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[llvm-complete.git] / lib / Transforms / Scalar / 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.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Transforms/Scalar/BDCE.h"
17 #include "llvm/ADT/SmallPtrSet.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/DemandedBits.h"
21 #include "llvm/Analysis/GlobalsModRef.h"
22 #include "llvm/Transforms/Utils/Local.h"
23 #include "llvm/IR/InstIterator.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28 #include "llvm/Transforms/Scalar.h"
29 using namespace llvm;
31 #define DEBUG_TYPE "bdce"
33 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
34 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
36 /// If an instruction is trivialized (dead), then the chain of users of that
37 /// instruction may need to be cleared of assumptions that can no longer be
38 /// guaranteed correct.
39 static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
40 assert(I->getType()->isIntOrIntVectorTy() &&
41 "Trivializing a non-integer value?");
43 // Initialize the worklist with eligible direct users.
44 SmallPtrSet<Instruction *, 16> Visited;
45 SmallVector<Instruction *, 16> WorkList;
46 for (User *JU : I->users()) {
47 // If all bits of a user are demanded, then we know that nothing below that
48 // in the def-use chain needs to be changed.
49 auto *J = dyn_cast<Instruction>(JU);
50 if (J && J->getType()->isIntOrIntVectorTy() &&
51 !DB.getDemandedBits(J).isAllOnesValue()) {
52 Visited.insert(J);
53 WorkList.push_back(J);
56 // Note that we need to check for non-int types above before asking for
57 // demanded bits. Normally, the only way to reach an instruction with an
58 // non-int type is via an instruction that has side effects (or otherwise
59 // will demand its input bits). However, if we have a readnone function
60 // that returns an unsized type (e.g., void), we must avoid asking for the
61 // demanded bits of the function call's return value. A void-returning
62 // readnone function is always dead (and so we can stop walking the use/def
63 // chain here), but the check is necessary to avoid asserting.
66 // DFS through subsequent users while tracking visits to avoid cycles.
67 while (!WorkList.empty()) {
68 Instruction *J = WorkList.pop_back_val();
70 // NSW, NUW, and exact are based on operands that might have changed.
71 J->dropPoisonGeneratingFlags();
73 // We do not have to worry about llvm.assume or range metadata:
74 // 1. llvm.assume demands its operand, so trivializing can't change it.
75 // 2. range metadata only applies to memory accesses which demand all bits.
77 for (User *KU : J->users()) {
78 // If all bits of a user are demanded, then we know that nothing below
79 // that in the def-use chain needs to be changed.
80 auto *K = dyn_cast<Instruction>(KU);
81 if (K && Visited.insert(K).second && K->getType()->isIntOrIntVectorTy() &&
82 !DB.getDemandedBits(K).isAllOnesValue())
83 WorkList.push_back(K);
88 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
89 SmallVector<Instruction*, 128> Worklist;
90 bool Changed = false;
91 for (Instruction &I : instructions(F)) {
92 // If the instruction has side effects and no non-dbg uses,
93 // skip it. This way we avoid computing known bits on an instruction
94 // that will not help us.
95 if (I.mayHaveSideEffects() && I.use_empty())
96 continue;
98 // Remove instructions that are dead, either because they were not reached
99 // during analysis or have no demanded bits.
100 if (DB.isInstructionDead(&I) ||
101 (I.getType()->isIntOrIntVectorTy() &&
102 DB.getDemandedBits(&I).isNullValue() &&
103 wouldInstructionBeTriviallyDead(&I))) {
104 salvageDebugInfo(I);
105 Worklist.push_back(&I);
106 I.dropAllReferences();
107 Changed = true;
108 continue;
111 for (Use &U : I.operands()) {
112 // DemandedBits only detects dead integer uses.
113 if (!U->getType()->isIntOrIntVectorTy())
114 continue;
116 if (!isa<Instruction>(U) && !isa<Argument>(U))
117 continue;
119 if (!DB.isUseDead(&U))
120 continue;
122 LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
124 clearAssumptionsOfUsers(&I, DB);
126 // FIXME: In theory we could substitute undef here instead of zero.
127 // This should be reconsidered once we settle on the semantics of
128 // undef, poison, etc.
129 U.set(ConstantInt::get(U->getType(), 0));
130 ++NumSimplified;
131 Changed = true;
135 for (Instruction *&I : Worklist) {
136 ++NumRemoved;
137 I->eraseFromParent();
140 return Changed;
143 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
144 auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
145 if (!bitTrackingDCE(F, DB))
146 return PreservedAnalyses::all();
148 PreservedAnalyses PA;
149 PA.preserveSet<CFGAnalyses>();
150 PA.preserve<GlobalsAA>();
151 return PA;
154 namespace {
155 struct BDCELegacyPass : public FunctionPass {
156 static char ID; // Pass identification, replacement for typeid
157 BDCELegacyPass() : FunctionPass(ID) {
158 initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
161 bool runOnFunction(Function &F) override {
162 if (skipFunction(F))
163 return false;
164 auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
165 return bitTrackingDCE(F, DB);
168 void getAnalysisUsage(AnalysisUsage &AU) const override {
169 AU.setPreservesCFG();
170 AU.addRequired<DemandedBitsWrapperPass>();
171 AU.addPreserved<GlobalsAAWrapperPass>();
176 char BDCELegacyPass::ID = 0;
177 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
178 "Bit-Tracking Dead Code Elimination", false, false)
179 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
180 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
181 "Bit-Tracking Dead Code Elimination", false, false)
183 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }