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<ScalarEvolutionWrapperPass>();
  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   const TargetLibraryInfo *TLI =
  81       &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
  82   auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
  83       *L->getHeader()->getParent());
  84
  85   SmallVector<BasicBlock*, 8> ExitBlocks;
  86   L->getUniqueExitBlocks(ExitBlocks);
  87   array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
  88
  89   SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  90
  91   // The bit we are stealing from the pointer represents whether this basic
  92   // block is the header of a subloop, in which case we only process its phis.
  93   typedef PointerIntPair<BasicBlock*, 1> WorklistItem;
  94   SmallVector<WorklistItem, 16> VisitStack;
  95   SmallPtrSet<BasicBlock*, 32> Visited;
  96
  97   bool Changed = false;
  98   bool LocalChanged;
  99   do {
 100     LocalChanged = false;
 101
 102     VisitStack.clear();
 103     Visited.clear();
 104
 105     VisitStack.push_back(WorklistItem(L->getHeader(), false));
 106
 107     while (!VisitStack.empty()) {
 108       WorklistItem Item = VisitStack.pop_back_val();
 109       BasicBlock *BB = Item.getPointer();
 110       bool IsSubloopHeader = Item.getInt();
 111       const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
 112
 113       // Simplify instructions in the current basic block.
 114       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
 115         Instruction *I = &*BI++;
 116
 117         // The first time through the loop ToSimplify is empty and we try to
 118         // simplify all instructions. On later iterations ToSimplify is not
 119         // empty and we only bother simplifying instructions that are in it.
 120         if (!ToSimplify->empty() && !ToSimplify->count(I))
 121           continue;
 122
 123         // Don't bother simplifying unused instructions.
 124         if (!I->use_empty()) {
 125           Value *V = SimplifyInstruction(I, DL, TLI, DT, &AC);
 126           if (V && LI->replacementPreservesLCSSAForm(I, V)) {
 127             // Mark all uses for resimplification next time round the loop.
 128             for (User *U : I->users())
 129               Next->insert(cast<Instruction>(U));
 130
 131             I->replaceAllUsesWith(V);
 132             LocalChanged = true;
 133             ++NumSimplified;
 134           }
 135         }
 136         bool res = RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
 137         if (res) {
 138           // RecursivelyDeleteTriviallyDeadInstruction can remove
 139           // more than one instruction, so simply incrementing the
 140           // iterator does not work. When instructions get deleted
 141           // re-iterate instead.
 142           BI = BB->begin(); BE = BB->end();
 143           LocalChanged |= res;
 144         }
 145
 146         if (IsSubloopHeader && !isa<PHINode>(I))
 147           break;
 148       }
 149
 150       // Add all successors to the worklist, except for loop exit blocks and the
 151       // bodies of subloops. We visit the headers of loops so that we can process
 152       // their phis, but we contract the rest of the subloop body and only follow
 153       // edges leading back to the original loop.
 154       for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
 155            ++SI) {
 156         BasicBlock *SuccBB = *SI;
 157         if (!Visited.insert(SuccBB).second)
 158           continue;
 159
 160         const Loop *SuccLoop = LI->getLoopFor(SuccBB);
 161         if (SuccLoop && SuccLoop->getHeader() == SuccBB
 162                      && L->contains(SuccLoop)) {
 163           VisitStack.push_back(WorklistItem(SuccBB, true));
 164
 165           SmallVector<BasicBlock*, 8> SubLoopExitBlocks;
 166           SuccLoop->getExitBlocks(SubLoopExitBlocks);
 167
 168           for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
 169             BasicBlock *ExitBB = SubLoopExitBlocks[i];
 170             if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB).second)
 171               VisitStack.push_back(WorklistItem(ExitBB, false));
 172           }
 173
 174           continue;
 175         }
 176
 177         bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
 178                                               ExitBlocks.end(), SuccBB);
 179         if (IsExitBlock)
 180           continue;
 181
 182         VisitStack.push_back(WorklistItem(SuccBB, false));
 183       }
 184     }
 185
 186     // Place the list of instructions to simplify on the next loop iteration
 187     // into ToSimplify.
 188     std::swap(ToSimplify, Next);
 189     Next->clear();
 190
 191     Changed |= LocalChanged;
 192   } while (LocalChanged);
 193
 194   return Changed;
 195 }