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