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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 //===----------------------------------------------------------------------===//
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/ADT/Statistic.h"
24 #include "llvm/IR/InstIterator.h"
25 #include "llvm/IR/Instructions.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/Debug.h"
29 #include "llvm/Transforms/Scalar.h"
30 using namespace llvm;
32 #define DEBUG_TYPE "bdce"
34 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
35 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
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.
41  assert(I->getType()->isIntegerTy() && "Trivializing a non-integer value?");
43  // Initialize the worklist with eligible direct users.
45  for (User *JU : I->users()) {
46  // If all bits of a user are demanded, then we know that nothing below that
47  // in the def-use chain needs to be changed.
48  auto *J = dyn_cast<Instruction>(JU);
49  if (J && J->getType()->isSized() &&
51  WorkList.push_back(J);
53  // Note that we need to check for unsized types above before asking for
54  // demanded bits. Normally, the only way to reach an instruction with an
55  // unsized type is via an instruction that has side effects (or otherwise
56  // will demand its input bits). However, if we have a readnone function
57  // that returns an unsized type (e.g., void), we must avoid asking for the
58  // demanded bits of the function call's return value. A void-returning
59  // readnone function is always dead (and so we can stop walking the use/def
60  // chain here), but the check is necessary to avoid asserting.
61  }
63  // DFS through subsequent users while tracking visits to avoid cycles.
65  while (!WorkList.empty()) {
66  Instruction *J = WorkList.pop_back_val();
68  // NSW, NUW, and exact are based on operands that might have changed.
71  // We do not have to worry about llvm.assume or range metadata:
72  // 1. llvm.assume demands its operand, so trivializing can't change it.
73  // 2. range metadata only applies to memory accesses which demand all bits.
75  Visited.insert(J);
77  for (User *KU : J->users()) {
78  // If all bits of a user are demanded, then we know that nothing below
79  // that in the def-use chain needs to be changed.
80  auto *K = dyn_cast<Instruction>(KU);
81  if (K && !Visited.count(K) && K->getType()->isSized() &&
83  WorkList.push_back(K);
84  }
85  }
86 }
88 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
90  bool Changed = false;
91  for (Instruction &I : instructions(F)) {
92  // If the instruction has side effects and no non-dbg uses,
93  // skip it. This way we avoid computing known bits on an instruction
94  // that will not help us.
95  if (I.mayHaveSideEffects() && I.use_empty())
96  continue;
98  if (I.getType()->isIntegerTy() &&
99  !DB.getDemandedBits(&I).getBoolValue()) {
100  // For live instructions that have all dead bits, first make them dead by
101  // replacing all uses with something else. Then, if they don't need to
102  // remain live (because they have side effects, etc.) we can remove them.
103  LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
107  // FIXME: In theory we could substitute undef here instead of zero.
108  // This should be reconsidered once we settle on the semantics of
109  // undef, poison, etc.
110  Value *Zero = ConstantInt::get(I.getType(), 0);
111  ++NumSimplified;
112  I.replaceNonMetadataUsesWith(Zero);
113  Changed = true;
114  }
115  if (!DB.isInstructionDead(&I))
116  continue;
119  Worklist.push_back(&I);
120  I.dropAllReferences();
121  Changed = true;
122  }
124  for (Instruction *&I : Worklist) {
125  ++NumRemoved;
126  I->eraseFromParent();
127  }
129  return Changed;
130 }
133  auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
134  if (!bitTrackingDCE(F, DB))
135  return PreservedAnalyses::all();
138  PA.preserveSet<CFGAnalyses>();
139  PA.preserve<GlobalsAA>();
140  return PA;
141 }
143 namespace {
144 struct BDCELegacyPass : public FunctionPass {
145  static char ID; // Pass identification, replacement for typeid
146  BDCELegacyPass() : FunctionPass(ID) {
148  }
150  bool runOnFunction(Function &F) override {
151  if (skipFunction(F))
152  return false;
153  auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
154  return bitTrackingDCE(F, DB);
155  }
157  void getAnalysisUsage(AnalysisUsage &AU) const override {
158  AU.setPreservesCFG();
161  }
162 };
163 }
165 char BDCELegacyPass::ID = 0;
167  "Bit-Tracking Dead Code Elimination", false, false)
169 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
170  "Bit-Tracking Dead Code Elimination", false, false)
172 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
Legacy wrapper pass to provide the GlobalsAAResult object.
void initializeBDCELegacyPassPass(PassRegistry &)
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:770
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
This is the interface for a simple mod/ref and alias analysis over globals.
bool isInstructionDead(Instruction *I)
Return true if, during analysis, I could not be reached.
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
Definition: BDCE.cpp:132
bool salvageDebugInfo(Instruction &I)
Assuming the instruction I is going to be deleted, attempt to salvage debug users of I by writing the...
Definition: Local.cpp:1609
STATISTIC(NumFunctions, "Total number of functions")
AnalysisUsage & addRequired()
Definition: PassSupport.h:51
static bool bitTrackingDCE(Function &F, DemandedBits &DB)
Definition: BDCE.cpp:88
bool isIntegerTy() const
True if this is an instance of IntegerType.
Definition: Type.h:197
An analysis that produces DemandedBits for a function.
Definition: DemandedBits.h:93
void dropPoisonGeneratingFlags()
Drops flags that may cause this instruction to evaluate to poison despite having non-poison inputs...
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:245
bool getBoolValue() const
Convert APInt to a boolean value.
Definition: APInt.h:478
static bool runOnFunction(Function &F, bool PostInlining)
bool isAllOnesValue() const
Determine if all bits are set.
Definition: APInt.h:396
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:154
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:371
Represent the analysis usage information of a pass.
Analysis pass providing a never-invalidated alias analysis result.
FunctionPass * createBitTrackingDCEPass()
Definition: BDCE.cpp:172
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:285
size_type count(ConstPtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:382
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:160
INITIALIZE_PASS_END(RegBankSelect, DEBUG_TYPE, "Assign register bank of generic virtual registers", false, false) RegBankSelect
Bit Tracking Dead Code Elimination
Definition: BDCE.cpp:169
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:418
This is a &#39;vector&#39; (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:847
static Constant * get(Type *Ty, uint64_t V, bool isSigned=false)
If Ty is a vector type, return a Constant with a splat of the given value.
Definition: Constants.cpp:621
void setPreservesCFG()
This function should be called by the pass, iff they do not:
Definition: Pass.cpp:286
static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB)
If an instruction is trivialized (dead), then the chain of users of that instruction may need to be c...
Definition: BDCE.cpp:40
APInt getDemandedBits(Instruction *I)
Return the bits demanded from instruction I.
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:133
Definition: BDCE.cpp:169
iterator_range< user_iterator > users()
Definition: Value.h:400
Represents analyses that only rely on functions&#39; control flow.
Definition: PassManager.h:115
void preserveSet()
Mark an analysis set as preserved.
Definition: PassManager.h:190
#define I(x, y, z)
Definition: MD5.cpp:58
INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce", "Bit-Tracking Dead Code Elimination", false, false) INITIALIZE_PASS_END(BDCELegacyPass
LLVM_NODISCARD std::enable_if<!is_simple_type< Y >::value, typename cast_retty< X, const Y >::ret_type >::type dyn_cast(const Y &Val)
Definition: Casting.h:323
void preserve()
Mark an analysis as preserved.
Definition: PassManager.h:175
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
LLVM Value Representation.
Definition: Value.h:73
inst_range instructions(Function *F)
Definition: InstIterator.h:134
A container for analyses that lazily runs them and caches their results.
#define LLVM_DEBUG(X)
Definition: Debug.h:123