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/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;
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 
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()->isIntegerTy() && "Trivializing a non-integer value?");
41 
42   // Initialize the worklist with eligible direct users.
43   SmallVector<Instruction *, 16> WorkList;
44   for (User *JU : I->users()) {
45     // If all bits of a user are demanded, then we know that nothing below that
46     // in the def-use chain needs to be changed.
47     auto *J = dyn_cast<Instruction>(JU);
48     if (J && J->getType()->isSized() &&
49         !DB.getDemandedBits(J).isAllOnesValue())
50       WorkList.push_back(J);
51 
52     // Note that we need to check for unsized types above before asking for
53     // demanded bits. Normally, the only way to reach an instruction with an
54     // unsized type is via an instruction that has side effects (or otherwise
55     // will demand its input bits). However, if we have a readnone function
56     // that returns an unsized type (e.g., void), we must avoid asking for the
57     // demanded bits of the function call's return value. A void-returning
58     // readnone function is always dead (and so we can stop walking the use/def
59     // chain here), but the check is necessary to avoid asserting.
60   }
61 
62   // DFS through subsequent users while tracking visits to avoid cycles.
63   SmallPtrSet<Instruction *, 16> Visited;
64   while (!WorkList.empty()) {
65     Instruction *J = WorkList.pop_back_val();
66 
67     // NSW, NUW, and exact are based on operands that might have changed.
68     J->dropPoisonGeneratingFlags();
69 
70     // We do not have to worry about llvm.assume or range metadata:
71     // 1. llvm.assume demands its operand, so trivializing can't change it.
72     // 2. range metadata only applies to memory accesses which demand all bits.
73 
74     Visited.insert(J);
75 
76     for (User *KU : J->users()) {
77       // If all bits of a user are demanded, then we know that nothing below
78       // that in the def-use chain needs to be changed.
79       auto *K = dyn_cast<Instruction>(KU);
80       if (K && !Visited.count(K) && K->getType()->isSized() &&
81           !DB.getDemandedBits(K).isAllOnesValue())
82         WorkList.push_back(K);
83     }
84   }
85 }
86 
87 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
88   SmallVector<Instruction*, 128> Worklist;
89   bool Changed = false;
90   for (Instruction &I : instructions(F)) {
91     // If the instruction has side effects and no non-dbg uses,
92     // skip it. This way we avoid computing known bits on an instruction
93     // that will not help us.
94     if (I.mayHaveSideEffects() && I.use_empty())
95       continue;
96 
97     if (I.getType()->isIntegerTy() &&
98         !DB.getDemandedBits(&I).getBoolValue()) {
99       // For live instructions that have all dead bits, first make them dead by
100       // replacing all uses with something else. Then, if they don't need to
101       // remain live (because they have side effects, etc.) we can remove them.
102       DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
103 
104       clearAssumptionsOfUsers(&I, DB);
105 
106       // FIXME: In theory we could substitute undef here instead of zero.
107       // This should be reconsidered once we settle on the semantics of
108       // undef, poison, etc.
109       Value *Zero = ConstantInt::get(I.getType(), 0);
110       ++NumSimplified;
111       I.replaceNonMetadataUsesWith(Zero);
112       Changed = true;
113     }
114     if (!DB.isInstructionDead(&I))
115       continue;
116 
117     Worklist.push_back(&I);
118     I.dropAllReferences();
119     Changed = true;
120   }
121 
122   for (Instruction *&I : Worklist) {
123     ++NumRemoved;
124     I->eraseFromParent();
125   }
126 
127   return Changed;
128 }
129 
130 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
131   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
132   if (!bitTrackingDCE(F, DB))
133     return PreservedAnalyses::all();
134 
135   PreservedAnalyses PA;
136   PA.preserveSet<CFGAnalyses>();
137   PA.preserve<GlobalsAA>();
138   return PA;
139 }
140 
141 namespace {
142 struct BDCELegacyPass : public FunctionPass {
143   static char ID; // Pass identification, replacement for typeid
144   BDCELegacyPass() : FunctionPass(ID) {
145     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
146   }
147 
148   bool runOnFunction(Function &F) override {
149     if (skipFunction(F))
150       return false;
151     auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
152     return bitTrackingDCE(F, DB);
153   }
154 
155   void getAnalysisUsage(AnalysisUsage &AU) const override {
156     AU.setPreservesCFG();
157     AU.addRequired<DemandedBitsWrapperPass>();
158     AU.addPreserved<GlobalsAAWrapperPass>();
159   }
160 };
161 }
162 
163 char BDCELegacyPass::ID = 0;
164 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
165                       "Bit-Tracking Dead Code Elimination", false, false)
166 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
167 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
168                     "Bit-Tracking Dead Code Elimination", false, false)
169 
170 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
171