Home | History | Annotate | Download | only in Scalar 
      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 }
    190