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 (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 
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
86   BDCELegacyPass() : FunctionPass(ID) {
87     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
88   }
89 
90   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 
97   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)
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