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