1 //==- ConstantHoisting.h - Prepare code for expensive constants --*- C++ -*-==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass identifies expensive constants to hoist and coalesces them to
11 // better prepare it for SelectionDAG-based code generation. This works around
12 // the limitations of the basic-block-at-a-time approach.
14 // First it scans all instructions for integer constants and calculates its
15 // cost. If the constant can be folded into the instruction (the cost is
16 // TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
17 // consider it expensive and leave it alone. This is the default behavior and
18 // the default implementation of getIntImmCost will always return TCC_Free.
20 // If the cost is more than TCC_BASIC, then the integer constant can't be folded
21 // into the instruction and it might be beneficial to hoist the constant.
22 // Similar constants are coalesced to reduce register pressure and
23 // materialization code.
25 // When a constant is hoisted, it is also hidden behind a bitcast to force it to
26 // be live-out of the basic block. Otherwise the constant would be just
27 // duplicated and each basic block would have its own copy in the SelectionDAG.
28 // The SelectionDAG recognizes such constants as opaque and doesn't perform
29 // certain transformations on them, which would create a new expensive constant.
31 // This optimization is only applied to integer constants in instructions and
32 // simple (this means not nested) constant cast expressions. For example:
33 // %0 = load i64* inttoptr (i64 big_constant to i64*)
35 //===----------------------------------------------------------------------===//
37 #ifndef LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H
38 #define LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H
40 #include "llvm/ADT/DenseMap.h"
41 #include "llvm/ADT/SmallPtrSet.h"
42 #include "llvm/ADT/SmallVector.h"
43 #include "llvm/IR/PassManager.h"
50 class BlockFrequencyInfo;
56 class TargetTransformInfo;
58 /// A private "module" namespace for types and utilities used by
59 /// ConstantHoisting. These are implementation details and should not be used by
61 namespace consthoist {
63 /// Keeps track of the user of a constant and the operand index where the
69 ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) {}
72 using ConstantUseListType = SmallVector<ConstantUser, 8>;
74 /// Keeps track of a constant candidate and its uses.
75 struct ConstantCandidate {
76 ConstantUseListType Uses;
77 ConstantInt *ConstInt;
78 unsigned CumulativeCost = 0;
80 ConstantCandidate(ConstantInt *ConstInt) : ConstInt(ConstInt) {}
82 /// Add the user to the use list and update the cost.
83 void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) {
84 CumulativeCost += Cost;
85 Uses.push_back(ConstantUser(Inst, Idx));
89 /// This represents a constant that has been rebased with respect to a
90 /// base constant. The difference to the base constant is recorded in Offset.
91 struct RebasedConstantInfo {
92 ConstantUseListType Uses;
95 RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset)
96 : Uses(std::move(Uses)), Offset(Offset) {}
99 using RebasedConstantListType = SmallVector<RebasedConstantInfo, 4>;
101 /// A base constant and all its rebased constants.
102 struct ConstantInfo {
103 ConstantInt *BaseConstant;
104 RebasedConstantListType RebasedConstants;
107 } // end namespace consthoist
109 class ConstantHoistingPass : public PassInfoMixin<ConstantHoistingPass> {
111 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
114 bool runImpl(Function &F, TargetTransformInfo &TTI, DominatorTree &DT,
115 BlockFrequencyInfo *BFI, BasicBlock &Entry);
117 void releaseMemory() {
119 ClonedCastMap.clear();
120 ConstCandVec.clear();
124 using ConstCandMapType = DenseMap<ConstantInt *, unsigned>;
125 using ConstCandVecType = std::vector<consthoist::ConstantCandidate>;
127 const TargetTransformInfo *TTI;
129 BlockFrequencyInfo *BFI;
132 /// Keeps track of constant candidates found in the function.
133 ConstCandVecType ConstCandVec;
135 /// Keep track of cast instructions we already cloned.
136 SmallDenseMap<Instruction *, Instruction *> ClonedCastMap;
138 /// These are the final constants we decided to hoist.
139 SmallVector<consthoist::ConstantInfo, 8> ConstantVec;
141 Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
142 SmallPtrSet<Instruction *, 8>
143 findConstantInsertionPoint(const consthoist::ConstantInfo &ConstInfo) const;
144 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
145 Instruction *Inst, unsigned Idx,
146 ConstantInt *ConstInt);
147 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
148 Instruction *Inst, unsigned Idx);
149 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
151 void collectConstantCandidates(Function &Fn);
152 void findAndMakeBaseConstant(ConstCandVecType::iterator S,
153 ConstCandVecType::iterator E);
154 unsigned maximizeConstantsInRange(ConstCandVecType::iterator S,
155 ConstCandVecType::iterator E,
156 ConstCandVecType::iterator &MaxCostItr);
157 void findBaseConstants();
158 void emitBaseConstants(Instruction *Base, Constant *Offset,
159 const consthoist::ConstantUser &ConstUser);
160 bool emitBaseConstants();
161 void deleteDeadCastInst() const;
162 bool optimizeConstants(Function &Fn);
165 } // end namespace llvm
167 #endif // LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H