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 
17 #include "llvm/Transforms/Scalar/BDCE.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Analysis/DemandedBits.h"
22 #include "llvm/Analysis/GlobalsModRef.h"
23 #include "llvm/IR/IRBuilder.h"
24 #include "llvm/IR/InstIterator.h"
25 #include "llvm/IR/Instructions.h"
26 #include "llvm/InitializePasses.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Transforms/Utils/Local.h"
32 using namespace llvm;
33 
34 #define DEBUG_TYPE "bdce"
35 
36 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
37 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
38 STATISTIC(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.
44 static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
45   assert(I->getType()->isIntOrIntVectorTy() &&
46          "Trivializing a non-integer value?");
47 
48   // Initialize the worklist with eligible direct users.
49   SmallPtrSet<Instruction *, 16> Visited;
50   SmallVector<Instruction *, 16> WorkList;
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.
76     J->dropPoisonGeneratingFlags();
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 
93 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
94   SmallVector<Instruction*, 128> Worklist;
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() &&
107          wouldInstructionBeTriviallyDead(&I))) {
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.countLeadingZeros() >= (DestBitSize - SrcBitSize)) {
120         clearAssumptionsOfUsers(SE, DB);
121         IRBuilder<> Builder(SE);
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 
144       clearAssumptionsOfUsers(&I, DB);
145 
146       // FIXME: In theory we could substitute undef here instead of zero.
147       // This should be reconsidered once we settle on the semantics of
148       // undef, poison, etc.
149       U.set(ConstantInt::get(U->getType(), 0));
150       ++NumSimplified;
151       Changed = true;
152     }
153   }
154 
155   for (Instruction *&I : llvm::reverse(Worklist)) {
156     salvageDebugInfo(*I);
157     I->dropAllReferences();
158   }
159 
160   for (Instruction *&I : Worklist) {
161     ++NumRemoved;
162     I->eraseFromParent();
163   }
164 
165   return Changed;
166 }
167 
168 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
169   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
170   if (!bitTrackingDCE(F, DB))
171     return PreservedAnalyses::all();
172 
173   PreservedAnalyses PA;
174   PA.preserveSet<CFGAnalyses>();
175   return PA;
176 }
177 
178 namespace {
179 struct BDCELegacyPass : public FunctionPass {
180   static char ID; // Pass identification, replacement for typeid
181   BDCELegacyPass() : FunctionPass(ID) {
182     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
183   }
184 
185   bool runOnFunction(Function &F) override {
186     if (skipFunction(F))
187       return false;
188     auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
189     return bitTrackingDCE(F, DB);
190   }
191 
192   void getAnalysisUsage(AnalysisUsage &AU) const override {
193     AU.setPreservesCFG();
194     AU.addRequired<DemandedBitsWrapperPass>();
195     AU.addPreserved<GlobalsAAWrapperPass>();
196   }
197 };
198 }
199 
200 char BDCELegacyPass::ID = 0;
201 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
202                       "Bit-Tracking Dead Code Elimination", false, false)
203 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
204 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
205                     "Bit-Tracking Dead Code Elimination", false, false)
206 
207 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
208