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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 the Aggressive Dead Code Elimination pass. This pass
// optimistically assumes that all instructions are dead until proven otherwise,
// allowing it to eliminate dead computations that other DCE passes do not
// catch, particularly involving loop computations.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Pass.h"
using namespace llvm;
#define DEBUG_TYPE "adce"
STATISTIC(NumRemoved, "Number of instructions removed");
namespace {
struct ADCE : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
ADCE() : FunctionPass(ID) {
initializeADCEPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function& F) override;
void getAnalysisUsage(AnalysisUsage& AU) const override {
AU.setPreservesCFG();
}
};
}
char ADCE::ID = 0;
INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false)
bool ADCE::runOnFunction(Function& F) {
if (skipOptnoneFunction(F))
return false;
SmallPtrSet<Instruction*, 128> Alive;
SmallVector<Instruction*, 128> Worklist;
// Collect the set of "root" instructions that are known live.
for (Instruction &I : inst_range(F)) {
if (isa<TerminatorInst>(I) || isa<DbgInfoIntrinsic>(I) ||
isa<LandingPadInst>(I) || I.mayHaveSideEffects()) {
Alive.insert(&I);
Worklist.push_back(&I);
}
}
// Propagate liveness backwards to operands.
while (!Worklist.empty()) {
Instruction *Curr = Worklist.pop_back_val();
for (Use &OI : Curr->operands()) {
if (Instruction *Inst = dyn_cast<Instruction>(OI))
if (Alive.insert(Inst).second)
Worklist.push_back(Inst);
}
}
// The inverse of the live set is the dead set. These are those instructions
// which have no side effects and do not influence the control flow or return
// value of the function, and may therefore be deleted safely.
// NOTE: We reuse the Worklist vector here for memory efficiency.
for (Instruction &I : inst_range(F)) {
if (!Alive.count(&I)) {
Worklist.push_back(&I);
I.dropAllReferences();
}
}
for (Instruction *&I : Worklist) {
++NumRemoved;
I->eraseFromParent();
}
return !Worklist.empty();
}
FunctionPass *llvm::createAggressiveDCEPass() {
return new ADCE();
}
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