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.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/ScalarEvolution.h"
28 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
29 #include "llvm/Analysis/TargetTransformInfo.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/Transforms/Utils/Local.h"
32 using namespace llvm;
33 
34 #define DEBUG_TYPE "loop-simplifycfg"
35 
36 namespace {
37 class LoopSimplifyCFG : public LoopPass {
38 public:
39   static char ID; // Pass ID, replacement for typeid
40   LoopSimplifyCFG() : LoopPass(ID) {
41     initializeLoopSimplifyCFGPass(*PassRegistry::getPassRegistry());
42   }
43 
44   bool runOnLoop(Loop *L, LPPassManager &) override;
45 
46   void getAnalysisUsage(AnalysisUsage &AU) const override {
47     AU.addRequired<DominatorTreeWrapperPass>();
48     AU.addRequired<LoopInfoWrapperPass>();
49 
50     AU.addPreserved<DominatorTreeWrapperPass>();
51     AU.addPreserved<LoopInfoWrapperPass>();
52     AU.addPreserved<GlobalsAAWrapperPass>();
53     AU.addPreserved<BasicAAWrapperPass>();
54     AU.addPreserved<AAResultsWrapperPass>();
55     AU.addPreserved<ScalarEvolutionWrapperPass>();
56     AU.addPreserved<SCEVAAWrapperPass>();
57     AU.addPreserved<DependenceAnalysis>();
58     AU.addPreservedID(LoopSimplifyID);
59     AU.addPreservedID(LCSSAID);
60   }
61 };
62 }
63 
64 char LoopSimplifyCFG::ID = 0;
65 INITIALIZE_PASS_BEGIN(LoopSimplifyCFG, "loop-simplifycfg", "Simplify loop CFG",
66                       false, false)
67 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
68 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
69 INITIALIZE_PASS_END(LoopSimplifyCFG, "loop-simplifycfg", "Simplify loop CFG",
70                     false, false)
71 
72 Pass *llvm::createLoopSimplifyCFGPass() { return new LoopSimplifyCFG(); }
73 
74 static bool simplifyLoopCFG(Loop *L, DominatorTree *DT, LoopInfo *LI) {
75   bool Changed = false;
76   // Copy blocks into a temporary array to avoid iterator invalidation issues
77   // as we remove them.
78   SmallVector<WeakVH, 16> Blocks(L->blocks());
79 
80   for (auto &Block : Blocks) {
81     // Attempt to merge blocks in the trivial case. Don't modify blocks which
82     // belong to other loops.
83     BasicBlock *Succ = cast_or_null<BasicBlock>(Block);
84     if (!Succ)
85       continue;
86 
87     BasicBlock *Pred = Succ->getSinglePredecessor();
88     if (!Pred || !Pred->getSingleSuccessor() || LI->getLoopFor(Pred) != L)
89       continue;
90 
91     // Pred is going to disappear, so we need to update the loop info.
92     if (L->getHeader() == Pred)
93       L->moveToHeader(Succ);
94     LI->removeBlock(Pred);
95     MergeBasicBlockIntoOnlyPred(Succ, DT);
96     Changed = true;
97   }
98 
99   return Changed;
100 }
101 
102 /// runOnLoop - Perform basic CFG simplifications to assist other loop passes.
103 /// For now, this only attempts to merge blocks in the trivial case.
104 bool LoopSimplifyCFG::runOnLoop(Loop *L, LPPassManager &) {
105   if (skipOptnoneFunction(L))
106     return false;
107 
108   DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
109   LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
110   return simplifyLoopCFG(L, DT, LI);
111 }
112