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