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/Instructions.h"
  16 #include "llvm/Analysis/Dominators.h"
  17 #include "llvm/Analysis/InstructionSimplify.h"
  18 #include "llvm/Analysis/LoopInfo.h"
  19 #include "llvm/Analysis/LoopPass.h"
  20 #include "llvm/Support/Debug.h"
  21 #include "llvm/Target/TargetData.h"
  22 #include "llvm/Transforms/Scalar.h"
  23 #include "llvm/Transforms/Utils/Local.h"
  24 #include "llvm/ADT/Statistic.h"
  25 using namespace llvm;
  26
  27 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
  28
  29 namespace {
  30   class LoopInstSimplify : public LoopPass {
  31   public:
  32     static char ID; // Pass ID, replacement for typeid
  33     LoopInstSimplify() : LoopPass(ID) {
  34       initializeLoopInstSimplifyPass(*PassRegistry::getPassRegistry());
  35     }
  36
  37     bool runOnLoop(Loop*, LPPassManager&);
  38
  39     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  40       AU.setPreservesCFG();
  41       AU.addRequired<LoopInfo>();
  42       AU.addRequiredID(LoopSimplifyID);
  43       AU.addPreservedID(LoopSimplifyID);
  44       AU.addPreservedID(LCSSAID);
  45       AU.addPreserved("scalar-evolution");
  46     }
  47   };
  48 }
  49
  50 char LoopInstSimplify::ID = 0;
  51 INITIALIZE_PASS_BEGIN(LoopInstSimplify, "loop-instsimplify",
  52                 "Simplify instructions in loops", false, false)
  53 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  54 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  55 INITIALIZE_PASS_DEPENDENCY(LCSSA)
  56 INITIALIZE_PASS_END(LoopInstSimplify, "loop-instsimplify",
  57                 "Simplify instructions in loops", false, false)
  58
  59 Pass *llvm::createLoopInstSimplifyPass() {
  60   return new LoopInstSimplify();
  61 }
  62
  63 bool LoopInstSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
  64   DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
  65   LoopInfo *LI = &getAnalysis<LoopInfo>();
  66   const TargetData *TD = getAnalysisIfAvailable<TargetData>();
  67
  68   SmallVector<BasicBlock*, 8> ExitBlocks;
  69   L->getUniqueExitBlocks(ExitBlocks);
  70   array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
  71
  72   SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  73
  74   // The bit we are stealing from the pointer represents whether this basic
  75   // block is the header of a subloop, in which case we only process its phis.
  76   typedef PointerIntPair<BasicBlock*, 1> WorklistItem;
  77   SmallVector<WorklistItem, 16> VisitStack;
  78   SmallPtrSet<BasicBlock*, 32> Visited;
  79
  80   bool Changed = false;
  81   bool LocalChanged;
  82   do {
  83     LocalChanged = false;
  84
  85     VisitStack.clear();
  86     Visited.clear();
  87
  88     VisitStack.push_back(WorklistItem(L->getHeader(), false));
  89
  90     while (!VisitStack.empty()) {
  91       WorklistItem Item = VisitStack.pop_back_val();
  92       BasicBlock *BB = Item.getPointer();
  93       bool IsSubloopHeader = Item.getInt();
  94
  95       // Simplify instructions in the current basic block.
  96       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
  97         Instruction *I = BI++;
  98
  99         // The first time through the loop ToSimplify is empty and we try to
 100         // simplify all instructions. On later iterations ToSimplify is not
 101         // empty and we only bother simplifying instructions that are in it.
 102         if (!ToSimplify->empty() && !ToSimplify->count(I))
 103           continue;
 104
 105         // Don't bother simplifying unused instructions.
 106         if (!I->use_empty()) {
 107           Value *V = SimplifyInstruction(I, TD, DT);
 108           if (V && LI->replacementPreservesLCSSAForm(I, V)) {
 109             // Mark all uses for resimplification next time round the loop.
 110             for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
 111                  UI != UE; ++UI)
 112               Next->insert(cast<Instruction>(*UI));
 113
 114             I->replaceAllUsesWith(V);
 115             LocalChanged = true;
 116             ++NumSimplified;
 117           }
 118         }
 119         LocalChanged |= RecursivelyDeleteTriviallyDeadInstructions(I);
 120
 121         if (IsSubloopHeader && !isa<PHINode>(I))
 122           break;
 123       }
 124
 125       // Add all successors to the worklist, except for loop exit blocks and the
 126       // bodies of subloops. We visit the headers of loops so that we can process
 127       // their phis, but we contract the rest of the subloop body and only follow
 128       // edges leading back to the original loop.
 129       for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
 130            ++SI) {
 131         BasicBlock *SuccBB = *SI;
 132         if (!Visited.insert(SuccBB))
 133           continue;
 134
 135         const Loop *SuccLoop = LI->getLoopFor(SuccBB);
 136         if (SuccLoop && SuccLoop->getHeader() == SuccBB
 137                      && L->contains(SuccLoop)) {
 138           VisitStack.push_back(WorklistItem(SuccBB, true));
 139
 140           SmallVector<BasicBlock*, 8> SubLoopExitBlocks;
 141           SuccLoop->getExitBlocks(SubLoopExitBlocks);
 142
 143           for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
 144             BasicBlock *ExitBB = SubLoopExitBlocks[i];
 145             if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB))
 146               VisitStack.push_back(WorklistItem(ExitBB, false));
 147           }
 148
 149           continue;
 150         }
 151
 152         bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
 153                                               ExitBlocks.end(), SuccBB);
 154         if (IsExitBlock)
 155           continue;
 156
 157         VisitStack.push_back(WorklistItem(SuccBB, false));
 158       }
 159     }
 160
 161     // Place the list of instructions to simplify on the next loop iteration
 162     // into ToSimplify.
 163     std::swap(ToSimplify, Next);
 164     Next->clear();
 165
 166     Changed |= LocalChanged;
 167   } while (LocalChanged);
 168
 169   return Changed;
 170 }