1 //===- JumpThreading.h - thread control through conditional BBs -*- C++ -*-===//
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 /// \file
10 /// See the comments on JumpThreadingPass.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_TRANSFORMS_SCALAR_JUMPTHREADING_H
15 #define LLVM_TRANSFORMS_SCALAR_JUMPTHREADING_H
16 
17 #include "llvm/ADT/ArrayRef.h"
18 #include "llvm/ADT/DenseSet.h"
19 #include "llvm/ADT/SmallSet.h"
20 #include "llvm/ADT/SmallVector.h"
21 #include "llvm/Analysis/BlockFrequencyInfo.h"
22 #include "llvm/Analysis/BranchProbabilityInfo.h"
23 #include "llvm/IR/ValueHandle.h"
24 #include <utility>
25 
26 namespace llvm {
27 
28 class AAResults;
29 class BasicBlock;
30 class BinaryOperator;
31 class BranchInst;
32 class CmpInst;
33 class Constant;
34 class DomTreeUpdater;
35 class Function;
36 class Instruction;
37 class IntrinsicInst;
38 class LazyValueInfo;
39 class LoadInst;
40 class PHINode;
41 class SelectInst;
42 class SwitchInst;
43 class TargetLibraryInfo;
44 class TargetTransformInfo;
45 class Value;
46 
47 /// A private "module" namespace for types and utilities used by
48 /// JumpThreading.
49 /// These are implementation details and should not be used by clients.
50 namespace jumpthreading {
51 
52 // These are at global scope so static functions can use them too.
53 using PredValueInfo = SmallVectorImpl<std::pair<Constant *, BasicBlock *>>;
54 using PredValueInfoTy = SmallVector<std::pair<Constant *, BasicBlock *>, 8>;
55 
56 // This is used to keep track of what kind of constant we're currently hoping
57 // to find.
58 enum ConstantPreference { WantInteger, WantBlockAddress };
59 
60 } // end namespace jumpthreading
61 
62 /// This pass performs 'jump threading', which looks at blocks that have
63 /// multiple predecessors and multiple successors.  If one or more of the
64 /// predecessors of the block can be proven to always jump to one of the
65 /// successors, we forward the edge from the predecessor to the successor by
66 /// duplicating the contents of this block.
67 ///
68 /// An example of when this can occur is code like this:
69 ///
70 ///   if () { ...
71 ///     X = 4;
72 ///   }
73 ///   if (X < 3) {
74 ///
75 /// In this case, the unconditional branch at the end of the first if can be
76 /// revectored to the false side of the second if.
77 class JumpThreadingPass : public PassInfoMixin<JumpThreadingPass> {
78   TargetLibraryInfo *TLI;
79   TargetTransformInfo *TTI;
80   LazyValueInfo *LVI;
81   AAResults *AA;
82   DomTreeUpdater *DTU;
83   std::unique_ptr<BlockFrequencyInfo> BFI;
84   std::unique_ptr<BranchProbabilityInfo> BPI;
85   bool HasProfileData = false;
86   bool HasGuards = false;
87 #ifndef LLVM_ENABLE_ABI_BREAKING_CHECKS
88   SmallPtrSet<const BasicBlock *, 16> LoopHeaders;
89 #else
90   SmallSet<AssertingVH<const BasicBlock>, 16> LoopHeaders;
91 #endif
92 
93   unsigned BBDupThreshold;
94   unsigned DefaultBBDupThreshold;
95 
96 public:
97   JumpThreadingPass(int T = -1);
98 
99   // Glue for old PM.
100   bool runImpl(Function &F, TargetLibraryInfo *TLI, TargetTransformInfo *TTI,
101                LazyValueInfo *LVI, AAResults *AA, DomTreeUpdater *DTU,
102                bool HasProfileData, std::unique_ptr<BlockFrequencyInfo> BFI,
103                std::unique_ptr<BranchProbabilityInfo> BPI);
104 
105   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
106 
107   void releaseMemory() {
108     BFI.reset();
109     BPI.reset();
110   }
111 
112   void findLoopHeaders(Function &F);
113   bool processBlock(BasicBlock *BB);
114   bool maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB);
115   void updateSSA(BasicBlock *BB, BasicBlock *NewBB,
116                  DenseMap<Instruction *, Value *> &ValueMapping);
117   DenseMap<Instruction *, Value *> cloneInstructions(BasicBlock::iterator BI,
118                                                      BasicBlock::iterator BE,
119                                                      BasicBlock *NewBB,
120                                                      BasicBlock *PredBB);
121   bool tryThreadEdge(BasicBlock *BB,
122                      const SmallVectorImpl<BasicBlock *> &PredBBs,
123                      BasicBlock *SuccBB);
124   void threadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
125                   BasicBlock *SuccBB);
126   bool duplicateCondBranchOnPHIIntoPred(
127       BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs);
128 
129   bool computeValueKnownInPredecessorsImpl(
130       Value *V, BasicBlock *BB, jumpthreading::PredValueInfo &Result,
131       jumpthreading::ConstantPreference Preference,
132       DenseSet<Value *> &RecursionSet, Instruction *CxtI = nullptr);
133   bool
134   computeValueKnownInPredecessors(Value *V, BasicBlock *BB,
135                                   jumpthreading::PredValueInfo &Result,
136                                   jumpthreading::ConstantPreference Preference,
137                                   Instruction *CxtI = nullptr) {
138     DenseSet<Value *> RecursionSet;
139     return computeValueKnownInPredecessorsImpl(V, BB, Result, Preference,
140                                                RecursionSet, CxtI);
141   }
142 
143   Constant *evaluateOnPredecessorEdge(BasicBlock *BB, BasicBlock *PredPredBB,
144                                       Value *cond);
145   bool maybethreadThroughTwoBasicBlocks(BasicBlock *BB, Value *Cond);
146   void threadThroughTwoBasicBlocks(BasicBlock *PredPredBB, BasicBlock *PredBB,
147                                    BasicBlock *BB, BasicBlock *SuccBB);
148   bool processThreadableEdges(Value *Cond, BasicBlock *BB,
149                               jumpthreading::ConstantPreference Preference,
150                               Instruction *CxtI = nullptr);
151 
152   bool processBranchOnPHI(PHINode *PN);
153   bool processBranchOnXOR(BinaryOperator *BO);
154   bool processImpliedCondition(BasicBlock *BB);
155 
156   bool simplifyPartiallyRedundantLoad(LoadInst *LI);
157   void unfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB, SelectInst *SI,
158                          PHINode *SIUse, unsigned Idx);
159 
160   bool tryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
161   bool tryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB);
162   bool tryToUnfoldSelectInCurrBB(BasicBlock *BB);
163 
164   bool processGuards(BasicBlock *BB);
165   bool threadGuard(BasicBlock *BB, IntrinsicInst *Guard, BranchInst *BI);
166 
167 private:
168   BasicBlock *splitBlockPreds(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
169                               const char *Suffix);
170   void updateBlockFreqAndEdgeWeight(BasicBlock *PredBB, BasicBlock *BB,
171                                     BasicBlock *NewBB, BasicBlock *SuccBB);
172   /// Check if the block has profile metadata for its outgoing edges.
173   bool doesBlockHaveProfileData(BasicBlock *BB);
174 };
175 
176 } // end namespace llvm
177 
178 #endif // LLVM_TRANSFORMS_SCALAR_JUMPTHREADING_H
179