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      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 }
    171