lib/Transforms/Utils/SimplifyInstructions.cpp

   1 //===------ SimplifyInstructions.cpp - Remove redundant instructions ------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This is a utility pass used for testing the InstructionSimplify analysis.
  11 // The analysis is applied to every instruction, and if it simplifies then the
  12 // instruction is replaced by the simplification.  If you are looking for a pass
  13 // that performs serious instruction folding, use the instcombine pass instead.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "llvm/Transforms/Scalar.h"
  18 #include "llvm/ADT/DepthFirstIterator.h"
  19 #include "llvm/ADT/SmallPtrSet.h"
  20 #include "llvm/ADT/Statistic.h"
  21 #include "llvm/Analysis/AssumptionCache.h"
  22 #include "llvm/Analysis/InstructionSimplify.h"
  23 #include "llvm/IR/DataLayout.h"
  24 #include "llvm/IR/Dominators.h"
  25 #include "llvm/IR/Function.h"
  26 #include "llvm/IR/Type.h"
  27 #include "llvm/Pass.h"
  28 #include "llvm/Analysis/TargetLibraryInfo.h"
  29 #include "llvm/Transforms/Utils/Local.h"
  30 using namespace llvm;
  31
  32 #define DEBUG_TYPE "instsimplify"
  33
  34 STATISTIC(NumSimplified, "Number of redundant instructions removed");
  35
  36 namespace {
  37   struct InstSimplifier : public FunctionPass {
  38     static char ID; // Pass identification, replacement for typeid
  39     InstSimplifier() : FunctionPass(ID) {
  40       initializeInstSimplifierPass(*PassRegistry::getPassRegistry());
  41     }
  42
  43     void getAnalysisUsage(AnalysisUsage &AU) const override {
  44       AU.setPreservesCFG();
  45       AU.addRequired<AssumptionCacheTracker>();
  46       AU.addRequired<TargetLibraryInfoWrapperPass>();
  47     }
  48
  49     /// runOnFunction - Remove instructions that simplify.
  50     bool runOnFunction(Function &F) override {
  51       const DominatorTreeWrapperPass *DTWP =
  52           getAnalysisIfAvailable<DominatorTreeWrapperPass>();
  53       const DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
  54       DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
  55       const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
  56       const TargetLibraryInfo *TLI =
  57           &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
  58       AssumptionCache *AC =
  59           &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
  60       SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  61       bool Changed = false;
  62
  63       do {
  64         for (BasicBlock *BB : depth_first(&F.getEntryBlock()))
  65           // Here be subtlety: the iterator must be incremented before the loop
  66           // body (not sure why), so a range-for loop won't work here.
  67           for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
  68             Instruction *I = BI++;
  69             // The first time through the loop ToSimplify is empty and we try to
  70             // simplify all instructions.  On later iterations ToSimplify is not
  71             // empty and we only bother simplifying instructions that are in it.
  72             if (!ToSimplify->empty() && !ToSimplify->count(I))
  73               continue;
  74             // Don't waste time simplifying unused instructions.
  75             if (!I->use_empty())
  76               if (Value *V = SimplifyInstruction(I, DL, TLI, DT, AC)) {
  77                 // Mark all uses for resimplification next time round the loop.
  78                 for (User *U : I->users())
  79                   Next->insert(cast<Instruction>(U));
  80                 I->replaceAllUsesWith(V);
  81                 ++NumSimplified;
  82                 Changed = true;
  83               }
  84             bool res = RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
  85             if (res)  {
  86               // RecursivelyDeleteTriviallyDeadInstruction can remove
  87               // more than one instruction, so simply incrementing the
  88               // iterator does not work. When instructions get deleted
  89               // re-iterate instead.
  90               BI = BB->begin(); BE = BB->end();
  91               Changed |= res;
  92             }
  93           }
  94
  95         // Place the list of instructions to simplify on the next loop iteration
  96         // into ToSimplify.
  97         std::swap(ToSimplify, Next);
  98         Next->clear();
  99       } while (!ToSimplify->empty());
 100
 101       return Changed;
 102     }
 103   };
 104 }
 105
 106 char InstSimplifier::ID = 0;
 107 INITIALIZE_PASS_BEGIN(InstSimplifier, "instsimplify",
 108                       "Remove redundant instructions", false, false)
 109 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 110 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
 111 INITIALIZE_PASS_END(InstSimplifier, "instsimplify",
 112                     "Remove redundant instructions", false, false)
 113 char &llvm::InstructionSimplifierID = InstSimplifier::ID;
 114
 115 // Public interface to the simplify instructions pass.
 116 FunctionPass *llvm::createInstructionSimplifierPass() {
 117   return new InstSimplifier();
 118 }