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 #include "llvm/Transforms/Scalar/LoopInstSimplify.h"
  15 #include "llvm/ADT/STLExtras.h"
  16 #include "llvm/ADT/Statistic.h"
  17 #include "llvm/Analysis/AssumptionCache.h"
  18 #include "llvm/Analysis/InstructionSimplify.h"
  19 #include "llvm/Analysis/LoopInfo.h"
  20 #include "llvm/Analysis/LoopPass.h"
  21 #include "llvm/Analysis/ScalarEvolution.h"
  22 #include "llvm/Analysis/TargetLibraryInfo.h"
  23 #include "llvm/IR/DataLayout.h"
  24 #include "llvm/IR/Dominators.h"
  25 #include "llvm/IR/Instructions.h"
  26 #include "llvm/Support/Debug.h"
  27 #include "llvm/Transforms/Scalar.h"
  28 #include "llvm/Transforms/Scalar/LoopPassManager.h"
  29 #include "llvm/Transforms/Utils/Local.h"
  30 #include "llvm/Transforms/Utils/LoopUtils.h"
  31 using namespace llvm;
  32
  33 #define DEBUG_TYPE "loop-instsimplify"
  34
  35 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
  36
  37 static bool SimplifyLoopInst(Loop *L, DominatorTree *DT, LoopInfo *LI,
  38                              AssumptionCache *AC,
  39                              const TargetLibraryInfo *TLI) {
  40   SmallVector<BasicBlock *, 8> ExitBlocks;
  41   L->getUniqueExitBlocks(ExitBlocks);
  42   array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
  43
  44   SmallPtrSet<const Instruction *, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  45
  46   // The bit we are stealing from the pointer represents whether this basic
  47   // block is the header of a subloop, in which case we only process its phis.
  48   typedef PointerIntPair<BasicBlock *, 1> WorklistItem;
  49   SmallVector<WorklistItem, 16> VisitStack;
  50   SmallPtrSet<BasicBlock *, 32> Visited;
  51
  52   bool Changed = false;
  53   bool LocalChanged;
  54   do {
  55     LocalChanged = false;
  56
  57     VisitStack.clear();
  58     Visited.clear();
  59
  60     VisitStack.push_back(WorklistItem(L->getHeader(), false));
  61
  62     while (!VisitStack.empty()) {
  63       WorklistItem Item = VisitStack.pop_back_val();
  64       BasicBlock *BB = Item.getPointer();
  65       bool IsSubloopHeader = Item.getInt();
  66       const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
  67
  68       // Simplify instructions in the current basic block.
  69       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
  70         Instruction *I = &*BI++;
  71
  72         // The first time through the loop ToSimplify is empty and we try to
  73         // simplify all instructions. On later iterations ToSimplify is not
  74         // empty and we only bother simplifying instructions that are in it.
  75         if (!ToSimplify->empty() && !ToSimplify->count(I))
  76           continue;
  77
  78         // Don't bother simplifying unused instructions.
  79         if (!I->use_empty()) {
  80           Value *V = SimplifyInstruction(I, DL, TLI, DT, AC);
  81           if (V && LI->replacementPreservesLCSSAForm(I, V)) {
  82             // Mark all uses for resimplification next time round the loop.
  83             for (User *U : I->users())
  84               Next->insert(cast<Instruction>(U));
  85
  86             I->replaceAllUsesWith(V);
  87             LocalChanged = true;
  88             ++NumSimplified;
  89           }
  90         }
  91         if (RecursivelyDeleteTriviallyDeadInstructions(I, TLI)) {
  92           // RecursivelyDeleteTriviallyDeadInstruction can remove more than one
  93           // instruction, so simply incrementing the iterator does not work.
  94           // When instructions get deleted re-iterate instead.
  95           BI = BB->begin();
  96           BE = BB->end();
  97           LocalChanged = true;
  98         }
  99
 100         if (IsSubloopHeader && !isa<PHINode>(I))
 101           break;
 102       }
 103
 104       // Add all successors to the worklist, except for loop exit blocks and the
 105       // bodies of subloops. We visit the headers of loops so that we can
 106       // process
 107       // their phis, but we contract the rest of the subloop body and only
 108       // follow
 109       // edges leading back to the original loop.
 110       for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
 111            ++SI) {
 112         BasicBlock *SuccBB = *SI;
 113         if (!Visited.insert(SuccBB).second)
 114           continue;
 115
 116         const Loop *SuccLoop = LI->getLoopFor(SuccBB);
 117         if (SuccLoop && SuccLoop->getHeader() == SuccBB &&
 118             L->contains(SuccLoop)) {
 119           VisitStack.push_back(WorklistItem(SuccBB, true));
 120
 121           SmallVector<BasicBlock *, 8> SubLoopExitBlocks;
 122           SuccLoop->getExitBlocks(SubLoopExitBlocks);
 123
 124           for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
 125             BasicBlock *ExitBB = SubLoopExitBlocks[i];
 126             if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB).second)
 127               VisitStack.push_back(WorklistItem(ExitBB, false));
 128           }
 129
 130           continue;
 131         }
 132
 133         bool IsExitBlock =
 134             std::binary_search(ExitBlocks.begin(), ExitBlocks.end(), SuccBB);
 135         if (IsExitBlock)
 136           continue;
 137
 138         VisitStack.push_back(WorklistItem(SuccBB, false));
 139       }
 140     }
 141
 142     // Place the list of instructions to simplify on the next loop iteration
 143     // into ToSimplify.
 144     std::swap(ToSimplify, Next);
 145     Next->clear();
 146
 147     Changed |= LocalChanged;
 148   } while (LocalChanged);
 149
 150   return Changed;
 151 }
 152
 153 namespace {
 154 class LoopInstSimplifyLegacyPass : public LoopPass {
 155 public:
 156   static char ID; // Pass ID, replacement for typeid
 157   LoopInstSimplifyLegacyPass() : LoopPass(ID) {
 158     initializeLoopInstSimplifyLegacyPassPass(*PassRegistry::getPassRegistry());
 159   }
 160
 161   bool runOnLoop(Loop *L, LPPassManager &LPM) override {
 162     if (skipLoop(L))
 163       return false;
 164     DominatorTreeWrapperPass *DTWP =
 165         getAnalysisIfAvailable<DominatorTreeWrapperPass>();
 166     DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
 167     LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
 168     AssumptionCache *AC =
 169         &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
 170             *L->getHeader()->getParent());
 171     const TargetLibraryInfo *TLI =
 172         &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
 173
 174     return SimplifyLoopInst(L, DT, LI, AC, TLI);
 175   }
 176
 177   void getAnalysisUsage(AnalysisUsage &AU) const override {
 178     AU.addRequired<AssumptionCacheTracker>();
 179     AU.addRequired<TargetLibraryInfoWrapperPass>();
 180     AU.setPreservesCFG();
 181     getLoopAnalysisUsage(AU);
 182   }
 183 };
 184 }
 185
 186 PreservedAnalyses LoopInstSimplifyPass::run(Loop &L, LoopAnalysisManager &AM,
 187                                             LoopStandardAnalysisResults &AR,
 188                                             LPMUpdater &) {
 189   if (!SimplifyLoopInst(&L, &AR.DT, &AR.LI, &AR.AC, &AR.TLI))
 190     return PreservedAnalyses::all();
 191
 192   return getLoopPassPreservedAnalyses();
 193 }
 194
 195 char LoopInstSimplifyLegacyPass::ID = 0;
 196 INITIALIZE_PASS_BEGIN(LoopInstSimplifyLegacyPass, "loop-instsimplify",
 197                       "Simplify instructions in loops", false, false)
 198 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 199 INITIALIZE_PASS_DEPENDENCY(LoopPass)
 200 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
 201 INITIALIZE_PASS_END(LoopInstSimplifyLegacyPass, "loop-instsimplify",
 202                     "Simplify instructions in loops", false, false)
 203
 204 Pass *llvm::createLoopInstSimplifyPass() {
 205   return new LoopInstSimplifyLegacyPass();
 206 }