| //===- 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(); |
| const DILocation *LastDIL = Last->getDebugLoc(); |
| if (!LastDIL) |
| continue; |
| |
| for (unsigned I = 0; I < Last->getNumSuccessors(); ++I) { |
| BasicBlock *Succ = Last->getSuccessor(I); |
| Instruction *First = Succ->getFirstNonPHIOrDbgOrLifetime(); |
| const DILocation *FirstDIL = First->getDebugLoc(); |
| if (!FirstDIL) |
| continue; |
| |
| // 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->canDiscriminate(*LastDIL)) { |
| // Create a new lexical scope and compute a new discriminator |
| // number for it. |
| StringRef Filename = FirstDIL->getFilename(); |
| auto *Scope = FirstDIL->getScope(); |
| auto *File = Builder.createFile(Filename, Scope->getDirectory()); |
| |
| // FIXME: Calculate the discriminator here, based on local information, |
| // and delete DILocation::computeNewDiscriminator(). The current |
| // solution gives different results depending on other modules in the |
| // same context. All we really need is to discriminate between |
| // FirstDIL and LastDIL -- a local map would suffice. |
| unsigned Discriminator = FirstDIL->computeNewDiscriminator(); |
| auto *NewScope = |
| Builder.createLexicalBlockFile(Scope, File, Discriminator); |
| auto *NewDIL = |
| DILocation::get(Ctx, FirstDIL->getLine(), FirstDIL->getColumn(), |
| NewScope, FirstDIL->getInlinedAt()); |
| DebugLoc newDebugLoc = 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().get() != FirstDIL) |
| break; |
| I1->setDebugLoc(newDebugLoc); |
| DEBUG(dbgs() << NewDIL->getFilename() << ":" << NewDIL->getLine() |
| << ":" << NewDIL->getColumn() << ":" |
| << NewDIL->getDiscriminator() << *I1 << "\n"); |
| } |
| DEBUG(dbgs() << "\n"); |
| Changed = true; |
| } |
| } |
| } |
| return Changed; |
| } |