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/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28 #include "llvm/Transforms/Utils/Local.h"
29 using namespace llvm;
30 
31 #define DEBUG_TYPE "bdce"
32 
33 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
34 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
35 STATISTIC(NumSExt2ZExt,
36           "Number of sign extension instructions converted to zero extension");
37 
38 /// If an instruction is trivialized (dead), then the chain of users of that
39 /// instruction may need to be cleared of assumptions that can no longer be
40 /// guaranteed correct.
41 static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
42   assert(I->getType()->isIntOrIntVectorTy() &&
43          "Trivializing a non-integer value?");
44 
45   // Initialize the worklist with eligible direct users.
46   SmallPtrSet<Instruction *, 16> Visited;
47   SmallVector<Instruction *, 16> WorkList;
48   for (User *JU : I->users()) {
49     // If all bits of a user are demanded, then we know that nothing below that
50     // in the def-use chain needs to be changed.
51     auto *J = dyn_cast<Instruction>(JU);
52     if (J && J->getType()->isIntOrIntVectorTy() &&
53         !DB.getDemandedBits(J).isAllOnes()) {
54       Visited.insert(J);
55       WorkList.push_back(J);
56     }
57 
58     // Note that we need to check for non-int types above before asking for
59     // demanded bits. Normally, the only way to reach an instruction with an
60     // non-int type is via an instruction that has side effects (or otherwise
61     // will demand its input bits). However, if we have a readnone function
62     // that returns an unsized type (e.g., void), we must avoid asking for the
63     // demanded bits of the function call's return value. A void-returning
64     // readnone function is always dead (and so we can stop walking the use/def
65     // chain here), but the check is necessary to avoid asserting.
66   }
67 
68   // DFS through subsequent users while tracking visits to avoid cycles.
69   while (!WorkList.empty()) {
70     Instruction *J = WorkList.pop_back_val();
71 
72     // NSW, NUW, and exact are based on operands that might have changed.
73     J->dropPoisonGeneratingFlags();
74 
75     // We do not have to worry about llvm.assume or range metadata:
76     // 1. llvm.assume demands its operand, so trivializing can't change it.
77     // 2. range metadata only applies to memory accesses which demand all bits.
78 
79     for (User *KU : J->users()) {
80       // If all bits of a user are demanded, then we know that nothing below
81       // that in the def-use chain needs to be changed.
82       auto *K = dyn_cast<Instruction>(KU);
83       if (K && Visited.insert(K).second && K->getType()->isIntOrIntVectorTy() &&
84           !DB.getDemandedBits(K).isAllOnes())
85         WorkList.push_back(K);
86     }
87   }
88 }
89 
90 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
91   SmallVector<Instruction*, 128> Worklist;
92   bool Changed = false;
93   for (Instruction &I : instructions(F)) {
94     // If the instruction has side effects and no non-dbg uses,
95     // skip it. This way we avoid computing known bits on an instruction
96     // that will not help us.
97     if (I.mayHaveSideEffects() && I.use_empty())
98       continue;
99 
100     // Remove instructions that are dead, either because they were not reached
101     // during analysis or have no demanded bits.
102     if (DB.isInstructionDead(&I) ||
103         (I.getType()->isIntOrIntVectorTy() && DB.getDemandedBits(&I).isZero() &&
104          wouldInstructionBeTriviallyDead(&I))) {
105       Worklist.push_back(&I);
106       Changed = true;
107       continue;
108     }
109 
110     // Convert SExt into ZExt if none of the extension bits is required
111     if (SExtInst *SE = dyn_cast<SExtInst>(&I)) {
112       APInt Demanded = DB.getDemandedBits(SE);
113       const uint32_t SrcBitSize = SE->getSrcTy()->getScalarSizeInBits();
114       auto *const DstTy = SE->getDestTy();
115       const uint32_t DestBitSize = DstTy->getScalarSizeInBits();
116       if (Demanded.countl_zero() >= (DestBitSize - SrcBitSize)) {
117         clearAssumptionsOfUsers(SE, DB);
118         IRBuilder<> Builder(SE);
119         I.replaceAllUsesWith(
120             Builder.CreateZExt(SE->getOperand(0), DstTy, SE->getName()));
121         Worklist.push_back(SE);
122         Changed = true;
123         NumSExt2ZExt++;
124         continue;
125       }
126     }
127 
128     for (Use &U : I.operands()) {
129       // DemandedBits only detects dead integer uses.
130       if (!U->getType()->isIntOrIntVectorTy())
131         continue;
132 
133       if (!isa<Instruction>(U) && !isa<Argument>(U))
134         continue;
135 
136       if (!DB.isUseDead(&U))
137         continue;
138 
139       LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
140 
141       clearAssumptionsOfUsers(&I, DB);
142 
143       // Substitute all uses with zero. In theory we could use `freeze poison`
144       // instead, but that seems unlikely to be profitable.
145       U.set(ConstantInt::get(U->getType(), 0));
146       ++NumSimplified;
147       Changed = true;
148     }
149   }
150 
151   for (Instruction *&I : llvm::reverse(Worklist)) {
152     salvageDebugInfo(*I);
153     I->dropAllReferences();
154   }
155 
156   for (Instruction *&I : Worklist) {
157     ++NumRemoved;
158     I->eraseFromParent();
159   }
160 
161   return Changed;
162 }
163 
164 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
165   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
166   if (!bitTrackingDCE(F, DB))
167     return PreservedAnalyses::all();
168 
169   PreservedAnalyses PA;
170   PA.preserveSet<CFGAnalyses>();
171   return PA;
172 }
173