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