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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 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 }
    196