LLVM 17.0.0git
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/Pass.h"
28#include "llvm/Support/Debug.h"
32using namespace llvm;
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 // Initialize the worklist with eligible direct users.
51 for (User *JU : I->users()) {
52 // If all bits of a user are demanded, then we know that nothing below that
53 // in the def-use chain needs to be changed.
54 auto *J = dyn_cast<Instruction>(JU);
55 if (J && J->getType()->isIntOrIntVectorTy() &&
56 !DB.getDemandedBits(J).isAllOnes()) {
57 Visited.insert(J);
58 WorkList.push_back(J);
59 }
60
61 // Note that we need to check for non-int types above before asking for
62 // demanded bits. Normally, the only way to reach an instruction with an
63 // non-int type is via an instruction that has side effects (or otherwise
64 // will demand its input bits). However, if we have a readnone function
65 // that returns an unsized type (e.g., void), we must avoid asking for the
66 // demanded bits of the function call's return value. A void-returning
67 // readnone function is always dead (and so we can stop walking the use/def
68 // chain here), but the check is necessary to avoid asserting.
69 }
70
71 // DFS through subsequent users while tracking visits to avoid cycles.
72 while (!WorkList.empty()) {
73 Instruction *J = WorkList.pop_back_val();
74
75 // NSW, NUW, and exact are based on operands that might have changed.
77
78 // We do not have to worry about llvm.assume or range metadata:
79 // 1. llvm.assume demands its operand, so trivializing can't change it.
80 // 2. range metadata only applies to memory accesses which demand all bits.
81
82 for (User *KU : J->users()) {
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 auto *K = dyn_cast<Instruction>(KU);
86 if (K && Visited.insert(K).second && K->getType()->isIntOrIntVectorTy() &&
87 !DB.getDemandedBits(K).isAllOnes())
88 WorkList.push_back(K);
89 }
90 }
91}
92
95 bool Changed = false;
96 for (Instruction &I : instructions(F)) {
97 // If the instruction has side effects and no non-dbg uses,
98 // skip it. This way we avoid computing known bits on an instruction
99 // that will not help us.
100 if (I.mayHaveSideEffects() && I.use_empty())
101 continue;
102
103 // Remove instructions that are dead, either because they were not reached
104 // during analysis or have no demanded bits.
105 if (DB.isInstructionDead(&I) ||
106 (I.getType()->isIntOrIntVectorTy() && DB.getDemandedBits(&I).isZero() &&
108 Worklist.push_back(&I);
109 Changed = true;
110 continue;
111 }
112
113 // Convert SExt into ZExt if none of the extension bits is required
114 if (SExtInst *SE = dyn_cast<SExtInst>(&I)) {
115 APInt Demanded = DB.getDemandedBits(SE);
116 const uint32_t SrcBitSize = SE->getSrcTy()->getScalarSizeInBits();
117 auto *const DstTy = SE->getDestTy();
118 const uint32_t DestBitSize = DstTy->getScalarSizeInBits();
119 if (Demanded.countl_zero() >= (DestBitSize - SrcBitSize)) {
122 I.replaceAllUsesWith(
123 Builder.CreateZExt(SE->getOperand(0), DstTy, SE->getName()));
124 Worklist.push_back(SE);
125 Changed = true;
126 NumSExt2ZExt++;
127 continue;
128 }
129 }
130
131 for (Use &U : I.operands()) {
132 // DemandedBits only detects dead integer uses.
133 if (!U->getType()->isIntOrIntVectorTy())
134 continue;
135
136 if (!isa<Instruction>(U) && !isa<Argument>(U))
137 continue;
138
139 if (!DB.isUseDead(&U))
140 continue;
141
142 LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
143
145
146 // Substitute all uses with zero. In theory we could use `freeze poison`
147 // instead, but that seems unlikely to be profitable.
148 U.set(ConstantInt::get(U->getType(), 0));
149 ++NumSimplified;
150 Changed = true;
151 }
152 }
153
154 for (Instruction *&I : llvm::reverse(Worklist)) {
156 I->dropAllReferences();
157 }
158
159 for (Instruction *&I : Worklist) {
160 ++NumRemoved;
161 I->eraseFromParent();
162 }
163
164 return Changed;
165}
166
168 auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
169 if (!bitTrackingDCE(F, DB))
170 return PreservedAnalyses::all();
171
174 return PA;
175}
176
177namespace {
178struct BDCELegacyPass : public FunctionPass {
179 static char ID; // Pass identification, replacement for typeid
180 BDCELegacyPass() : FunctionPass(ID) {
182 }
183
184 bool runOnFunction(Function &F) override {
185 if (skipFunction(F))
186 return false;
187 auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
188 return bitTrackingDCE(F, DB);
189 }
190
191 void getAnalysisUsage(AnalysisUsage &AU) const override {
192 AU.setPreservesCFG();
195 }
196};
197}
198
199char BDCELegacyPass::ID = 0;
200INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
201 "Bit-Tracking Dead Code Elimination", false, false)
203INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
204 "Bit-Tracking Dead Code Elimination", false, false)
205
206FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
assume Assume Builder
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
bdce
Definition: BDCE.cpp:203
static bool bitTrackingDCE(Function &F, DemandedBits &DB)
Definition: BDCE.cpp:93
#define LLVM_DEBUG(X)
Definition: Debug.h:101
static bool runOnFunction(Function &F, bool PostInlining)
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
Machine Common Subexpression Elimination
Definition: MachineCSE.cpp:169
print must be executed print the must be executed context for all instructions
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:55
#define INITIALIZE_PASS_END(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:59
#define INITIALIZE_PASS_BEGIN(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:52
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:75
unsigned countl_zero() const
The APInt version of std::countl_zero.
Definition: APInt.h:1551
A container for analyses that lazily runs them and caches their results.
Definition: PassManager.h:620
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:774
Represent the analysis usage information of a pass.
AnalysisUsage & addRequired()
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
void setPreservesCFG()
This function should be called by the pass, iff they do not:
Definition: Pass.cpp:265
Represents analyses that only rely on functions' control flow.
Definition: PassManager.h:113
static Constant * get(Type *Ty, uint64_t V, bool IsSigned=false)
If Ty is a vector type, return a Constant with a splat of the given value.
Definition: Constants.cpp:888
An analysis that produces DemandedBits for a function.
Definition: DemandedBits.h:123
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:308
Legacy wrapper pass to provide the GlobalsAAResult object.
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:2558
void dropPoisonGeneratingFlags()
Drops flags that may cause this instruction to evaluate to poison despite having non-poison inputs.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:152
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:158
void preserveSet()
Mark an analysis set as preserved.
Definition: PassManager.h:188
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:365
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements.
Definition: SmallPtrSet.h:450
bool empty() const
Definition: SmallVector.h:94
void push_back(const T &Elt)
Definition: SmallVector.h:416
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
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
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
FunctionPass * createBitTrackingDCEPass()
Definition: BDCE.cpp:206
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:1367
void initializeBDCELegacyPassPass(PassRegistry &)
bool wouldInstructionBeTriviallyDead(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:417
auto reverse(ContainerTy &&C)
Definition: STLExtras.h:511
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
Definition: BDCE.cpp:167