1 //===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements Loop Rotation Pass.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "llvm/Transforms/Scalar/LoopRotation.h"
14 #include "llvm/Analysis/AssumptionCache.h"
15 #include "llvm/Analysis/InstructionSimplify.h"
16 #include "llvm/Analysis/LazyBlockFrequencyInfo.h"
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/Analysis/LoopPass.h"
19 #include "llvm/Analysis/MemorySSA.h"
20 #include "llvm/Analysis/MemorySSAUpdater.h"
21 #include "llvm/Analysis/ScalarEvolution.h"
22 #include "llvm/Analysis/TargetTransformInfo.h"
23 #include "llvm/InitializePasses.h"
24 #include "llvm/Support/CommandLine.h"
25 #include "llvm/Transforms/Scalar.h"
26 #include "llvm/Transforms/Utils/LoopRotationUtils.h"
27 #include "llvm/Transforms/Utils/LoopUtils.h"
28 #include <optional>
29 using namespace llvm;
30 
31 #define DEBUG_TYPE "loop-rotate"
32 
33 static cl::opt<unsigned> DefaultRotationThreshold(
34     "rotation-max-header-size", cl::init(16), cl::Hidden,
35     cl::desc("The default maximum header size for automatic loop rotation"));
36 
37 static cl::opt<bool> PrepareForLTOOption(
38     "rotation-prepare-for-lto", cl::init(false), cl::Hidden,
39     cl::desc("Run loop-rotation in the prepare-for-lto stage. This option "
40              "should be used for testing only."));
41 
42 LoopRotatePass::LoopRotatePass(bool EnableHeaderDuplication, bool PrepareForLTO)
43     : EnableHeaderDuplication(EnableHeaderDuplication),
44       PrepareForLTO(PrepareForLTO) {}
45 
46 PreservedAnalyses LoopRotatePass::run(Loop &L, LoopAnalysisManager &AM,
47                                       LoopStandardAnalysisResults &AR,
48                                       LPMUpdater &) {
49   // Vectorization requires loop-rotation. Use default threshold for loops the
50   // user explicitly marked for vectorization, even when header duplication is
51   // disabled.
52   int Threshold = EnableHeaderDuplication ||
53                           hasVectorizeTransformation(&L) == TM_ForcedByUser
54                       ? DefaultRotationThreshold
55                       : 0;
56   const DataLayout &DL = L.getHeader()->getModule()->getDataLayout();
57   const SimplifyQuery SQ = getBestSimplifyQuery(AR, DL);
58 
59   std::optional<MemorySSAUpdater> MSSAU;
60   if (AR.MSSA)
61     MSSAU = MemorySSAUpdater(AR.MSSA);
62   bool Changed = LoopRotation(&L, &AR.LI, &AR.TTI, &AR.AC, &AR.DT, &AR.SE,
63                               MSSAU ? &*MSSAU : nullptr, SQ, false, Threshold,
64                               false, PrepareForLTO || PrepareForLTOOption);
65 
66   if (!Changed)
67     return PreservedAnalyses::all();
68 
69   if (AR.MSSA && VerifyMemorySSA)
70     AR.MSSA->verifyMemorySSA();
71 
72   auto PA = getLoopPassPreservedAnalyses();
73   if (AR.MSSA)
74     PA.preserve<MemorySSAAnalysis>();
75   return PA;
76 }
77 
78 namespace {
79 
80 class LoopRotateLegacyPass : public LoopPass {
81   unsigned MaxHeaderSize;
82   bool PrepareForLTO;
83 
84 public:
85   static char ID; // Pass ID, replacement for typeid
86   LoopRotateLegacyPass(int SpecifiedMaxHeaderSize = -1,
87                        bool PrepareForLTO = false)
88       : LoopPass(ID), PrepareForLTO(PrepareForLTO) {
89     initializeLoopRotateLegacyPassPass(*PassRegistry::getPassRegistry());
90     if (SpecifiedMaxHeaderSize == -1)
91       MaxHeaderSize = DefaultRotationThreshold;
92     else
93       MaxHeaderSize = unsigned(SpecifiedMaxHeaderSize);
94   }
95 
96   // LCSSA form makes instruction renaming easier.
97   void getAnalysisUsage(AnalysisUsage &AU) const override {
98     AU.addRequired<AssumptionCacheTracker>();
99     AU.addRequired<TargetTransformInfoWrapperPass>();
100     AU.addPreserved<MemorySSAWrapperPass>();
101     getLoopAnalysisUsage(AU);
102 
103     // Lazy BFI and BPI are marked as preserved here so LoopRotate
104     // can remain part of the same loop pass manager as LICM.
105     AU.addPreserved<LazyBlockFrequencyInfoPass>();
106     AU.addPreserved<LazyBranchProbabilityInfoPass>();
107   }
108 
109   bool runOnLoop(Loop *L, LPPassManager &LPM) override {
110     if (skipLoop(L))
111       return false;
112     Function &F = *L->getHeader()->getParent();
113 
114     auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
115     const auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
116     auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
117     auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
118     auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
119     const SimplifyQuery SQ = getBestSimplifyQuery(*this, F);
120     std::optional<MemorySSAUpdater> MSSAU;
121     // Not requiring MemorySSA and getting it only if available will split
122     // the loop pass pipeline when LoopRotate is being run first.
123     auto *MSSAA = getAnalysisIfAvailable<MemorySSAWrapperPass>();
124     if (MSSAA)
125       MSSAU = MemorySSAUpdater(&MSSAA->getMSSA());
126     // Vectorization requires loop-rotation. Use default threshold for loops the
127     // user explicitly marked for vectorization, even when header duplication is
128     // disabled.
129     int Threshold = hasVectorizeTransformation(L) == TM_ForcedByUser
130                         ? DefaultRotationThreshold
131                         : MaxHeaderSize;
132 
133     return LoopRotation(L, LI, TTI, AC, &DT, &SE, MSSAU ? &*MSSAU : nullptr, SQ,
134                         false, Threshold, false,
135                         PrepareForLTO || PrepareForLTOOption);
136   }
137 };
138 } // end namespace
139 
140 char LoopRotateLegacyPass::ID = 0;
141 INITIALIZE_PASS_BEGIN(LoopRotateLegacyPass, "loop-rotate", "Rotate Loops",
142                       false, false)
143 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
144 INITIALIZE_PASS_DEPENDENCY(LoopPass)
145 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
146 INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
147 INITIALIZE_PASS_END(LoopRotateLegacyPass, "loop-rotate", "Rotate Loops", false,
148                     false)
149 
150 Pass *llvm::createLoopRotatePass(int MaxHeaderSize, bool PrepareForLTO) {
151   return new LoopRotateLegacyPass(MaxHeaderSize, PrepareForLTO);
152 }
153