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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 (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
if (isa<TerminatorInst>(I.getInstructionIterator()) ||
isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
isa<LandingPadInst>(I.getInstructionIterator()) ||
I->mayHaveSideEffects()) {
alive.insert(I.getInstructionIterator());
worklist.push_back(I.getInstructionIterator());
}
// Propagate liveness backwards to operands.
while (!worklist.empty()) {
Instruction* curr = worklist.pop_back_val();
for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
OI != OE; ++OI)
if (Instruction* Inst = dyn_cast<Instruction>(OI))
if (alive.insert(Inst))
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 (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
if (!alive.count(I.getInstructionIterator())) {
worklist.push_back(I.getInstructionIterator());
I->dropAllReferences();
}
for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(),
E = worklist.end(); I != E; ++I) {
++NumRemoved;
(*I)->eraseFromParent();
}
return !worklist.empty();
}
FunctionPass *llvm::createAggressiveDCEPass() {
return new ADCE();
}
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