llvm/lib/Transforms/Utils/SimplifyInstructions.cpp

//===------ SimplifyInstructions.cpp - Remove redundant instructions ------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This is a utility pass used for testing the InstructionSimplify analysis.
// The analysis is applied to every instruction, and if it simplifies then the
// instruction is replaced by the simplification.  If you are looking for a pass
// that performs serious instruction folding, use the instcombine pass instead.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/SimplifyInstructions.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Type.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Scalar.h"
using namespace llvm;

#define DEBUG_TYPE "instsimplify"

STATISTIC(NumSimplified, "Number of redundant instructions removed");

static bool runImpl(Function &F, const DominatorTree *DT, const TargetLibraryInfo *TLI,
                    AssumptionCache *AC) {
  const DataLayout &DL = F.getParent()->getDataLayout();
  SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  bool Changed = false;

  do {
    for (BasicBlock *BB : depth_first(&F.getEntryBlock()))
      // Here be subtlety: the iterator must be incremented before the loop
      // body (not sure why), so a range-for loop won't work here.
      for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
        Instruction *I = &*BI++;
        // The first time through the loop ToSimplify is empty and we try to
        // simplify all instructions.  On later iterations ToSimplify is not
        // empty and we only bother simplifying instructions that are in it.
        if (!ToSimplify->empty() && !ToSimplify->count(I))
          continue;
        // Don't waste time simplifying unused instructions.
        if (!I->use_empty())
          if (Value *V = SimplifyInstruction(I, DL, TLI, DT, AC)) {
            // Mark all uses for resimplification next time round the loop.
            for (User *U : I->users())
              Next->insert(cast<Instruction>(U));
            I->replaceAllUsesWith(V);
            ++NumSimplified;
            Changed = true;
          }
        bool res = RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
        if (res)  {
          // RecursivelyDeleteTriviallyDeadInstruction can remove
          // more than one instruction, so simply incrementing the
          // iterator does not work. When instructions get deleted
          // re-iterate instead.
          BI = BB->begin(); BE = BB->end();
          Changed |= res;
        }
      }

    // Place the list of instructions to simplify on the next loop iteration
    // into ToSimplify.
    std::swap(ToSimplify, Next);
    Next->clear();
  } while (!ToSimplify->empty());

  return Changed;
}

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

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

    /// runOnFunction - Remove instructions that simplify.
    bool runOnFunction(Function &F) override {
      if (skipFunction(F))
        return false;

      const DominatorTree *DT =
          &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
      const TargetLibraryInfo *TLI =
          &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
      AssumptionCache *AC =
          &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
      return runImpl(F, DT, TLI, AC);
    }
  };
}

char InstSimplifier::ID = 0;
INITIALIZE_PASS_BEGIN(InstSimplifier, "instsimplify",
                      "Remove redundant instructions", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(InstSimplifier, "instsimplify",
                    "Remove redundant instructions", false, false)
char &llvm::InstructionSimplifierID = InstSimplifier::ID;

// Public interface to the simplify instructions pass.
FunctionPass *llvm::createInstructionSimplifierPass() {
  return new InstSimplifier();
}

PreservedAnalyses InstSimplifierPass::run(Function &F,
                                      FunctionAnalysisManager &AM) {
  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
  auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
  auto &AC = AM.getResult<AssumptionAnalysis>(F);
  bool Changed = runImpl(F, &DT, &TLI, &AC);
  if (!Changed)
    return PreservedAnalyses::all();
  // FIXME: This should also 'preserve the CFG'.
  return PreservedAnalyses::none();
}