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/BasicAliasAnalysis.h"
22 #include "llvm/Analysis/AssumptionCache.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/LoopPassManager.h"
28 #include "llvm/Analysis/ScalarEvolution.h"
29 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
30 #include "llvm/Analysis/TargetTransformInfo.h"
31 #include "llvm/IR/Dominators.h"
32 #include "llvm/Transforms/Scalar.h"
33 #include "llvm/Transforms/Utils/Local.h"
34 #include "llvm/Transforms/Utils/LoopUtils.h"
35 using namespace llvm;
36
37 #define DEBUG_TYPE "loop-simplifycfg"
38
simplifyLoopCFG(Loop & L,DominatorTree & DT,LoopInfo & LI)39 static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI) {
40 bool Changed = false;
41 // Copy blocks into a temporary array to avoid iterator invalidation issues
42 // as we remove them.
43 SmallVector<WeakVH, 16> Blocks(L.blocks());
44
45 for (auto &Block : Blocks) {
46 // Attempt to merge blocks in the trivial case. Don't modify blocks which
47 // belong to other loops.
48 BasicBlock *Succ = cast_or_null<BasicBlock>(Block);
49 if (!Succ)
50 continue;
51
52 BasicBlock *Pred = Succ->getSinglePredecessor();
53 if (!Pred || !Pred->getSingleSuccessor() || LI.getLoopFor(Pred) != &L)
54 continue;
55
56 // Pred is going to disappear, so we need to update the loop info.
57 if (L.getHeader() == Pred)
58 L.moveToHeader(Succ);
59 LI.removeBlock(Pred);
60 MergeBasicBlockIntoOnlyPred(Succ, &DT);
61 Changed = true;
62 }
63
64 return Changed;
65 }
66
run(Loop & L,AnalysisManager<Loop> & AM)67 PreservedAnalyses LoopSimplifyCFGPass::run(Loop &L, AnalysisManager<Loop> &AM) {
68 const auto &FAM =
69 AM.getResult<FunctionAnalysisManagerLoopProxy>(L).getManager();
70 Function *F = L.getHeader()->getParent();
71
72 auto *LI = FAM.getCachedResult<LoopAnalysis>(*F);
73 auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(*F);
74 assert((LI && DT) && "Analyses for LoopSimplifyCFG not available");
75
76 if (!simplifyLoopCFG(L, *DT, *LI))
77 return PreservedAnalyses::all();
78 return getLoopPassPreservedAnalyses();
79 }
80
81 namespace {
82 class LoopSimplifyCFGLegacyPass : public LoopPass {
83 public:
84 static char ID; // Pass ID, replacement for typeid
LoopSimplifyCFGLegacyPass()85 LoopSimplifyCFGLegacyPass() : LoopPass(ID) {
86 initializeLoopSimplifyCFGLegacyPassPass(*PassRegistry::getPassRegistry());
87 }
88
runOnLoop(Loop * L,LPPassManager &)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 return simplifyLoopCFG(*L, DT, LI);
96 }
97
getAnalysisUsage(AnalysisUsage & AU) const98 void getAnalysisUsage(AnalysisUsage &AU) const override {
99 AU.addPreserved<DependenceAnalysisWrapperPass>();
100 getLoopAnalysisUsage(AU);
101 }
102 };
103 }
104
105 char LoopSimplifyCFGLegacyPass::ID = 0;
106 INITIALIZE_PASS_BEGIN(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
107 "Simplify loop CFG", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopPass)108 INITIALIZE_PASS_DEPENDENCY(LoopPass)
109 INITIALIZE_PASS_END(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
110 "Simplify loop CFG", false, false)
111
112 Pass *llvm::createLoopSimplifyCFGPass() {
113 return new LoopSimplifyCFGLegacyPass();
114 }
115