llvm-project/llvm/lib/Transforms/ObjCARC/DependencyAnalysis.cpp

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//===- DependencyAnalysis.cpp - ObjC ARC Optimization ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
/// \file
///
/// This file defines special dependency analysis routines used in Objective C
/// ARC Optimizations.
///
/// WARNING: This file knows about certain library functions. It recognizes them
/// by name, and hardwires knowledge of their semantics.
///
/// WARNING: This file knows about how certain Objective-C library functions are
/// used. Naive LLVM IR transformations which would otherwise be
/// behavior-preserving may break these assumptions.
///
//===----------------------------------------------------------------------===//
#include "ObjCARC.h"
#include "DependencyAnalysis.h"
#include "ProvenanceAnalysis.h"
#include "llvm/IR/CFG.h"
using namespace llvm;
using namespace llvm::objcarc;
#define DEBUG_TYPE "objc-arc-dependency"
/// Test whether the given instruction can result in a reference count
/// modification (positive or negative) for the pointer's object.
bool
llvm::objcarc::CanAlterRefCount(const Instruction *Inst, const Value *Ptr,
ProvenanceAnalysis &PA,
InstructionClass Class) {
switch (Class) {
case IC_Autorelease:
case IC_AutoreleaseRV:
case IC_IntrinsicUser:
case IC_User:
// These operations never directly modify a reference count.
return false;
default: break;
}
ImmutableCallSite CS = static_cast<const Value *>(Inst);
assert(CS && "Only calls can alter reference counts!");
// See if AliasAnalysis can help us with the call.
AliasAnalysis::ModRefBehavior MRB = PA.getAA()->getModRefBehavior(CS);
if (AliasAnalysis::onlyReadsMemory(MRB))
return false;
if (AliasAnalysis::onlyAccessesArgPointees(MRB)) {
for (ImmutableCallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
I != E; ++I) {
const Value *Op = *I;
if (IsPotentialRetainableObjPtr(Op, *PA.getAA()) && PA.related(Ptr, Op))
return true;
}
return false;
}
// Assume the worst.
return true;
}
/// Test whether the given instruction can "use" the given pointer's object in a
/// way that requires the reference count to be positive.
bool
llvm::objcarc::CanUse(const Instruction *Inst, const Value *Ptr,
ProvenanceAnalysis &PA, InstructionClass Class) {
// IC_Call operations (as opposed to IC_CallOrUser) never "use" objc pointers.
if (Class == IC_Call)
return false;
// Consider various instructions which may have pointer arguments which are
// not "uses".
if (const ICmpInst *ICI = dyn_cast<ICmpInst>(Inst)) {
// Comparing a pointer with null, or any other constant, isn't really a use,
// because we don't care what the pointer points to, or about the values
// of any other dynamic reference-counted pointers.
if (!IsPotentialRetainableObjPtr(ICI->getOperand(1), *PA.getAA()))
return false;
} else if (ImmutableCallSite CS = static_cast<const Value *>(Inst)) {
// For calls, just check the arguments (and not the callee operand).
for (ImmutableCallSite::arg_iterator OI = CS.arg_begin(),
OE = CS.arg_end(); OI != OE; ++OI) {
const Value *Op = *OI;
if (IsPotentialRetainableObjPtr(Op, *PA.getAA()) && PA.related(Ptr, Op))
return true;
}
return false;
} else if (const StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
// Special-case stores, because we don't care about the stored value, just
// the store address.
const Value *Op = GetUnderlyingObjCPtr(SI->getPointerOperand());
// If we can't tell what the underlying object was, assume there is a
// dependence.
return IsPotentialRetainableObjPtr(Op, *PA.getAA()) && PA.related(Op, Ptr);
}
// Check each operand for a match.
for (User::const_op_iterator OI = Inst->op_begin(), OE = Inst->op_end();
OI != OE; ++OI) {
const Value *Op = *OI;
if (IsPotentialRetainableObjPtr(Op, *PA.getAA()) && PA.related(Ptr, Op))
return true;
}
return false;
}
/// Test if there can be dependencies on Inst through Arg. This function only
/// tests dependencies relevant for removing pairs of calls.
bool
llvm::objcarc::Depends(DependenceKind Flavor, Instruction *Inst,
const Value *Arg, ProvenanceAnalysis &PA) {
// If we've reached the definition of Arg, stop.
if (Inst == Arg)
return true;
switch (Flavor) {
case NeedsPositiveRetainCount: {
InstructionClass Class = GetInstructionClass(Inst);
switch (Class) {
case IC_AutoreleasepoolPop:
case IC_AutoreleasepoolPush:
case IC_None:
return false;
default:
return CanUse(Inst, Arg, PA, Class);
}
}
case AutoreleasePoolBoundary: {
InstructionClass Class = GetInstructionClass(Inst);
switch (Class) {
case IC_AutoreleasepoolPop:
case IC_AutoreleasepoolPush:
// These mark the end and begin of an autorelease pool scope.
return true;
default:
// Nothing else does this.
return false;
}
}
case CanChangeRetainCount: {
InstructionClass Class = GetInstructionClass(Inst);
switch (Class) {
case IC_AutoreleasepoolPop:
// Conservatively assume this can decrement any count.
return true;
case IC_AutoreleasepoolPush:
case IC_None:
return false;
default:
return CanAlterRefCount(Inst, Arg, PA, Class);
}
}
case RetainAutoreleaseDep:
switch (GetBasicInstructionClass(Inst)) {
case IC_AutoreleasepoolPop:
case IC_AutoreleasepoolPush:
// Don't merge an objc_autorelease with an objc_retain inside a different
// autoreleasepool scope.
return true;
case IC_Retain:
case IC_RetainRV:
// Check for a retain of the same pointer for merging.
return GetArgRCIdentityRoot(Inst) == Arg;
default:
// Nothing else matters for objc_retainAutorelease formation.
return false;
}
case RetainAutoreleaseRVDep: {
InstructionClass Class = GetBasicInstructionClass(Inst);
switch (Class) {
case IC_Retain:
case IC_RetainRV:
// Check for a retain of the same pointer for merging.
return GetArgRCIdentityRoot(Inst) == Arg;
default:
// Anything that can autorelease interrupts
// retainAutoreleaseReturnValue formation.
return CanInterruptRV(Class);
}
}
case RetainRVDep:
return CanInterruptRV(GetBasicInstructionClass(Inst));
}
llvm_unreachable("Invalid dependence flavor");
}
/// Walk up the CFG from StartPos (which is in StartBB) and find local and
/// non-local dependencies on Arg.
///
/// TODO: Cache results?
void
llvm::objcarc::FindDependencies(DependenceKind Flavor,
const Value *Arg,
BasicBlock *StartBB, Instruction *StartInst,
SmallPtrSetImpl<Instruction *> &DependingInsts,
SmallPtrSetImpl<const BasicBlock *> &Visited,
ProvenanceAnalysis &PA) {
BasicBlock::iterator StartPos = StartInst;
SmallVector<std::pair<BasicBlock *, BasicBlock::iterator>, 4> Worklist;
Worklist.push_back(std::make_pair(StartBB, StartPos));
do {
std::pair<BasicBlock *, BasicBlock::iterator> Pair =
Worklist.pop_back_val();
BasicBlock *LocalStartBB = Pair.first;
BasicBlock::iterator LocalStartPos = Pair.second;
BasicBlock::iterator StartBBBegin = LocalStartBB->begin();
for (;;) {
if (LocalStartPos == StartBBBegin) {
pred_iterator PI(LocalStartBB), PE(LocalStartBB, false);
if (PI == PE)
// If we've reached the function entry, produce a null dependence.
DependingInsts.insert(nullptr);
else
// Add the predecessors to the worklist.
do {
BasicBlock *PredBB = *PI;
if (Visited.insert(PredBB).second)
Worklist.push_back(std::make_pair(PredBB, PredBB->end()));
} while (++PI != PE);
break;
}
Instruction *Inst = --LocalStartPos;
if (Depends(Flavor, Inst, Arg, PA)) {
DependingInsts.insert(Inst);
break;
}
}
} while (!Worklist.empty());
// Determine whether the original StartBB post-dominates all of the blocks we
// visited. If not, insert a sentinal indicating that most optimizations are
// not safe.
for (const BasicBlock *BB : Visited) {
if (BB == StartBB)
continue;
const TerminatorInst *TI = cast<TerminatorInst>(&BB->back());
for (succ_const_iterator SI(TI), SE(TI, false); SI != SE; ++SI) {
const BasicBlock *Succ = *SI;
if (Succ != StartBB && !Visited.count(Succ)) {
DependingInsts.insert(reinterpret_cast<Instruction *>(-1));
return;
}
}
}
}