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/Transforms/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()->isIntOrIntVectorTy() &&
42          "Trivializing a non-integer value?");
43 
44   // Initialize the worklist with eligible direct users.
45   SmallVector<Instruction *, 16> WorkList;
46   for (User *JU : I->users()) {
47     // If all bits of a user are demanded, then we know that nothing below that
48     // in the def-use chain needs to be changed.
49     auto *J = dyn_cast<Instruction>(JU);
50     if (J && J->getType()->isIntOrIntVectorTy() &&
51         !DB.getDemandedBits(J).isAllOnesValue())
52       WorkList.push_back(J);
53 
54     // Note that we need to check for non-int types above before asking for
55     // demanded bits. Normally, the only way to reach an instruction with an
56     // non-int type is via an instruction that has side effects (or otherwise
57     // will demand its input bits). However, if we have a readnone function
58     // that returns an unsized type (e.g., void), we must avoid asking for the
59     // demanded bits of the function call's return value. A void-returning
60     // readnone function is always dead (and so we can stop walking the use/def
61     // chain here), but the check is necessary to avoid asserting.
62   }
63 
64   // DFS through subsequent users while tracking visits to avoid cycles.
65   SmallPtrSet<Instruction *, 16> Visited;
66   while (!WorkList.empty()) {
67     Instruction *J = WorkList.pop_back_val();
68 
69     // NSW, NUW, and exact are based on operands that might have changed.
70     J->dropPoisonGeneratingFlags();
71 
72     // We do not have to worry about llvm.assume or range metadata:
73     // 1. llvm.assume demands its operand, so trivializing can't change it.
74     // 2. range metadata only applies to memory accesses which demand all bits.
75 
76     Visited.insert(J);
77 
78     for (User *KU : J->users()) {
79       // If all bits of a user are demanded, then we know that nothing below
80       // that in the def-use chain needs to be changed.
81       auto *K = dyn_cast<Instruction>(KU);
82       if (K && !Visited.count(K) && K->getType()->isIntOrIntVectorTy() &&
83           !DB.getDemandedBits(K).isAllOnesValue())
84         WorkList.push_back(K);
85     }
86   }
87 }
88 
89 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
90   SmallVector<Instruction*, 128> Worklist;
91   bool Changed = false;
92   for (Instruction &I : instructions(F)) {
93     // If the instruction has side effects and no non-dbg uses,
94     // skip it. This way we avoid computing known bits on an instruction
95     // that will not help us.
96     if (I.mayHaveSideEffects() && I.use_empty())
97       continue;
98 
99     // Remove instructions that are dead, either because they were not reached
100     // during analysis or have no demanded bits.
101     if (DB.isInstructionDead(&I) ||
102         (I.getType()->isIntOrIntVectorTy() &&
103          DB.getDemandedBits(&I).isNullValue() &&
104          wouldInstructionBeTriviallyDead(&I))) {
105       salvageDebugInfo(I);
106       Worklist.push_back(&I);
107       I.dropAllReferences();
108       Changed = true;
109       continue;
110     }
111 
112     for (Use &U : I.operands()) {
113       // DemandedBits only detects dead integer uses.
114       if (!U->getType()->isIntOrIntVectorTy())
115         continue;
116 
117       if (!isa<Instruction>(U) && !isa<Argument>(U))
118         continue;
119 
120       if (!DB.isUseDead(&U))
121         continue;
122 
123       LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
124 
125       clearAssumptionsOfUsers(&I, DB);
126 
127       // FIXME: In theory we could substitute undef here instead of zero.
128       // This should be reconsidered once we settle on the semantics of
129       // undef, poison, etc.
130       U.set(ConstantInt::get(U->getType(), 0));
131       ++NumSimplified;
132       Changed = true;
133     }
134   }
135 
136   for (Instruction *&I : Worklist) {
137     ++NumRemoved;
138     I->eraseFromParent();
139   }
140 
141   return Changed;
142 }
143 
144 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
145   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
146   if (!bitTrackingDCE(F, DB))
147     return PreservedAnalyses::all();
148 
149   PreservedAnalyses PA;
150   PA.preserveSet<CFGAnalyses>();
151   PA.preserve<GlobalsAA>();
152   return PA;
153 }
154 
155 namespace {
156 struct BDCELegacyPass : public FunctionPass {
157   static char ID; // Pass identification, replacement for typeid
158   BDCELegacyPass() : FunctionPass(ID) {
159     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
160   }
161 
162   bool runOnFunction(Function &F) override {
163     if (skipFunction(F))
164       return false;
165     auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
166     return bitTrackingDCE(F, DB);
167   }
168 
169   void getAnalysisUsage(AnalysisUsage &AU) const override {
170     AU.setPreservesCFG();
171     AU.addRequired<DemandedBitsWrapperPass>();
172     AU.addPreserved<GlobalsAAWrapperPass>();
173   }
174 };
175 }
176 
177 char BDCELegacyPass::ID = 0;
178 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
179                       "Bit-Tracking Dead Code Elimination", false, false)
180 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
181 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
182                     "Bit-Tracking Dead Code Elimination", false, false)
183 
184 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
185