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 
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 the instruction has side effects and no non-dbg uses,
43     // skip it. This way we avoid computing known bits on an instruction
44     // that will not help us.
45     if (I.mayHaveSideEffects() && I.use_empty())
46       continue;
47 
48     if (I.getType()->isIntegerTy() &&
49         !DB.getDemandedBits(&I).getBoolValue()) {
50       // For live instructions that have all dead bits, first make them dead by
51       // replacing all uses with something else. Then, if they don't need to
52       // remain live (because they have side effects, etc.) we can remove them.
53       DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
54       // FIXME: In theory we could substitute undef here instead of zero.
55       // This should be reconsidered once we settle on the semantics of
56       // undef, poison, etc.
57       Value *Zero = ConstantInt::get(I.getType(), 0);
58       ++NumSimplified;
59       I.replaceNonMetadataUsesWith(Zero);
60       Changed = true;
61     }
62     if (!DB.isInstructionDead(&I))
63       continue;
64 
65     Worklist.push_back(&I);
66     I.dropAllReferences();
67     Changed = true;
68   }
69 
70   for (Instruction *&I : Worklist) {
71     ++NumRemoved;
72     I->eraseFromParent();
73   }
74 
75   return Changed;
76 }
77 
78 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
79   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
80   if (!bitTrackingDCE(F, DB))
81     return PreservedAnalyses::all();
82 
83   PreservedAnalyses PA;
84   PA.preserveSet<CFGAnalyses>();
85   PA.preserve<GlobalsAA>();
86   return PA;
87 }
88 
89 namespace {
90 struct BDCELegacyPass : public FunctionPass {
91   static char ID; // Pass identification, replacement for typeid
92   BDCELegacyPass() : FunctionPass(ID) {
93     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
94   }
95 
96   bool runOnFunction(Function &F) override {
97     if (skipFunction(F))
98       return false;
99     auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
100     return bitTrackingDCE(F, DB);
101   }
102 
103   void getAnalysisUsage(AnalysisUsage &AU) const override {
104     AU.setPreservesCFG();
105     AU.addRequired<DemandedBitsWrapperPass>();
106     AU.addPreserved<GlobalsAAWrapperPass>();
107   }
108 };
109 }
110 
111 char BDCELegacyPass::ID = 0;
112 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
113                       "Bit-Tracking Dead Code Elimination", false, false)
114 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
115 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
116                     "Bit-Tracking Dead Code Elimination", false, false)
117 
118 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
119