1 //===---- BDCE.cpp - Bit-tracking dead code elimination -------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the Bit-Tracking Dead Code Elimination pass. Some
11 // instructions (shifts, some ands, ors, etc.) kill some of their input bits.
12 // We track these dead bits and remove instructions that compute only these
13 // dead bits.
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/Analysis/Utils/Local.h"
24 #include "llvm/IR/InstIterator.h"
25 #include "llvm/IR/Instructions.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/raw_ostream.h"
29 #include "llvm/Transforms/Scalar.h"
30 using namespace llvm;
31 
32 #define DEBUG_TYPE "bdce"
33 
34 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
35 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
36 
37 /// If an instruction is trivialized (dead), then the chain of users of that
38 /// instruction may need to be cleared of assumptions that can no longer be
39 /// guaranteed correct.
40 static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
41   assert(I->getType()->isIntegerTy() && "Trivializing a non-integer value?");
42 
43   // Initialize the worklist with eligible direct users.
44   SmallVector<Instruction *, 16> WorkList;
45   for (User *JU : I->users()) {
46     // If all bits of a user are demanded, then we know that nothing below that
47     // in the def-use chain needs to be changed.
48     auto *J = dyn_cast<Instruction>(JU);
49     if (J && J->getType()->isSized() &&
50         !DB.getDemandedBits(J).isAllOnesValue())
51       WorkList.push_back(J);
52 
53     // Note that we need to check for unsized types above before asking for
54     // demanded bits. Normally, the only way to reach an instruction with an
55     // unsized type is via an instruction that has side effects (or otherwise
56     // will demand its input bits). However, if we have a readnone function
57     // that returns an unsized type (e.g., void), we must avoid asking for the
58     // demanded bits of the function call's return value. A void-returning
59     // readnone function is always dead (and so we can stop walking the use/def
60     // chain here), but the check is necessary to avoid asserting.
61   }
62 
63   // DFS through subsequent users while tracking visits to avoid cycles.
64   SmallPtrSet<Instruction *, 16> Visited;
65   while (!WorkList.empty()) {
66     Instruction *J = WorkList.pop_back_val();
67 
68     // NSW, NUW, and exact are based on operands that might have changed.
69     J->dropPoisonGeneratingFlags();
70 
71     // We do not have to worry about llvm.assume or range metadata:
72     // 1. llvm.assume demands its operand, so trivializing can't change it.
73     // 2. range metadata only applies to memory accesses which demand all bits.
74 
75     Visited.insert(J);
76 
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.count(K) && K->getType()->isSized() &&
82           !DB.getDemandedBits(K).isAllOnesValue())
83         WorkList.push_back(K);
84     }
85   }
86 }
87 
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;
97 
98     if (I.getType()->isIntegerTy() &&
99         !DB.getDemandedBits(&I).getBoolValue()) {
100       // For live instructions that have all dead bits, first make them dead by
101       // replacing all uses with something else. Then, if they don't need to
102       // remain live (because they have side effects, etc.) we can remove them.
103       DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
104 
105       clearAssumptionsOfUsers(&I, DB);
106 
107       // FIXME: In theory we could substitute undef here instead of zero.
108       // This should be reconsidered once we settle on the semantics of
109       // undef, poison, etc.
110       Value *Zero = ConstantInt::get(I.getType(), 0);
111       ++NumSimplified;
112       I.replaceNonMetadataUsesWith(Zero);
113       Changed = true;
114     }
115     if (!DB.isInstructionDead(&I))
116       continue;
117 
118     salvageDebugInfo(I);
119     Worklist.push_back(&I);
120     I.dropAllReferences();
121     Changed = true;
122   }
123 
124   for (Instruction *&I : Worklist) {
125     ++NumRemoved;
126     I->eraseFromParent();
127   }
128 
129   return Changed;
130 }
131 
132 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
133   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
134   if (!bitTrackingDCE(F, DB))
135     return PreservedAnalyses::all();
136 
137   PreservedAnalyses PA;
138   PA.preserveSet<CFGAnalyses>();
139   PA.preserve<GlobalsAA>();
140   return PA;
141 }
142 
143 namespace {
144 struct BDCELegacyPass : public FunctionPass {
145   static char ID; // Pass identification, replacement for typeid
146   BDCELegacyPass() : FunctionPass(ID) {
147     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
148   }
149 
150   bool runOnFunction(Function &F) override {
151     if (skipFunction(F))
152       return false;
153     auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
154     return bitTrackingDCE(F, DB);
155   }
156 
157   void getAnalysisUsage(AnalysisUsage &AU) const override {
158     AU.setPreservesCFG();
159     AU.addRequired<DemandedBitsWrapperPass>();
160     AU.addPreserved<GlobalsAAWrapperPass>();
161   }
162 };
163 }
164 
165 char BDCELegacyPass::ID = 0;
166 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
167                       "Bit-Tracking Dead Code Elimination", false, false)
168 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
169 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
170                     "Bit-Tracking Dead Code Elimination", false, false)
171 
172 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
173