lib/Transforms/Scalar/LoopInstSimplify.cpp

   1 //===- LoopInstSimplify.cpp - Loop Instruction Simplification Pass --------===//
   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 pass performs lightweight instruction simplification on loop bodies.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #define DEBUG_TYPE "loop-instsimplify"
  15 #include "llvm/Analysis/Dominators.h"
  16 #include "llvm/Analysis/InstructionSimplify.h"
  17 #include "llvm/Analysis/LoopInfo.h"
  18 #include "llvm/Analysis/LoopPass.h"
  19 #include "llvm/Support/Debug.h"
  20 #include "llvm/Target/TargetData.h"
  21 #include "llvm/Transforms/Scalar.h"
  22 #include "llvm/Transforms/Utils/Local.h"
  23 #include "llvm/ADT/Statistic.h"
  24 using namespace llvm;
  25
  26 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
  27
  28 namespace {
  29   class LoopInstSimplify : public LoopPass {
  30   public:
  31     static char ID; // Pass ID, replacement for typeid
  32     LoopInstSimplify() : LoopPass(ID) {
  33       initializeLoopInstSimplifyPass(*PassRegistry::getPassRegistry());
  34     }
  35
  36     bool runOnLoop(Loop*, LPPassManager&);
  37
  38     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  39       AU.setPreservesCFG();
  40       AU.addRequired<LoopInfo>();
  41       AU.addRequiredID(LoopSimplifyID);
  42       AU.addPreservedID(LCSSAID);
  43     }
  44   };
  45 }
  46
  47 char LoopInstSimplify::ID = 0;
  48 INITIALIZE_PASS_BEGIN(LoopInstSimplify, "loop-instsimplify",
  49                 "Simplify instructions in loops", false, false)
  50 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  51 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  52 INITIALIZE_PASS_DEPENDENCY(LCSSA)
  53 INITIALIZE_PASS_END(LoopInstSimplify, "loop-instsimplify",
  54                 "Simplify instructions in loops", false, false)
  55
  56 Pass* llvm::createLoopInstSimplifyPass() {
  57   return new LoopInstSimplify();
  58 }
  59
  60 bool LoopInstSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
  61   DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
  62   LoopInfo *LI = &getAnalysis<LoopInfo>();
  63   const TargetData *TD = getAnalysisIfAvailable<TargetData>();
  64
  65   SmallVector<BasicBlock*, 8> ExitBlocks;
  66   L->getUniqueExitBlocks(ExitBlocks);
  67   array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
  68
  69   SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  70
  71   SmallVector<BasicBlock*, 16> VisitStack;
  72   SmallPtrSet<BasicBlock*, 32> Visited;
  73
  74   bool Changed = false;
  75   bool LocalChanged;
  76   do {
  77     LocalChanged = false;
  78
  79     VisitStack.clear();
  80     Visited.clear();
  81
  82     VisitStack.push_back(L->getHeader());
  83
  84     while (!VisitStack.empty()) {
  85       BasicBlock *BB = VisitStack.pop_back_val();
  86
  87       // Simplify instructions in the current basic block.
  88       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
  89         Instruction *I = BI++;
  90
  91         // The first time through the loop ToSimplify is empty and we try to
  92         // simplify all instructions. On later iterations ToSimplify is not
  93         // empty and we only bother simplifying instructions that are in it.
  94         if (!ToSimplify->empty() && !ToSimplify->count(I))
  95           continue;
  96
  97         // Don't bother simplifying unused instructions.
  98         if (!I->use_empty()) {
  99           Value *V = SimplifyInstruction(I, TD, DT);
 100           if (V && LI->replacementPreservesLCSSAForm(I, V)) {
 101             // Mark all uses for resimplification next time round the loop.
 102             for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
 103                  UI != UE; ++UI)
 104               Next->insert(cast<Instruction>(*UI));
 105
 106             I->replaceAllUsesWith(V);
 107             LocalChanged = true;
 108             ++NumSimplified;
 109           }
 110         }
 111         LocalChanged |= RecursivelyDeleteTriviallyDeadInstructions(I);
 112       }
 113
 114       // Add all successors to the worklist, except for loop exit blocks.
 115       for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
 116            ++SI) {
 117         BasicBlock *SuccBB = *SI;
 118         bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
 119                                              ExitBlocks.end(), SuccBB);
 120         if (!IsExitBlock && Visited.insert(SuccBB))
 121           VisitStack.push_back(SuccBB);
 122       }
 123     }
 124
 125     // Place the list of instructions to simplify on the next loop iteration
 126     // into ToSimplify.
 127     std::swap(ToSimplify, Next);
 128     Next->clear();
 129
 130     Changed |= LocalChanged;
 131   } while (LocalChanged);
 132
 133   return Changed;
 134 }