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