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/CFG.h"
24 #include "llvm/IR/InstIterator.h"
25 #include "llvm/IR/Instructions.h"
26 #include "llvm/IR/IntrinsicInst.h"
27 #include "llvm/IR/Operator.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Support/Debug.h"
30 #include "llvm/Support/raw_ostream.h"
31 #include "llvm/Transforms/Scalar.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 
39 /// If an instruction is trivialized (dead), then the chain of users of that
40 /// instruction may need to be cleared of assumptions that can no longer be
41 /// guaranteed correct.
42 static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
43   assert(I->getType()->isIntegerTy() && "Trivializing a non-integer value?");
44 
45   // Initialize the worklist with eligible direct users.
46   SmallVector<Instruction *, 16> WorkList;
47   for (User *JU : I->users()) {
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     auto *J = dyn_cast<Instruction>(JU);
51     if (J && J->getType()->isSized() &&
52         !DB.getDemandedBits(J).isAllOnesValue())
53       WorkList.push_back(J);
54 
55     // Note that we need to check for unsized types above before asking for
56     // demanded bits. Normally, the only way to reach an instruction with an
57     // unsized type is via an instruction that has side effects (or otherwise
58     // will demand its input bits). However, if we have a readnone function
59     // that returns an unsized type (e.g., void), we must avoid asking for the
60     // demanded bits of the function call's return value. A void-returning
61     // readnone function is always dead (and so we can stop walking the use/def
62     // chain here), but the check is necessary to avoid asserting.
63   }
64 
65   // DFS through subsequent users while tracking visits to avoid cycles.
66   SmallPtrSet<Instruction *, 16> Visited;
67   while (!WorkList.empty()) {
68     Instruction *J = WorkList.pop_back_val();
69 
70     // NSW, NUW, and exact are based on operands that might have changed.
71     J->dropPoisonGeneratingFlags();
72 
73     // We do not have to worry about llvm.assume or range metadata:
74     // 1. llvm.assume demands its operand, so trivializing can't change it.
75     // 2. range metadata only applies to memory accesses which demand all bits.
76 
77     Visited.insert(J);
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.count(K) && K->getType()->isSized() &&
84           !DB.getDemandedBits(K).isAllOnesValue())
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     if (I.getType()->isIntegerTy() &&
101         !DB.getDemandedBits(&I).getBoolValue()) {
102       // For live instructions that have all dead bits, first make them dead by
103       // replacing all uses with something else. Then, if they don't need to
104       // remain live (because they have side effects, etc.) we can remove them.
105       DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
106 
107       clearAssumptionsOfUsers(&I, DB);
108 
109       // FIXME: In theory we could substitute undef here instead of zero.
110       // This should be reconsidered once we settle on the semantics of
111       // undef, poison, etc.
112       Value *Zero = ConstantInt::get(I.getType(), 0);
113       ++NumSimplified;
114       I.replaceNonMetadataUsesWith(Zero);
115       Changed = true;
116     }
117     if (!DB.isInstructionDead(&I))
118       continue;
119 
120     Worklist.push_back(&I);
121     I.dropAllReferences();
122     Changed = true;
123   }
124 
125   for (Instruction *&I : Worklist) {
126     ++NumRemoved;
127     I->eraseFromParent();
128   }
129 
130   return Changed;
131 }
132 
133 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
134   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
135   if (!bitTrackingDCE(F, DB))
136     return PreservedAnalyses::all();
137 
138   PreservedAnalyses PA;
139   PA.preserveSet<CFGAnalyses>();
140   PA.preserve<GlobalsAA>();
141   return PA;
142 }
143 
144 namespace {
145 struct BDCELegacyPass : public FunctionPass {
146   static char ID; // Pass identification, replacement for typeid
147   BDCELegacyPass() : FunctionPass(ID) {
148     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
149   }
150 
151   bool runOnFunction(Function &F) override {
152     if (skipFunction(F))
153       return false;
154     auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
155     return bitTrackingDCE(F, DB);
156   }
157 
158   void getAnalysisUsage(AnalysisUsage &AU) const override {
159     AU.setPreservesCFG();
160     AU.addRequired<DemandedBitsWrapperPass>();
161     AU.addPreserved<GlobalsAAWrapperPass>();
162   }
163 };
164 }
165 
166 char BDCELegacyPass::ID = 0;
167 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
168                       "Bit-Tracking Dead Code Elimination", false, false)
169 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
170 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
171                     "Bit-Tracking Dead Code Elimination", false, false)
172 
173 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
174