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