xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/Scalar/BDCE.cpp (revision b1879975794772ee51f0b4865753364c7d7626c3)
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/IR/PatternMatch.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/raw_ostream.h"
29 #include "llvm/Transforms/Utils/Local.h"
30 
31 using namespace llvm;
32 using namespace PatternMatch;
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   // 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.
54   SmallPtrSet<Instruction *, 16> Visited;
55   SmallVector<Instruction *, 16> WorkList;
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.
78     J->dropPoisonGeneratingAnnotations();
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 
96 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
97   SmallVector<Instruction*, 128> Worklist;
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() &&
110          wouldInstructionBeTriviallyDead(&I))) {
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)) {
123         clearAssumptionsOfUsers(SE, DB);
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) {
155             clearAssumptionsOfUsers(BO, DB);
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 
179       clearAssumptionsOfUsers(&I, DB);
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)) {
190     salvageDebugInfo(*I);
191     I->dropAllReferences();
192   }
193 
194   for (Instruction *&I : Worklist) {
195     ++NumRemoved;
196     I->eraseFromParent();
197   }
198 
199   return Changed;
200 }
201 
202 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
203   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
204   if (!bitTrackingDCE(F, DB))
205     return PreservedAnalyses::all();
206 
207   PreservedAnalyses PA;
208   PA.preserveSet<CFGAnalyses>();
209   return PA;
210 }
211