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/Target/TargetLibraryInfo.h"
  23 #include "llvm/Transforms/Scalar.h"
  24 #include "llvm/Transforms/Utils/Local.h"
  25 #include "llvm/ADT/Statistic.h"
  26 using namespace llvm;
  27
  28 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
  29
  30 namespace {
  31   class LoopInstSimplify : public LoopPass {
  32   public:
  33     static char ID; // Pass ID, replacement for typeid
  34     LoopInstSimplify() : LoopPass(ID) {
  35       initializeLoopInstSimplifyPass(*PassRegistry::getPassRegistry());
  36     }
  37
  38     bool runOnLoop(Loop*, LPPassManager&);
  39
  40     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  41       AU.setPreservesCFG();
  42       AU.addRequired<LoopInfo>();
  43       AU.addRequiredID(LoopSimplifyID);
  44       AU.addPreservedID(LoopSimplifyID);
  45       AU.addPreservedID(LCSSAID);
  46       AU.addPreserved("scalar-evolution");
  47       AU.addRequired<TargetLibraryInfo>();
  48     }
  49   };
  50 }
  51
  52 char LoopInstSimplify::ID = 0;
  53 INITIALIZE_PASS_BEGIN(LoopInstSimplify, "loop-instsimplify",
  54                 "Simplify instructions in loops", false, false)
  55 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
  56 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  57 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  58 INITIALIZE_PASS_DEPENDENCY(LCSSA)
  59 INITIALIZE_PASS_END(LoopInstSimplify, "loop-instsimplify",
  60                 "Simplify instructions in loops", false, false)
  61
  62 Pass *llvm::createLoopInstSimplifyPass() {
  63   return new LoopInstSimplify();
  64 }
  65
  66 bool LoopInstSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
  67   DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
  68   LoopInfo *LI = &getAnalysis<LoopInfo>();
  69   const TargetData *TD = getAnalysisIfAvailable<TargetData>();
  70   const TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
  71
  72   SmallVector<BasicBlock*, 8> ExitBlocks;
  73   L->getUniqueExitBlocks(ExitBlocks);
  74   array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
  75
  76   SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
  77
  78   // The bit we are stealing from the pointer represents whether this basic
  79   // block is the header of a subloop, in which case we only process its phis.
  80   typedef PointerIntPair<BasicBlock*, 1> WorklistItem;
  81   SmallVector<WorklistItem, 16> VisitStack;
  82   SmallPtrSet<BasicBlock*, 32> Visited;
  83
  84   bool Changed = false;
  85   bool LocalChanged;
  86   do {
  87     LocalChanged = false;
  88
  89     VisitStack.clear();
  90     Visited.clear();
  91
  92     VisitStack.push_back(WorklistItem(L->getHeader(), false));
  93
  94     while (!VisitStack.empty()) {
  95       WorklistItem Item = VisitStack.pop_back_val();
  96       BasicBlock *BB = Item.getPointer();
  97       bool IsSubloopHeader = Item.getInt();
  98
  99       // Simplify instructions in the current basic block.
 100       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
 101         Instruction *I = BI++;
 102
 103         // The first time through the loop ToSimplify is empty and we try to
 104         // simplify all instructions. On later iterations ToSimplify is not
 105         // empty and we only bother simplifying instructions that are in it.
 106         if (!ToSimplify->empty() && !ToSimplify->count(I))
 107           continue;
 108
 109         // Don't bother simplifying unused instructions.
 110         if (!I->use_empty()) {
 111           Value *V = SimplifyInstruction(I, TD, TLI, DT);
 112           if (V && LI->replacementPreservesLCSSAForm(I, V)) {
 113             // Mark all uses for resimplification next time round the loop.
 114             for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
 115                  UI != UE; ++UI)
 116               Next->insert(cast<Instruction>(*UI));
 117
 118             I->replaceAllUsesWith(V);
 119             LocalChanged = true;
 120             ++NumSimplified;
 121           }
 122         }
 123         LocalChanged |= RecursivelyDeleteTriviallyDeadInstructions(I);
 124
 125         if (IsSubloopHeader && !isa<PHINode>(I))
 126           break;
 127       }
 128
 129       // Add all successors to the worklist, except for loop exit blocks and the
 130       // bodies of subloops. We visit the headers of loops so that we can process
 131       // their phis, but we contract the rest of the subloop body and only follow
 132       // edges leading back to the original loop.
 133       for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
 134            ++SI) {
 135         BasicBlock *SuccBB = *SI;
 136         if (!Visited.insert(SuccBB))
 137           continue;
 138
 139         const Loop *SuccLoop = LI->getLoopFor(SuccBB);
 140         if (SuccLoop && SuccLoop->getHeader() == SuccBB
 141                      && L->contains(SuccLoop)) {
 142           VisitStack.push_back(WorklistItem(SuccBB, true));
 143
 144           SmallVector<BasicBlock*, 8> SubLoopExitBlocks;
 145           SuccLoop->getExitBlocks(SubLoopExitBlocks);
 146
 147           for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
 148             BasicBlock *ExitBB = SubLoopExitBlocks[i];
 149             if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB))
 150               VisitStack.push_back(WorklistItem(ExitBB, false));
 151           }
 152
 153           continue;
 154         }
 155
 156         bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
 157                                               ExitBlocks.end(), SuccBB);
 158         if (IsExitBlock)
 159           continue;
 160
 161         VisitStack.push_back(WorklistItem(SuccBB, false));
 162       }
 163     }
 164
 165     // Place the list of instructions to simplify on the next loop iteration
 166     // into ToSimplify.
 167     std::swap(ToSimplify, Next);
 168     Next->clear();
 169
 170     Changed |= LocalChanged;
 171   } while (LocalChanged);
 172
 173   return Changed;
 174 }