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//===- DCE.cpp - Code to perform dead code elimination --------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements dead inst elimination and dead code elimination.
//
// Dead Inst Elimination performs a single pass over the function removing
// instructions that are obviously dead. Dead Code Elimination is similar, but
// it rechecks instructions that were used by removed instructions to see if
// they are newly dead.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "dce"
STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
STATISTIC(DCEEliminated, "Number of insts removed");
namespace {
//===--------------------------------------------------------------------===//
// DeadInstElimination pass implementation
//
struct DeadInstElimination : public BasicBlockPass {
static char ID; // Pass identification, replacement for typeid
DeadInstElimination() : BasicBlockPass(ID) {
initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
}
bool runOnBasicBlock(BasicBlock &BB) override {
if (skipOptnoneFunction(BB))
return false;
auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
bool Changed = false;
for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
Instruction *Inst = DI++;
if (isInstructionTriviallyDead(Inst, TLI)) {
Inst->eraseFromParent();
Changed = true;
++DIEEliminated;
}
}
return Changed;
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
}
};
}
char DeadInstElimination::ID = 0;
INITIALIZE_PASS(DeadInstElimination, "die",
"Dead Instruction Elimination", false, false)
Pass *llvm::createDeadInstEliminationPass() {
return new DeadInstElimination();
}
namespace {
//===--------------------------------------------------------------------===//
// DeadCodeElimination pass implementation
//
struct DCE : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
DCE() : FunctionPass(ID) {
initializeDCEPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
}
};
}
char DCE::ID = 0;
INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
bool DCE::runOnFunction(Function &F) {
if (skipOptnoneFunction(F))
return false;
auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
// Start out with all of the instructions in the worklist...
std::vector<Instruction*> WorkList;
for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
WorkList.push_back(&*i);
// Loop over the worklist finding instructions that are dead. If they are
// dead make them drop all of their uses, making other instructions
// potentially dead, and work until the worklist is empty.
//
bool MadeChange = false;
while (!WorkList.empty()) {
Instruction *I = WorkList.back();
WorkList.pop_back();
if (isInstructionTriviallyDead(I, TLI)) { // If the instruction is dead.
// Loop over all of the values that the instruction uses, if there are
// instructions being used, add them to the worklist, because they might
// go dead after this one is removed.
//
for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
if (Instruction *Used = dyn_cast<Instruction>(*OI))
WorkList.push_back(Used);
// Remove the instruction.
I->eraseFromParent();
// Remove the instruction from the worklist if it still exists in it.
WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I),
WorkList.end());
MadeChange = true;
++DCEEliminated;
}
}
return MadeChange;
}
FunctionPass *llvm::createDeadCodeEliminationPass() {
return new DCE();
}