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      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 #include "llvm/Transforms/Utils/LoopUtils.h"
     29 using namespace llvm;
     30 
     31 #define DEBUG_TYPE "loop-instsimplify"
     32 
     33 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
     34 
     35 namespace {
     36   class LoopInstSimplify : public LoopPass {
     37   public:
     38     static char ID; // Pass ID, replacement for typeid
     39     LoopInstSimplify() : LoopPass(ID) {
     40       initializeLoopInstSimplifyPass(*PassRegistry::getPassRegistry());
     41     }
     42 
     43     bool runOnLoop(Loop*, LPPassManager&) override;
     44 
     45     void getAnalysisUsage(AnalysisUsage &AU) const override {
     46       AU.addRequired<AssumptionCacheTracker>();
     47       AU.addRequired<TargetLibraryInfoWrapperPass>();
     48       AU.setPreservesCFG();
     49       getLoopAnalysisUsage(AU);
     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(AssumptionCacheTracker)
     58 INITIALIZE_PASS_DEPENDENCY(LoopPass)
     59 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
     60 INITIALIZE_PASS_END(LoopInstSimplify, "loop-instsimplify",
     61                 "Simplify instructions in loops", false, false)
     62 
     63 Pass *llvm::createLoopInstSimplifyPass() {
     64   return new LoopInstSimplify();
     65 }
     66 
     67 bool LoopInstSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
     68   if (skipLoop(L))
     69     return false;
     70 
     71   DominatorTreeWrapperPass *DTWP =
     72       getAnalysisIfAvailable<DominatorTreeWrapperPass>();
     73   DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
     74   LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
     75   const TargetLibraryInfo *TLI =
     76       &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
     77   auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
     78       *L->getHeader()->getParent());
     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       const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
    107 
    108       // Simplify instructions in the current basic block.
    109       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
    110         Instruction *I = &*BI++;
    111 
    112         // The first time through the loop ToSimplify is empty and we try to
    113         // simplify all instructions. On later iterations ToSimplify is not
    114         // empty and we only bother simplifying instructions that are in it.
    115         if (!ToSimplify->empty() && !ToSimplify->count(I))
    116           continue;
    117 
    118         // Don't bother simplifying unused instructions.
    119         if (!I->use_empty()) {
    120           Value *V = SimplifyInstruction(I, DL, TLI, DT, &AC);
    121           if (V && LI->replacementPreservesLCSSAForm(I, V)) {
    122             // Mark all uses for resimplification next time round the loop.
    123             for (User *U : I->users())
    124               Next->insert(cast<Instruction>(U));
    125 
    126             I->replaceAllUsesWith(V);
    127             LocalChanged = true;
    128             ++NumSimplified;
    129           }
    130         }
    131         if (RecursivelyDeleteTriviallyDeadInstructions(I, TLI)) {
    132           // RecursivelyDeleteTriviallyDeadInstruction can remove more than one
    133           // instruction, so simply incrementing the iterator does not work.
    134           // When instructions get deleted re-iterate instead.
    135           BI = BB->begin(); BE = BB->end();
    136           LocalChanged = true;
    137         }
    138 
    139         if (IsSubloopHeader && !isa<PHINode>(I))
    140           break;
    141       }
    142 
    143       // Add all successors to the worklist, except for loop exit blocks and the
    144       // bodies of subloops. We visit the headers of loops so that we can process
    145       // their phis, but we contract the rest of the subloop body and only follow
    146       // edges leading back to the original loop.
    147       for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
    148            ++SI) {
    149         BasicBlock *SuccBB = *SI;
    150         if (!Visited.insert(SuccBB).second)
    151           continue;
    152 
    153         const Loop *SuccLoop = LI->getLoopFor(SuccBB);
    154         if (SuccLoop && SuccLoop->getHeader() == SuccBB
    155                      && L->contains(SuccLoop)) {
    156           VisitStack.push_back(WorklistItem(SuccBB, true));
    157 
    158           SmallVector<BasicBlock*, 8> SubLoopExitBlocks;
    159           SuccLoop->getExitBlocks(SubLoopExitBlocks);
    160 
    161           for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
    162             BasicBlock *ExitBB = SubLoopExitBlocks[i];
    163             if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB).second)
    164               VisitStack.push_back(WorklistItem(ExitBB, false));
    165           }
    166 
    167           continue;
    168         }
    169 
    170         bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
    171                                               ExitBlocks.end(), SuccBB);
    172         if (IsExitBlock)
    173           continue;
    174 
    175         VisitStack.push_back(WorklistItem(SuccBB, false));
    176       }
    177     }
    178 
    179     // Place the list of instructions to simplify on the next loop iteration
    180     // into ToSimplify.
    181     std::swap(ToSimplify, Next);
    182     Next->clear();
    183 
    184     Changed |= LocalChanged;
    185   } while (LocalChanged);
    186 
    187   return Changed;
    188 }
    189