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/Transforms/Scalar.h"
  16 #include "llvm/ADT/STLExtras.h"
  17 #include "llvm/ADT/Statistic.h"
  18 #include "llvm/Analysis/InstructionSimplify.h"
  19 #include "llvm/Analysis/LoopInfo.h"
  20 #include "llvm/Analysis/LoopPass.h"
  21 #include "llvm/IR/DataLayout.h"
  22 #include "llvm/IR/Dominators.h"
  23 #include "llvm/IR/Instructions.h"
  24 #include "llvm/Support/Debug.h"
  25 #include "llvm/Target/TargetLibraryInfo.h"
  26 #include "llvm/Transforms/Utils/Local.h"
  27 using namespace llvm;
  28
  29 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
  30
  31 namespace {
  32   class LoopInstSimplify : public LoopPass {
  33   public:
  34     static char ID; // Pass ID, replacement for typeid
  35     LoopInstSimplify() : LoopPass(ID) {
  36       initializeLoopInstSimplifyPass(*PassRegistry::getPassRegistry());
  37     }
  38
  39     bool runOnLoop(Loop*, LPPassManager&);
  40
  41     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  42       AU.setPreservesCFG();
  43       AU.addRequired<LoopInfo>();
  44       AU.addRequiredID(LoopSimplifyID);
  45       AU.addPreservedID(LoopSimplifyID);
  46       AU.addPreservedID(LCSSAID);
  47       AU.addPreserved("scalar-evolution");
  48       AU.addRequired<TargetLibraryInfo>();
  49     }
  50   };
  51 }
  52
  53 char LoopInstSimplify::ID = 0;
  54 INITIALIZE_PASS_BEGIN(LoopInstSimplify, "loop-instsimplify",
  55                 "Simplify instructions in loops", false, false)
  56 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
  57 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  58 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  59 INITIALIZE_PASS_DEPENDENCY(LCSSA)
  60 INITIALIZE_PASS_END(LoopInstSimplify, "loop-instsimplify",
  61                 "Simplify instructions in loops", false, false)
  62
  63 Pass *llvm::createLoopInstSimplifyPass() {
  64   return new LoopInstSimplify();
  65 }
  66
  67 bool LoopInstSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
  68   DominatorTreeWrapperPass *DTWP =
  69       getAnalysisIfAvailable<DominatorTreeWrapperPass>();
  70   DominatorTree *DT = DTWP ? &DTWP->getDomTree() : 0;
  71   LoopInfo *LI = &getAnalysis<LoopInfo>();
  72   const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
  73   const TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
  74
  75   SmallVector<BasicBlock*, 8> ExitBlocks;
  76   L->getUniqueExitBlocks(ExitBlocks);
  77   array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
  78
  79   SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  80
  81   // The bit we are stealing from the pointer represents whether this basic
  82   // block is the header of a subloop, in which case we only process its phis.
  83   typedef PointerIntPair<BasicBlock*, 1> WorklistItem;
  84   SmallVector<WorklistItem, 16> VisitStack;
  85   SmallPtrSet<BasicBlock*, 32> Visited;
  86
  87   bool Changed = false;
  88   bool LocalChanged;
  89   do {
  90     LocalChanged = false;
  91
  92     VisitStack.clear();
  93     Visited.clear();
  94
  95     VisitStack.push_back(WorklistItem(L->getHeader(), false));
  96
  97     while (!VisitStack.empty()) {
  98       WorklistItem Item = VisitStack.pop_back_val();
  99       BasicBlock *BB = Item.getPointer();
 100       bool IsSubloopHeader = Item.getInt();
 101
 102       // Simplify instructions in the current basic block.
 103       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
 104         Instruction *I = BI++;
 105
 106         // The first time through the loop ToSimplify is empty and we try to
 107         // simplify all instructions. On later iterations ToSimplify is not
 108         // empty and we only bother simplifying instructions that are in it.
 109         if (!ToSimplify->empty() && !ToSimplify->count(I))
 110           continue;
 111
 112         // Don't bother simplifying unused instructions.
 113         if (!I->use_empty()) {
 114           Value *V = SimplifyInstruction(I, TD, TLI, DT);
 115           if (V && LI->replacementPreservesLCSSAForm(I, V)) {
 116             // Mark all uses for resimplification next time round the loop.
 117             for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
 118                  UI != UE; ++UI)
 119               Next->insert(cast<Instruction>(*UI));
 120
 121             I->replaceAllUsesWith(V);
 122             LocalChanged = true;
 123             ++NumSimplified;
 124           }
 125         }
 126         LocalChanged |= RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
 127
 128         if (IsSubloopHeader && !isa<PHINode>(I))
 129           break;
 130       }
 131
 132       // Add all successors to the worklist, except for loop exit blocks and the
 133       // bodies of subloops. We visit the headers of loops so that we can process
 134       // their phis, but we contract the rest of the subloop body and only follow
 135       // edges leading back to the original loop.
 136       for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
 137            ++SI) {
 138         BasicBlock *SuccBB = *SI;
 139         if (!Visited.insert(SuccBB))
 140           continue;
 141
 142         const Loop *SuccLoop = LI->getLoopFor(SuccBB);
 143         if (SuccLoop && SuccLoop->getHeader() == SuccBB
 144                      && L->contains(SuccLoop)) {
 145           VisitStack.push_back(WorklistItem(SuccBB, true));
 146
 147           SmallVector<BasicBlock*, 8> SubLoopExitBlocks;
 148           SuccLoop->getExitBlocks(SubLoopExitBlocks);
 149
 150           for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
 151             BasicBlock *ExitBB = SubLoopExitBlocks[i];
 152             if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB))
 153               VisitStack.push_back(WorklistItem(ExitBB, false));
 154           }
 155
 156           continue;
 157         }
 158
 159         bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
 160                                               ExitBlocks.end(), SuccBB);
 161         if (IsExitBlock)
 162           continue;
 163
 164         VisitStack.push_back(WorklistItem(SuccBB, false));
 165       }
 166     }
 167
 168     // Place the list of instructions to simplify on the next loop iteration
 169     // into ToSimplify.
 170     std::swap(ToSimplify, Next);
 171     Next->clear();
 172
 173     Changed |= LocalChanged;
 174   } while (LocalChanged);
 175
 176   return Changed;
 177 }