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/InstructionSimplify.h"
  22 #include "llvm/IR/DataLayout.h"
  23 #include "llvm/IR/Dominators.h"
  24 #include "llvm/IR/Function.h"
  25 #include "llvm/IR/Type.h"
  26 #include "llvm/Pass.h"
  27 #include "llvm/Target/TargetLibraryInfo.h"
  28 #include "llvm/Transforms/Utils/Local.h"
  29 using namespace llvm;
  30
  31 #define DEBUG_TYPE "instsimplify"
  32
  33 STATISTIC(NumSimplified, "Number of redundant instructions removed");
  34
  35 namespace {
  36   struct InstSimplifier : public FunctionPass {
  37     static char ID; // Pass identification, replacement for typeid
  38     InstSimplifier() : FunctionPass(ID) {
  39       initializeInstSimplifierPass(*PassRegistry::getPassRegistry());
  40     }
  41
  42     void getAnalysisUsage(AnalysisUsage &AU) const override {
  43       AU.setPreservesCFG();
  44       AU.addRequired<TargetLibraryInfo>();
  45     }
  46
  47     /// runOnFunction - Remove instructions that simplify.
  48     bool runOnFunction(Function &F) override {
  49       const DominatorTreeWrapperPass *DTWP =
  50           getAnalysisIfAvailable<DominatorTreeWrapperPass>();
  51       const DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
  52       DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
  53       const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
  54       const TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
  55       SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  56       bool Changed = false;
  57
  58       do {
  59         for (BasicBlock *BB : depth_first(&F.getEntryBlock()))
  60           // Here be subtlety: the iterator must be incremented before the loop
  61           // body (not sure why), so a range-for loop won't work here.
  62           for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
  63             Instruction *I = BI++;
  64             // The first time through the loop ToSimplify is empty and we try to
  65             // simplify all instructions.  On later iterations ToSimplify is not
  66             // empty and we only bother simplifying instructions that are in it.
  67             if (!ToSimplify->empty() && !ToSimplify->count(I))
  68               continue;
  69             // Don't waste time simplifying unused instructions.
  70             if (!I->use_empty())
  71               if (Value *V = SimplifyInstruction(I, DL, TLI, DT)) {
  72                 // Mark all uses for resimplification next time round the loop.
  73                 for (User *U : I->users())
  74                   Next->insert(cast<Instruction>(U));
  75                 I->replaceAllUsesWith(V);
  76                 ++NumSimplified;
  77                 Changed = true;
  78               }
  79             Changed |= RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
  80           }
  81
  82         // Place the list of instructions to simplify on the next loop iteration
  83         // into ToSimplify.
  84         std::swap(ToSimplify, Next);
  85         Next->clear();
  86       } while (!ToSimplify->empty());
  87
  88       return Changed;
  89     }
  90   };
  91 }
  92
  93 char InstSimplifier::ID = 0;
  94 INITIALIZE_PASS_BEGIN(InstSimplifier, "instsimplify",
  95                       "Remove redundant instructions", false, false)
  96 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
  97 INITIALIZE_PASS_END(InstSimplifier, "instsimplify",
  98                     "Remove redundant instructions", false, false)
  99 char &llvm::InstructionSimplifierID = InstSimplifier::ID;
 100
 101 // Public interface to the simplify instructions pass.
 102 FunctionPass *llvm::createInstructionSimplifierPass() {
 103   return new InstSimplifier();
 104 }