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/DomTreeUpdater.h"
31 #include "llvm/IR/Dominators.h"
32 #include "llvm/Transforms/Scalar.h"
33 #include "llvm/Transforms/Scalar/LoopPassManager.h"
34 #include "llvm/Transforms/Utils.h"
35 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include "llvm/Transforms/Utils/LoopUtils.h"
38 using namespace llvm;
39 
40 #define DEBUG_TYPE "loop-simplifycfg"
41 
42 static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI,
43                             ScalarEvolution &SE) {
44   bool Changed = false;
45   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
46   // Copy blocks into a temporary array to avoid iterator invalidation issues
47   // as we remove them.
48   SmallVector<WeakTrackingVH, 16> Blocks(L.blocks());
49 
50   for (auto &Block : Blocks) {
51     // Attempt to merge blocks in the trivial case. Don't modify blocks which
52     // belong to other loops.
53     BasicBlock *Succ = cast_or_null<BasicBlock>(Block);
54     if (!Succ)
55       continue;
56 
57     BasicBlock *Pred = Succ->getSinglePredecessor();
58     if (!Pred || !Pred->getSingleSuccessor() || LI.getLoopFor(Pred) != &L)
59       continue;
60 
61     // Merge Succ into Pred and delete it.
62     MergeBlockIntoPredecessor(Succ, &DTU, &LI);
63 
64     SE.forgetTopmostLoop(&L);
65 
66     Changed = true;
67   }
68 
69   return Changed;
70 }
71 
72 PreservedAnalyses LoopSimplifyCFGPass::run(Loop &L, LoopAnalysisManager &AM,
73                                            LoopStandardAnalysisResults &AR,
74                                            LPMUpdater &) {
75   if (!simplifyLoopCFG(L, AR.DT, AR.LI, AR.SE))
76     return PreservedAnalyses::all();
77 
78   return getLoopPassPreservedAnalyses();
79 }
80 
81 namespace {
82 class LoopSimplifyCFGLegacyPass : public LoopPass {
83 public:
84   static char ID; // Pass ID, replacement for typeid
85   LoopSimplifyCFGLegacyPass() : LoopPass(ID) {
86     initializeLoopSimplifyCFGLegacyPassPass(*PassRegistry::getPassRegistry());
87   }
88 
89   bool runOnLoop(Loop *L, LPPassManager &) override {
90     if (skipLoop(L))
91       return false;
92 
93     DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
94     LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
95     ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
96     return simplifyLoopCFG(*L, DT, LI, SE);
97   }
98 
99   void getAnalysisUsage(AnalysisUsage &AU) const override {
100     AU.addPreserved<DependenceAnalysisWrapperPass>();
101     getLoopAnalysisUsage(AU);
102   }
103 };
104 }
105 
106 char LoopSimplifyCFGLegacyPass::ID = 0;
107 INITIALIZE_PASS_BEGIN(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
108                       "Simplify loop CFG", false, false)
109 INITIALIZE_PASS_DEPENDENCY(LoopPass)
110 INITIALIZE_PASS_END(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
111                     "Simplify loop CFG", false, false)
112 
113 Pass *llvm::createLoopSimplifyCFGPass() {
114   return new LoopSimplifyCFGLegacyPass();
115 }
116