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/SmallPtrSet.h"
20 #include "llvm/ADT/SmallSet.h"
21 #include "llvm/ADT/SmallVector.h"
22 #include "llvm/Analysis/BlockFrequencyInfo.h"
23 #include "llvm/Analysis/BranchProbabilityInfo.h"
24 #include "llvm/Analysis/DomTreeUpdater.h"
25 #include "llvm/IR/ValueHandle.h"
26 #include <memory>
27 #include <utility>
28 
29 namespace llvm {
30 
31 class AAResults;
32 class BasicBlock;
33 class BinaryOperator;
34 class BranchInst;
35 class CmpInst;
36 class Constant;
37 class DomTreeUpdater;
38 class Function;
39 class Instruction;
40 class IntrinsicInst;
41 class LazyValueInfo;
42 class LoadInst;
43 class PHINode;
44 class SelectInst;
45 class SwitchInst;
46 class TargetLibraryInfo;
47 class Value;
48 
49 /// A private "module" namespace for types and utilities used by
50 /// JumpThreading.
51 /// These are implementation details and should not be used by clients.
52 namespace jumpthreading {
53 
54 // These are at global scope so static functions can use them too.
55 using PredValueInfo = SmallVectorImpl<std::pair<Constant *, BasicBlock *>>;
56 using PredValueInfoTy = SmallVector<std::pair<Constant *, BasicBlock *>, 8>;
57 
58 // This is used to keep track of what kind of constant we're currently hoping
59 // to find.
60 enum ConstantPreference { WantInteger, WantBlockAddress };
61 
62 } // end namespace jumpthreading
63 
64 /// This pass performs 'jump threading', which looks at blocks that have
65 /// multiple predecessors and multiple successors.  If one or more of the
66 /// predecessors of the block can be proven to always jump to one of the
67 /// successors, we forward the edge from the predecessor to the successor by
68 /// duplicating the contents of this block.
69 ///
70 /// An example of when this can occur is code like this:
71 ///
72 ///   if () { ...
73 ///     X = 4;
74 ///   }
75 ///   if (X < 3) {
76 ///
77 /// In this case, the unconditional branch at the end of the first if can be
78 /// revectored to the false side of the second if.
79 class JumpThreadingPass : public PassInfoMixin<JumpThreadingPass> {
80   TargetLibraryInfo *TLI;
81   LazyValueInfo *LVI;
82   AAResults *AA;
83   DomTreeUpdater *DTU;
84   std::unique_ptr<BlockFrequencyInfo> BFI;
85   std::unique_ptr<BranchProbabilityInfo> BPI;
86   bool HasProfileData = false;
87   bool HasGuards = false;
88 #ifndef LLVM_ENABLE_ABI_BREAKING_CHECKS
89   SmallPtrSet<const BasicBlock *, 16> LoopHeaders;
90 #else
91   SmallSet<AssertingVH<const BasicBlock>, 16> LoopHeaders;
92 #endif
93 
94   unsigned BBDupThreshold;
95   unsigned DefaultBBDupThreshold;
96   bool InsertFreezeWhenUnfoldingSelect;
97 
98 public:
99   JumpThreadingPass(bool InsertFreezeWhenUnfoldingSelect = false, int T = -1);
100 
101   // Glue for old PM.
102   bool runImpl(Function &F, TargetLibraryInfo *TLI, LazyValueInfo *LVI,
103                AAResults *AA, DomTreeUpdater *DTU, bool HasProfileData,
104                std::unique_ptr<BlockFrequencyInfo> BFI,
105                std::unique_ptr<BranchProbabilityInfo> BPI);
106 
107   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
108 
109   void releaseMemory() {
110     BFI.reset();
111     BPI.reset();
112   }
113 
114   void findLoopHeaders(Function &F);
115   bool processBlock(BasicBlock *BB);
116   bool maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB);
117   void updateSSA(BasicBlock *BB, BasicBlock *NewBB,
118                  DenseMap<Instruction *, Value *> &ValueMapping);
119   DenseMap<Instruction *, Value *> cloneInstructions(BasicBlock::iterator BI,
120                                                      BasicBlock::iterator BE,
121                                                      BasicBlock *NewBB,
122                                                      BasicBlock *PredBB);
123   bool tryThreadEdge(BasicBlock *BB,
124                      const SmallVectorImpl<BasicBlock *> &PredBBs,
125                      BasicBlock *SuccBB);
126   void threadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
127                   BasicBlock *SuccBB);
128   bool duplicateCondBranchOnPHIIntoPred(
129       BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs);
130 
131   bool computeValueKnownInPredecessorsImpl(
132       Value *V, BasicBlock *BB, jumpthreading::PredValueInfo &Result,
133       jumpthreading::ConstantPreference Preference,
134       DenseSet<Value *> &RecursionSet, Instruction *CxtI = nullptr);
135   bool
136   computeValueKnownInPredecessors(Value *V, BasicBlock *BB,
137                                   jumpthreading::PredValueInfo &Result,
138                                   jumpthreading::ConstantPreference Preference,
139                                   Instruction *CxtI = nullptr) {
140     DenseSet<Value *> RecursionSet;
141     return computeValueKnownInPredecessorsImpl(V, BB, Result, Preference,
142                                                RecursionSet, CxtI);
143   }
144 
145   Constant *evaluateOnPredecessorEdge(BasicBlock *BB, BasicBlock *PredPredBB,
146                                       Value *cond);
147   bool maybethreadThroughTwoBasicBlocks(BasicBlock *BB, Value *Cond);
148   void threadThroughTwoBasicBlocks(BasicBlock *PredPredBB, BasicBlock *PredBB,
149                                    BasicBlock *BB, BasicBlock *SuccBB);
150   bool processThreadableEdges(Value *Cond, BasicBlock *BB,
151                               jumpthreading::ConstantPreference Preference,
152                               Instruction *CxtI = nullptr);
153 
154   bool processBranchOnPHI(PHINode *PN);
155   bool processBranchOnXOR(BinaryOperator *BO);
156   bool processImpliedCondition(BasicBlock *BB);
157 
158   bool simplifyPartiallyRedundantLoad(LoadInst *LI);
159   void unfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB, SelectInst *SI,
160                          PHINode *SIUse, unsigned Idx);
161 
162   bool tryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
163   bool tryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB);
164   bool tryToUnfoldSelectInCurrBB(BasicBlock *BB);
165 
166   bool processGuards(BasicBlock *BB);
167   bool threadGuard(BasicBlock *BB, IntrinsicInst *Guard, BranchInst *BI);
168 
169 private:
170   BasicBlock *splitBlockPreds(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
171                               const char *Suffix);
172   void updateBlockFreqAndEdgeWeight(BasicBlock *PredBB, BasicBlock *BB,
173                                     BasicBlock *NewBB, BasicBlock *SuccBB);
174   /// Check if the block has profile metadata for its outgoing edges.
175   bool doesBlockHaveProfileData(BasicBlock *BB);
176 };
177 
178 } // end namespace llvm
179 
180 #endif // LLVM_TRANSFORMS_SCALAR_JUMPTHREADING_H
181