lib/Transforms/Utils/AddDiscriminators.cpp - llvm - Git at Google

 //===- 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;
 }
	//===- 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;
	}