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 #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, TLI);
    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 }
    175