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      1 //===--------- LoopSimplifyCFG.cpp - Loop CFG 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 file implements the Loop SimplifyCFG Pass. This pass is responsible for
     11 // basic loop CFG cleanup, primarily to assist other loop passes. If you
     12 // encounter a noncanonical CFG construct that causes another loop pass to
     13 // perform suboptimally, this is the place to fix it up.
     14 //
     15 //===----------------------------------------------------------------------===//
     16 
     17 #include "llvm/Transforms/Scalar/LoopSimplifyCFG.h"
     18 #include "llvm/ADT/SmallVector.h"
     19 #include "llvm/ADT/Statistic.h"
     20 #include "llvm/Analysis/AliasAnalysis.h"
     21 #include "llvm/Analysis/AssumptionCache.h"
     22 #include "llvm/Analysis/BasicAliasAnalysis.h"
     23 #include "llvm/Analysis/DependenceAnalysis.h"
     24 #include "llvm/Analysis/GlobalsModRef.h"
     25 #include "llvm/Analysis/LoopInfo.h"
     26 #include "llvm/Analysis/LoopPass.h"
     27 #include "llvm/Analysis/ScalarEvolution.h"
     28 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
     29 #include "llvm/Analysis/TargetTransformInfo.h"
     30 #include "llvm/IR/Dominators.h"
     31 #include "llvm/Transforms/Scalar.h"
     32 #include "llvm/Transforms/Scalar/LoopPassManager.h"
     33 #include "llvm/Transforms/Utils.h"
     34 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
     35 #include "llvm/Transforms/Utils/Local.h"
     36 #include "llvm/Transforms/Utils/LoopUtils.h"
     37 using namespace llvm;
     38 
     39 #define DEBUG_TYPE "loop-simplifycfg"
     40 
     41 static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI,
     42                             ScalarEvolution &SE) {
     43   bool Changed = false;
     44   // Copy blocks into a temporary array to avoid iterator invalidation issues
     45   // as we remove them.
     46   SmallVector<WeakTrackingVH, 16> Blocks(L.blocks());
     47 
     48   for (auto &Block : Blocks) {
     49     // Attempt to merge blocks in the trivial case. Don't modify blocks which
     50     // belong to other loops.
     51     BasicBlock *Succ = cast_or_null<BasicBlock>(Block);
     52     if (!Succ)
     53       continue;
     54 
     55     BasicBlock *Pred = Succ->getSinglePredecessor();
     56     if (!Pred || !Pred->getSingleSuccessor() || LI.getLoopFor(Pred) != &L)
     57       continue;
     58 
     59     // Merge Succ into Pred and delete it.
     60     MergeBlockIntoPredecessor(Succ, &DT, &LI);
     61 
     62     SE.forgetLoop(&L);
     63     Changed = true;
     64   }
     65 
     66   return Changed;
     67 }
     68 
     69 PreservedAnalyses LoopSimplifyCFGPass::run(Loop &L, LoopAnalysisManager &AM,
     70                                            LoopStandardAnalysisResults &AR,
     71                                            LPMUpdater &) {
     72   if (!simplifyLoopCFG(L, AR.DT, AR.LI, AR.SE))
     73     return PreservedAnalyses::all();
     74 
     75   return getLoopPassPreservedAnalyses();
     76 }
     77 
     78 namespace {
     79 class LoopSimplifyCFGLegacyPass : public LoopPass {
     80 public:
     81   static char ID; // Pass ID, replacement for typeid
     82   LoopSimplifyCFGLegacyPass() : LoopPass(ID) {
     83     initializeLoopSimplifyCFGLegacyPassPass(*PassRegistry::getPassRegistry());
     84   }
     85 
     86   bool runOnLoop(Loop *L, LPPassManager &) override {
     87     if (skipLoop(L))
     88       return false;
     89 
     90     DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     91     LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
     92     ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
     93     return simplifyLoopCFG(*L, DT, LI, SE);
     94   }
     95 
     96   void getAnalysisUsage(AnalysisUsage &AU) const override {
     97     AU.addPreserved<DependenceAnalysisWrapperPass>();
     98     getLoopAnalysisUsage(AU);
     99   }
    100 };
    101 }
    102 
    103 char LoopSimplifyCFGLegacyPass::ID = 0;
    104 INITIALIZE_PASS_BEGIN(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
    105                       "Simplify loop CFG", false, false)
    106 INITIALIZE_PASS_DEPENDENCY(LoopPass)
    107 INITIALIZE_PASS_END(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
    108                     "Simplify loop CFG", false, false)
    109 
    110 Pass *llvm::createLoopSimplifyCFGPass() {
    111   return new LoopSimplifyCFGLegacyPass();
    112 }
    113