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//===- DCE.cpp - Code to perform dead code elimination --------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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/DCE.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/Pass.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Transforms/Scalar.h"
using namespace llvm;
#define DEBUG_TYPE "dce"
STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
STATISTIC(DCEEliminated, "Number of insts removed");
DEBUG_COUNTER(DCECounter, "dce-transform",
"Controls which instructions are eliminated");
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 (skipBasicBlock(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)) {
if (!DebugCounter::shouldExecute(DCECounter))
continue;
salvageDebugInfo(*Inst);
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();
}
static bool DCEInstruction(Instruction *I,
SmallSetVector<Instruction *, 16> &WorkList,
const TargetLibraryInfo *TLI) {
if (isInstructionTriviallyDead(I, TLI)) {
if (!DebugCounter::shouldExecute(DCECounter))
return false;
salvageDebugInfo(*I);
// Null out all of the instruction's operands to see if any operand becomes
// dead as we go.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
Value *OpV = I->getOperand(i);
I->setOperand(i, nullptr);
if (!OpV->use_empty() || I == OpV)
continue;
// If the operand is an instruction that became dead as we nulled out the
// operand, and if it is 'trivially' dead, delete it in a future loop
// iteration.
if (Instruction *OpI = dyn_cast<Instruction>(OpV))
if (isInstructionTriviallyDead(OpI, TLI))
WorkList.insert(OpI);
}
I->eraseFromParent();
++DCEEliminated;
return true;
}
return false;
}
static bool eliminateDeadCode(Function &F, TargetLibraryInfo *TLI) {
bool MadeChange = false;
SmallSetVector<Instruction *, 16> WorkList;
// Iterate over the original function, only adding insts to the worklist
// if they actually need to be revisited. This avoids having to pre-init
// the worklist with the entire function's worth of instructions.
for (inst_iterator FI = inst_begin(F), FE = inst_end(F); FI != FE;) {
Instruction *I = &*FI;
++FI;
// We're visiting this instruction now, so make sure it's not in the
// worklist from an earlier visit.
if (!WorkList.count(I))
MadeChange |= DCEInstruction(I, WorkList, TLI);
}
while (!WorkList.empty()) {
Instruction *I = WorkList.pop_back_val();
MadeChange |= DCEInstruction(I, WorkList, TLI);
}
return MadeChange;
}
PreservedAnalyses DCEPass::run(Function &F, FunctionAnalysisManager &AM) {
if (!eliminateDeadCode(F, AM.getCachedResult<TargetLibraryAnalysis>(F)))
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
return PA;
}
namespace {
struct DCELegacyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
DCELegacyPass() : FunctionPass(ID) {
initializeDCELegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
return eliminateDeadCode(F, TLI);
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
}
};
}
char DCELegacyPass::ID = 0;
INITIALIZE_PASS(DCELegacyPass, "dce", "Dead Code Elimination", false, false)
FunctionPass *llvm::createDeadCodeEliminationPass() {
return new DCELegacyPass();
}
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