LLVM 19.0.0git
BDCE.cpp
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1//===---- BDCE.cpp - Bit-tracking dead code elimination -------------------===//
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 the Bit-Tracking Dead Code Elimination pass. Some
10// instructions (shifts, some ands, ors, etc.) kill some of their input bits.
11// We track these dead bits and remove instructions that compute only these
12// dead bits. We also simplify sext that generates unused extension bits,
13// converting it to a zext.
14//
15//===----------------------------------------------------------------------===//
16
20#include "llvm/ADT/Statistic.h"
23#include "llvm/IR/IRBuilder.h"
27#include "llvm/Support/Debug.h"
30
31using namespace llvm;
32using namespace PatternMatch;
33
34#define DEBUG_TYPE "bdce"
35
36STATISTIC(NumRemoved, "Number of instructions removed (unused)");
37STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
38STATISTIC(NumSExt2ZExt,
39 "Number of sign extension instructions converted to zero extension");
40
41/// If an instruction is trivialized (dead), then the chain of users of that
42/// instruction may need to be cleared of assumptions that can no longer be
43/// guaranteed correct.
45 assert(I->getType()->isIntOrIntVectorTy() &&
46 "Trivializing a non-integer value?");
47
48 // If all bits of a user are demanded, then we know that nothing below that
49 // in the def-use chain needs to be changed.
50 if (DB.getDemandedBits(I).isAllOnes())
51 return;
52
53 // Initialize the worklist with eligible direct users.
56 for (User *JU : I->users()) {
57 auto *J = cast<Instruction>(JU);
58 if (J->getType()->isIntOrIntVectorTy()) {
59 Visited.insert(J);
60 WorkList.push_back(J);
61 }
62
63 // Note that we need to check for non-int types above before asking for
64 // demanded bits. Normally, the only way to reach an instruction with an
65 // non-int type is via an instruction that has side effects (or otherwise
66 // will demand its input bits). However, if we have a readnone function
67 // that returns an unsized type (e.g., void), we must avoid asking for the
68 // demanded bits of the function call's return value. A void-returning
69 // readnone function is always dead (and so we can stop walking the use/def
70 // chain here), but the check is necessary to avoid asserting.
71 }
72
73 // DFS through subsequent users while tracking visits to avoid cycles.
74 while (!WorkList.empty()) {
75 Instruction *J = WorkList.pop_back_val();
76
77 // NSW, NUW, and exact are based on operands that might have changed.
79
80 // We do not have to worry about llvm.assume, because it demands its
81 // operand, so trivializing can't change it.
82
83 // If all bits of a user are demanded, then we know that nothing below
84 // that in the def-use chain needs to be changed.
85 if (DB.getDemandedBits(J).isAllOnes())
86 continue;
87
88 for (User *KU : J->users()) {
89 auto *K = cast<Instruction>(KU);
90 if (Visited.insert(K).second && K->getType()->isIntOrIntVectorTy())
91 WorkList.push_back(K);
92 }
93 }
94}
95
98 bool Changed = false;
99 for (Instruction &I : instructions(F)) {
100 // If the instruction has side effects and no non-dbg uses,
101 // skip it. This way we avoid computing known bits on an instruction
102 // that will not help us.
103 if (I.mayHaveSideEffects() && I.use_empty())
104 continue;
105
106 // Remove instructions that are dead, either because they were not reached
107 // during analysis or have no demanded bits.
108 if (DB.isInstructionDead(&I) ||
109 (I.getType()->isIntOrIntVectorTy() && DB.getDemandedBits(&I).isZero() &&
111 Worklist.push_back(&I);
112 Changed = true;
113 continue;
114 }
115
116 // Convert SExt into ZExt if none of the extension bits is required
117 if (SExtInst *SE = dyn_cast<SExtInst>(&I)) {
118 APInt Demanded = DB.getDemandedBits(SE);
119 const uint32_t SrcBitSize = SE->getSrcTy()->getScalarSizeInBits();
120 auto *const DstTy = SE->getDestTy();
121 const uint32_t DestBitSize = DstTy->getScalarSizeInBits();
122 if (Demanded.countl_zero() >= (DestBitSize - SrcBitSize)) {
124 IRBuilder<> Builder(SE);
125 I.replaceAllUsesWith(
126 Builder.CreateZExt(SE->getOperand(0), DstTy, SE->getName()));
127 Worklist.push_back(SE);
128 Changed = true;
129 NumSExt2ZExt++;
130 continue;
131 }
132 }
133
134 // Simplify and, or, xor when their mask does not affect the demanded bits.
135 if (auto *BO = dyn_cast<BinaryOperator>(&I)) {
136 APInt Demanded = DB.getDemandedBits(BO);
137 if (!Demanded.isAllOnes()) {
138 const APInt *Mask;
139 if (match(BO->getOperand(1), m_APInt(Mask))) {
140 bool CanBeSimplified = false;
141 switch (BO->getOpcode()) {
142 case Instruction::Or:
143 case Instruction::Xor:
144 CanBeSimplified = !Demanded.intersects(*Mask);
145 break;
146 case Instruction::And:
147 CanBeSimplified = Demanded.isSubsetOf(*Mask);
148 break;
149 default:
150 // TODO: Handle more cases here.
151 break;
152 }
153
154 if (CanBeSimplified) {
156 BO->replaceAllUsesWith(BO->getOperand(0));
157 Worklist.push_back(BO);
158 ++NumSimplified;
159 Changed = true;
160 continue;
161 }
162 }
163 }
164 }
165
166 for (Use &U : I.operands()) {
167 // DemandedBits only detects dead integer uses.
168 if (!U->getType()->isIntOrIntVectorTy())
169 continue;
170
171 if (!isa<Instruction>(U) && !isa<Argument>(U))
172 continue;
173
174 if (!DB.isUseDead(&U))
175 continue;
176
177 LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
178
180
181 // Substitute all uses with zero. In theory we could use `freeze poison`
182 // instead, but that seems unlikely to be profitable.
183 U.set(ConstantInt::get(U->getType(), 0));
184 ++NumSimplified;
185 Changed = true;
186 }
187 }
188
189 for (Instruction *&I : llvm::reverse(Worklist)) {
191 I->dropAllReferences();
192 }
193
194 for (Instruction *&I : Worklist) {
195 ++NumRemoved;
196 I->eraseFromParent();
197 }
198
199 return Changed;
200}
201
203 auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
204 if (!bitTrackingDCE(F, DB))
205 return PreservedAnalyses::all();
206
209 return PA;
210}
Expand Atomic instructions
static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB)
If an instruction is trivialized (dead), then the chain of users of that instruction may need to be c...
Definition: BDCE.cpp:44
static bool bitTrackingDCE(Function &F, DemandedBits &DB)
Definition: BDCE.cpp:96
#define LLVM_DEBUG(X)
Definition: Debug.h:101
This is the interface for a simple mod/ref and alias analysis over globals.
#define F(x, y, z)
Definition: MD5.cpp:55
#define I(x, y, z)
Definition: MD5.cpp:58
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
This file defines the SmallPtrSet class.
This file defines the SmallVector class.
This file defines the 'Statistic' class, which is designed to be an easy way to expose various metric...
#define STATISTIC(VARNAME, DESC)
Definition: Statistic.h:167
Class for arbitrary precision integers.
Definition: APInt.h:77
bool isAllOnes() const
Determine if all bits are set. This is true for zero-width values.
Definition: APInt.h:350
bool intersects(const APInt &RHS) const
This operation tests if there are any pairs of corresponding bits between this APInt and RHS that are...
Definition: APInt.h:1228
unsigned countl_zero() const
The APInt version of std::countl_zero.
Definition: APInt.h:1556
bool isSubsetOf(const APInt &RHS) const
This operation checks that all bits set in this APInt are also set in RHS.
Definition: APInt.h:1236
A container for analyses that lazily runs them and caches their results.
Definition: PassManager.h:321
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:473
Represents analyses that only rely on functions' control flow.
Definition: Analysis.h:70
An analysis that produces DemandedBits for a function.
Definition: DemandedBits.h:101
Value * CreateZExt(Value *V, Type *DestTy, const Twine &Name="", bool IsNonNeg=false)
Definition: IRBuilder.h:2021
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:2666
void dropPoisonGeneratingAnnotations()
Drops flags, return attributes and metadata that may generate poison.
Definition: Instruction.h:514
A set of analyses that are preserved following a run of a transformation pass.
Definition: Analysis.h:109
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: Analysis.h:115
void preserveSet()
Mark an analysis set as preserved.
Definition: Analysis.h:144
This class represents a sign extension of integer types.
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:342
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements.
Definition: SmallPtrSet.h:427
bool empty() const
Definition: SmallVector.h:94
void push_back(const T &Elt)
Definition: SmallVector.h:426
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1209
A Use represents the edge between a Value definition and its users.
Definition: Use.h:43
iterator_range< user_iterator > users()
Definition: Value.h:421
bool match(Val *V, const Pattern &P)
Definition: PatternMatch.h:49
apint_match m_APInt(const APInt *&Res)
Match a ConstantInt or splatted ConstantVector, binding the specified pointer to the contained APInt.
Definition: PatternMatch.h:299
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
void salvageDebugInfo(const MachineRegisterInfo &MRI, MachineInstr &MI)
Assuming the instruction MI is going to be deleted, attempt to salvage debug users of MI by writing t...
Definition: Utils.cpp:1665
auto reverse(ContainerTy &&C)
Definition: STLExtras.h:419
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
bool wouldInstructionBeTriviallyDead(const Instruction *I, const TargetLibraryInfo *TLI=nullptr)
Return true if the result produced by the instruction would have no side effects if it was not used.
Definition: Local.cpp:419
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
Definition: BDCE.cpp:202