ConstantProp.cpp

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//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include <set>
using namespace llvm;

#define DEBUG_TYPE "constprop"

STATISTIC(NumInstKilled, "Number of instructions killed");

namespace {
  struct ConstantPropagation : public FunctionPass {
    static char ID; // Pass identification, replacement for typeid
    ConstantPropagation() : FunctionPass(ID) {
      initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
    }

    bool runOnFunction(Function &F) override;

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.setPreservesCFG();
      AU.addRequired<TargetLibraryInfo>();
    }
  };
}

char ConstantPropagation::ID = 0;
INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
                "Simple constant propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
INITIALIZE_PASS_END(ConstantPropagation, "constprop",
                "Simple constant propagation", false, false)

FunctionPass *llvm::createConstantPropagationPass() {
  return new ConstantPropagation();
}

bool ConstantPropagation::runOnFunction(Function &F) {
  // Initialize the worklist to all of the instructions ready to process...
  std::set<Instruction*> WorkList;
  for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
      WorkList.insert(&*i);
  }
  bool Changed = false;
  DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
  const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
  TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();

  while (!WorkList.empty()) {
    Instruction *I = *WorkList.begin();
    WorkList.erase(WorkList.begin());    // Get an element from the worklist...

    if (!I->use_empty())                 // Don't muck with dead instructions...
      if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
        // Add all of the users of this instruction to the worklist, they might
        // be constant propagatable now...
        for (User *U : I->users())
          WorkList.insert(cast<Instruction>(U));

        // Replace all of the uses of a variable with uses of the constant.
        I->replaceAllUsesWith(C);

        // Remove the dead instruction.
        WorkList.erase(I);
        I->eraseFromParent();

        // We made a change to the function...
        Changed = true;
        ++NumInstKilled;
      }
  }
  return Changed;
}