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"
35using namespace llvm;
36
37#define DEBUG_TYPE "loop-simplifycfg"
38
39static 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
67PreservedAnalyses 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
81namespace {
82class LoopSimplifyCFGLegacyPass : public LoopPass {
83public:
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    return simplifyLoopCFG(*L, DT, LI);
96  }
97
98  void getAnalysisUsage(AnalysisUsage &AU) const override {
99    AU.addPreserved<DependenceAnalysisWrapperPass>();
100    getLoopAnalysisUsage(AU);
101  }
102};
103}
104
105char LoopSimplifyCFGLegacyPass::ID = 0;
106INITIALIZE_PASS_BEGIN(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
107                      "Simplify loop CFG", false, false)
108INITIALIZE_PASS_DEPENDENCY(LoopPass)
109INITIALIZE_PASS_END(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
110                    "Simplify loop CFG", false, false)
111
112Pass *llvm::createLoopSimplifyCFGPass() {
113  return new LoopSimplifyCFGLegacyPass();
114}
115