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/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/DemandedBits.h"
21 #include "llvm/Analysis/GlobalsModRef.h"
22 #include "llvm/IR/CFG.h"
23 #include "llvm/IR/InstIterator.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/IR/IntrinsicInst.h"
26 #include "llvm/IR/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Transforms/Scalar.h"
31 using namespace llvm;
32
33 #define DEBUG_TYPE "bdce"
34
35 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
36 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
37
bitTrackingDCE(Function & F,DemandedBits & DB)38 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
39 SmallVector<Instruction*, 128> Worklist;
40 bool Changed = false;
41 for (Instruction &I : instructions(F)) {
42 if (I.getType()->isIntegerTy() &&
43 !DB.getDemandedBits(&I).getBoolValue()) {
44 // For live instructions that have all dead bits, first make them dead by
45 // replacing all uses with something else. Then, if they don't need to
46 // remain live (because they have side effects, etc.) we can remove them.
47 DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
48 // FIXME: In theory we could substitute undef here instead of zero.
49 // This should be reconsidered once we settle on the semantics of
50 // undef, poison, etc.
51 Value *Zero = ConstantInt::get(I.getType(), 0);
52 ++NumSimplified;
53 I.replaceAllUsesWith(Zero);
54 Changed = true;
55 }
56 if (!DB.isInstructionDead(&I))
57 continue;
58
59 Worklist.push_back(&I);
60 I.dropAllReferences();
61 Changed = true;
62 }
63
64 for (Instruction *&I : Worklist) {
65 ++NumRemoved;
66 I->eraseFromParent();
67 }
68
69 return Changed;
70 }
71
run(Function & F,FunctionAnalysisManager & AM)72 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
73 auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
74 if (!bitTrackingDCE(F, DB))
75 return PreservedAnalyses::all();
76
77 // FIXME: This should also 'preserve the CFG'.
78 auto PA = PreservedAnalyses();
79 PA.preserve<GlobalsAA>();
80 return PA;
81 }
82
83 namespace {
84 struct BDCELegacyPass : public FunctionPass {
85 static char ID; // Pass identification, replacement for typeid
BDCELegacyPass__anon262648ac0111::BDCELegacyPass86 BDCELegacyPass() : FunctionPass(ID) {
87 initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
88 }
89
runOnFunction__anon262648ac0111::BDCELegacyPass90 bool runOnFunction(Function &F) override {
91 if (skipFunction(F))
92 return false;
93 auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
94 return bitTrackingDCE(F, DB);
95 }
96
getAnalysisUsage__anon262648ac0111::BDCELegacyPass97 void getAnalysisUsage(AnalysisUsage &AU) const override {
98 AU.setPreservesCFG();
99 AU.addRequired<DemandedBitsWrapperPass>();
100 AU.addPreserved<GlobalsAAWrapperPass>();
101 }
102 };
103 }
104
105 char BDCELegacyPass::ID = 0;
106 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
107 "Bit-Tracking Dead Code Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)108 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
109 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
110 "Bit-Tracking Dead Code Elimination", false, false)
111
112 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
113