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/AssumptionTracker.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/Target/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<AssumptionTracker>();
  46       AU.addRequired<TargetLibraryInfo>();
  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 = &getAnalysis<TargetLibraryInfo>();
  57       AssumptionTracker *AT = &getAnalysis<AssumptionTracker>();
  58       SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  59       bool Changed = false;
  60
  61       do {
  62         for (BasicBlock *BB : depth_first(&F.getEntryBlock()))
  63           // Here be subtlety: the iterator must be incremented before the loop
  64           // body (not sure why), so a range-for loop won't work here.
  65           for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
  66             Instruction *I = BI++;
  67             // The first time through the loop ToSimplify is empty and we try to
  68             // simplify all instructions.  On later iterations ToSimplify is not
  69             // empty and we only bother simplifying instructions that are in it.
  70             if (!ToSimplify->empty() && !ToSimplify->count(I))
  71               continue;
  72             // Don't waste time simplifying unused instructions.
  73             if (!I->use_empty())
  74               if (Value *V = SimplifyInstruction(I, DL, TLI, DT, AT)) {
  75                 // Mark all uses for resimplification next time round the loop.
  76                 for (User *U : I->users())
  77                   Next->insert(cast<Instruction>(U));
  78                 I->replaceAllUsesWith(V);
  79                 ++NumSimplified;
  80                 Changed = true;
  81               }
  82             bool res = RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
  83             if (res)  {
  84               // RecursivelyDeleteTriviallyDeadInstruction can remove
  85               // more than one instruction, so simply incrementing the
  86               // iterator does not work. When instructions get deleted
  87               // re-iterate instead.
  88               BI = BB->begin(); BE = BB->end();
  89               Changed |= res;
  90             }
  91           }
  92
  93         // Place the list of instructions to simplify on the next loop iteration
  94         // into ToSimplify.
  95         std::swap(ToSimplify, Next);
  96         Next->clear();
  97       } while (!ToSimplify->empty());
  98
  99       return Changed;
 100     }
 101   };
 102 }
 103
 104 char InstSimplifier::ID = 0;
 105 INITIALIZE_PASS_BEGIN(InstSimplifier, "instsimplify",
 106                       "Remove redundant instructions", false, false)
 107 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
 108 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
 109 INITIALIZE_PASS_END(InstSimplifier, "instsimplify",
 110                     "Remove redundant instructions", false, false)
 111 char &llvm::InstructionSimplifierID = InstSimplifier::ID;
 112
 113 // Public interface to the simplify instructions pass.
 114 FunctionPass *llvm::createInstructionSimplifierPass() {
 115   return new InstSimplifier();
 116 }