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//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements constant propagation and merging:
// Specifically, this:
// * Converts instructions like "add int 1, 2" into 3
// Notice that:
// * This pass has a habit of making definitions be dead. It is a good idea
// to run a DIE pass sometime after running this pass.
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/IR/Constant.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"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "constprop"
STATISTIC(NumInstKilled, "Number of instructions killed");
DEBUG_COUNTER(CPCounter, "constprop-transform",
"Controls which instructions are killed");
namespace {
struct ConstantPropagation : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
ConstantPropagation() : FunctionPass(ID) {
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
char ConstantPropagation::ID = 0;
INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
"Simple constant propagation", false, false)
INITIALIZE_PASS_END(ConstantPropagation, "constprop",
"Simple constant propagation", false, false)
FunctionPass *llvm::createConstantPropagationPass() {
return new ConstantPropagation();
bool ConstantPropagation::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
// Initialize the worklist to all of the instructions ready to process...
SmallPtrSet<Instruction *, 16> WorkList;
// The SmallVector of WorkList ensures that we do iteration at stable order.
// We use two containers rather than one SetVector, since remove is
// linear-time, and we don't care enough to remove from Vec.
SmallVector<Instruction *, 16> WorkListVec;
for (Instruction &I : instructions(&F)) {
bool Changed = false;
const DataLayout &DL = F.getParent()->getDataLayout();
TargetLibraryInfo *TLI =
while (!WorkList.empty()) {
SmallVector<Instruction*, 16> NewWorkListVec;
for (auto *I : WorkListVec) {
WorkList.erase(I); // Remove element from the worklist...
if (!I->use_empty()) // Don't muck with dead instructions...
if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
if (!DebugCounter::shouldExecute(CPCounter))
// Add all of the users of this instruction to the worklist, they might
// be constant propagatable now...
for (User *U : I->users()) {
// If user not in the set, then add it to the vector.
if (WorkList.insert(cast<Instruction>(U)).second)
// Replace all of the uses of a variable with uses of the constant.
if (isInstructionTriviallyDead(I, TLI)) {
// We made a change to the function...
Changed = true;
WorkListVec = std::move(NewWorkListVec);
return Changed;