirstDIL.getLineNumber();
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unsigned ColumnNumber = FirstDIL.getColumnNumber();
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DIScope Scope = FirstDIL.getScope();
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DIFile File = Builder.createFile(Filename, Scope.getDirectory());
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unsigned Discriminator = FirstDIL.computeNewDiscriminator(Ctx);
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DILexicalBlock NewScope = Builder.createLexicalBlock(
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Scope, File, LineNumber, ColumnNumber, Discriminator);
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DILocation NewDIL = FirstDIL.copyWithNewScope(Ctx, NewScope);
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DebugLoc newDebugLoc = DebugLoc::getFromDILocation(NewDIL);
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// Attach this new debug location to First and every
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// instruction following First that shares the same location.
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for (BasicBlock::iterator I1(*First), E1 = Succ->end(); I1 != E1;
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++I1) {
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if (I1->getDebugLoc() != FirstLoc) break;
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I1->setDebugLoc(newDebugLoc);
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DEBUG(dbgs() << NewDIL.getFilename() << ":" << NewDIL.getLineNumber()
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<< ":" << NewDIL.getColumnNumber() << ":"
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<< NewDIL.getDiscriminator() << *I1 << "\n");
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}
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DEBUG(dbgs() << "\n");
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Changed = true;
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}
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}
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}
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return Changed;
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}
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