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ObjCARCAPElim.cpp
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1 //===- ObjCARCAPElim.cpp - ObjC ARC Optimization --------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 /// \file
9 ///
10 /// This file defines ObjC ARC optimizations. ARC stands for Automatic
11 /// Reference Counting and is a system for managing reference counts for objects
12 /// in Objective C.
13 ///
14 /// This specific file implements optimizations which remove extraneous
15 /// autorelease pools.
16 ///
17 /// WARNING: This file knows about certain library functions. It recognizes them
18 /// by name, and hardwires knowledge of their semantics.
19 ///
20 /// WARNING: This file knows about how certain Objective-C library functions are
21 /// used. Naive LLVM IR transformations which would otherwise be
22 /// behavior-preserving may break these assumptions.
23 ///
24 //===----------------------------------------------------------------------===//
25 
26 #include "llvm/ADT/STLExtras.h"
29 #include "llvm/IR/Constants.h"
30 #include "llvm/IR/InstrTypes.h"
31 #include "llvm/IR/PassManager.h"
32 #include "llvm/Support/Debug.h"
35 
36 using namespace llvm;
37 using namespace llvm::objcarc;
38 
39 #define DEBUG_TYPE "objc-arc-ap-elim"
40 
41 namespace {
42 
43 /// Interprocedurally determine if calls made by the given call site can
44 /// possibly produce autoreleases.
45 bool MayAutorelease(const CallBase &CB, unsigned Depth = 0) {
46  if (const Function *Callee = CB.getCalledFunction()) {
47  if (!Callee->hasExactDefinition())
48  return true;
49  for (const BasicBlock &BB : *Callee) {
50  for (const Instruction &I : BB)
51  if (const CallBase *JCB = dyn_cast<CallBase>(&I))
52  // This recursion depth limit is arbitrary. It's just great
53  // enough to cover known interesting testcases.
54  if (Depth < 3 && !JCB->onlyReadsMemory() &&
55  MayAutorelease(*JCB, Depth + 1))
56  return true;
57  }
58  return false;
59  }
60 
61  return true;
62 }
63 
64 bool OptimizeBB(BasicBlock *BB) {
65  bool Changed = false;
66 
67  Instruction *Push = nullptr;
68  for (Instruction &Inst : llvm::make_early_inc_range(*BB)) {
69  switch (GetBasicARCInstKind(&Inst)) {
71  Push = &Inst;
72  break;
74  // If this pop matches a push and nothing in between can autorelease,
75  // zap the pair.
76  if (Push && cast<CallInst>(&Inst)->getArgOperand(0) == Push) {
77  Changed = true;
78  LLVM_DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
79  "autorelease pair:\n"
80  " Pop: "
81  << Inst << "\n"
82  << " Push: " << *Push
83  << "\n");
84  Inst.eraseFromParent();
85  Push->eraseFromParent();
86  }
87  Push = nullptr;
88  break;
90  if (MayAutorelease(cast<CallBase>(Inst)))
91  Push = nullptr;
92  break;
93  default:
94  break;
95  }
96  }
97 
98  return Changed;
99 }
100 
101 bool runImpl(Module &M) {
102  if (!EnableARCOpts)
103  return false;
104 
105  // If nothing in the Module uses ARC, don't do anything.
106  if (!ModuleHasARC(M))
107  return false;
108  // Find the llvm.global_ctors variable, as the first step in
109  // identifying the global constructors. In theory, unnecessary autorelease
110  // pools could occur anywhere, but in practice it's pretty rare. Global
111  // ctors are a place where autorelease pools get inserted automatically,
112  // so it's pretty common for them to be unnecessary, and it's pretty
113  // profitable to eliminate them.
114  GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
115  if (!GV)
116  return false;
117 
119  "llvm.global_ctors is uncooperative!");
120 
121  bool Changed = false;
122 
123  // Dig the constructor functions out of GV's initializer.
124  ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
125  for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
126  OI != OE; ++OI) {
127  Value *Op = *OI;
128  // llvm.global_ctors is an array of three-field structs where the second
129  // members are constructor functions.
130  Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
131  // If the user used a constructor function with the wrong signature and
132  // it got bitcasted or whatever, look the other way.
133  if (!F)
134  continue;
135  // Only look at function definitions.
136  if (F->isDeclaration())
137  continue;
138  // Only look at functions with one basic block.
139  if (std::next(F->begin()) != F->end())
140  continue;
141  // Ok, a single-block constructor function definition. Try to optimize it.
142  Changed |= OptimizeBB(&F->front());
143  }
144 
145  return Changed;
146 }
147 
148 } // namespace
149 
151  if (!runImpl(M))
152  return PreservedAnalyses::all();
154  PA.preserveSet<CFGAnalyses>();
155  return PA;
156 }
llvm::objcarc::GetBasicARCInstKind
ARCInstKind GetBasicARCInstKind(const Value *V)
Determine which objc runtime call instruction class V belongs to.
Definition: ObjCARCInstKind.h:104
llvm::PreservedAnalyses
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:152
llvm
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
M
We currently emits eax Perhaps this is what we really should generate is Is imull three or four cycles eax eax The current instruction priority is based on pattern complexity The former is more complex because it folds a load so the latter will not be emitted Perhaps we should use AddedComplexity to give LEA32r a higher priority We should always try to match LEA first since the LEA matching code does some estimate to determine whether the match is profitable if we care more about code then imull is better It s two bytes shorter than movl leal On a Pentium M
Definition: README.txt:252
llvm::Function
Definition: Function.h:60
llvm::ObjCARCAPElimPass::run
PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM)
Definition: ObjCARCAPElim.cpp:150
llvm::GlobalVariable
Definition: GlobalVariable.h:39
llvm::Depth
@ Depth
Definition: SIMachineScheduler.h:36
STLExtras.h
LLVM_DEBUG
#define LLVM_DEBUG(X)
Definition: Debug.h:101
F
#define F(x, y, z)
Definition: MD5.cpp:55
llvm::BasicBlock
LLVM Basic Block Representation.
Definition: BasicBlock.h:55
llvm::dbgs
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
Constants.h
llvm::objcarc::ARCInstKind::CallOrUser
@ CallOrUser
could call objc_release and/or "use" pointers
InstrTypes.h
llvm::CallBase::getCalledFunction
Function * getCalledFunction() const
Returns the function called, or null if this is an indirect function invocation or the function signa...
Definition: InstrTypes.h:1396
llvm::ConstantArray
ConstantArray - Constant Array Declarations.
Definition: Constants.h:410
llvm::Instruction
Definition: Instruction.h:42
llvm::objcarc::ARCInstKind::AutoreleasepoolPop
@ AutoreleasepoolPop
objc_autoreleasePoolPop
llvm::GlobalVariable::getInitializer
const Constant * getInitializer() const
getInitializer - Return the initializer for this global variable.
Definition: GlobalVariable.h:135
llvm::Instruction::eraseFromParent
SymbolTableList< Instruction >::iterator eraseFromParent()
This method unlinks 'this' from the containing basic block and deletes it.
Definition: Instruction.cpp:81
llvm::GlobalVariable::hasDefinitiveInitializer
bool hasDefinitiveInitializer() const
hasDefinitiveInitializer - Whether the global variable has an initializer, and any other instances of...
Definition: GlobalVariable.h:109
I
#define I(x, y, z)
Definition: MD5.cpp:58
llvm::make_early_inc_range
iterator_range< early_inc_iterator_impl< detail::IterOfRange< RangeT > > > make_early_inc_range(RangeT &&Range)
Make a range that does early increment to allow mutation of the underlying range without disrupting i...
Definition: STLExtras.h:716
ObjCARCInstKind.h
assert
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
llvm::objcarc::EnableARCOpts
bool EnableARCOpts
A handy option to enable/disable all ARC Optimizations.
Definition: ObjCARCAnalysisUtils.cpp:23
llvm::objcarc
Definition: ObjCARCAliasAnalysis.h:29
llvm::Module
A Module instance is used to store all the information related to an LLVM module.
Definition: Module.h:65
llvm::CFGAnalyses
Represents analyses that only rely on functions' control flow.
Definition: PassManager.h:113
llvm::Init
Definition: Record.h:282
ObjCARCAnalysisUtils.h
Callee
amdgpu Simplify well known AMD library false FunctionCallee Callee
Definition: AMDGPULibCalls.cpp:187
llvm::objcarc::ARCInstKind::AutoreleasepoolPush
@ AutoreleasepoolPush
objc_autoreleasePoolPush
llvm::AMDGPU::SendMsg::Op
Op
Definition: SIDefines.h:351
llvm::PreservedAnalyses::all
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:158
PassManager.h
llvm::objcarc::ModuleHasARC
bool ModuleHasARC(const Module &M)
Test if the given module looks interesting to run ARC optimization on.
Definition: ObjCARCAnalysisUtils.h:44
runImpl
static bool runImpl(Function &F, const TargetLowering &TLI)
Definition: ExpandLargeDivRem.cpp:56
llvm::PreservedAnalyses::preserveSet
void preserveSet()
Mark an analysis set as preserved.
Definition: PassManager.h:188
ObjCARC.h
llvm::CallBase
Base class for all callable instructions (InvokeInst and CallInst) Holds everything related to callin...
Definition: InstrTypes.h:1174
llvm::AnalysisManager
A container for analyses that lazily runs them and caches their results.
Definition: InstructionSimplify.h:42
BB
Common register allocation spilling lr str ldr sxth r3 ldr mla r4 can lr mov lr str ldr sxth r3 mla r4 and then merge mul and lr str ldr sxth r3 mla r4 It also increase the likelihood the store may become dead bb27 Successors according to LLVM BB
Definition: README.txt:39
raw_ostream.h
llvm::Value
LLVM Value Representation.
Definition: Value.h:74
Debug.h
llvm::Use
A Use represents the edge between a Value definition and its users.
Definition: Use.h:43