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//===--- LLVMARCOpts.cpp - LLVM Reference Counting Optimizations ----------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This file implements optimizations for reference counting, object allocation,
// and other runtime entrypoints. Most of this code will be removed once the SIL
// level ARC optimizer causes it to no longer be needed.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "swift-llvm-arc-opts"
#include "swift/LLVMPasses/Passes.h"
#include "ARCEntryPointBuilder.h"
#include "LLVMARCOpts.h"
#include "swift/Basic/NullablePtr.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/ADT/APInt.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Utils/Local.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/ADT/TinyPtrVector.h"
#include "llvm/TargetParser/Triple.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Support/CommandLine.h"
using namespace llvm;
using namespace swift;
using swift::SwiftARCOpt;
STATISTIC(NumNoopDeleted,
"Number of no-op swift calls eliminated");
STATISTIC(NumRetainReleasePairs,
"Number of swift retain/release pairs eliminated");
STATISTIC(NumObjCRetainReleasePairs,
"Number of objc retain/release pairs eliminated");
STATISTIC(NumAllocateReleasePairs,
"Number of swift allocate/release pairs eliminated");
STATISTIC(NumStoreOnlyObjectsEliminated,
"Number of swift stored-only objects eliminated");
STATISTIC(NumUnknownObjectRetainReleaseSRed,
"Number of unknownretain/release strength reduced to retain/release");
llvm::cl::opt<bool>
DisableARCOpts("disable-llvm-arc-opts", llvm::cl::init(false));
//===----------------------------------------------------------------------===//
// Input Function Canonicalizer
//===----------------------------------------------------------------------===//
/// canonicalizeInputFunction - Functions like swift_retain return an
/// argument as a low-level performance optimization. This makes it difficult
/// to reason about pointer equality though, so undo it as an initial
/// canonicalization step. After this step, all swift_retain's have been
/// replaced with swift_retain.
///
/// This also does some trivial peep-hole optimizations as we go.
static bool canonicalizeInputFunction(Function &F, ARCEntryPointBuilder &B,
SwiftRCIdentity &RC) {
bool Changed = false;
DenseSet<Value *> NativeRefs;
DenseMap<Value *, TinyPtrVector<Instruction *>> UnknownObjectRetains;
DenseMap<Value *, TinyPtrVector<Instruction *>> UnknownObjectReleases;
for (auto &BB : F) {
UnknownObjectRetains.clear();
UnknownObjectReleases.clear();
NativeRefs.clear();
for (auto I = BB.begin(); I != BB.end(); ) {
Instruction &Inst = *I++;
switch (classifyInstruction(Inst)) {
// These instructions should not reach here based on the pass ordering.
// i.e. LLVMARCOpt -> LLVMContractOpt.
case RT_RetainN:
case RT_UnknownObjectRetainN:
case RT_BridgeRetainN:
case RT_ReleaseN:
case RT_UnknownObjectReleaseN:
case RT_BridgeReleaseN:
llvm_unreachable("These are only created by LLVMARCContract !");
case RT_Unknown:
case RT_BridgeRelease:
case RT_AllocObject:
case RT_FixLifetime:
case RT_EndBorrow:
case RT_NoMemoryAccessed:
case RT_RetainUnowned:
case RT_CheckUnowned:
break;
case RT_Retain: {
CallInst &CI = cast<CallInst>(Inst);
Value *ArgVal = RC.getSwiftRCIdentityRoot(CI.getArgOperand(0));
// retain(null) is a no-op.
if (isa<ConstantPointerNull>(ArgVal)) {
CI.eraseFromParent();
Changed = true;
++NumNoopDeleted;
continue;
}
if (!CI.use_empty()) {
// Do not get RC identical value here, could end up with a
// crash in replaceAllUsesWith as the type maybe different.
CI.replaceAllUsesWith(CI.getArgOperand(0));
Changed = true;
}
// Rewrite unknown retains into swift_retains.
NativeRefs.insert(ArgVal);
for (auto &X : UnknownObjectRetains[ArgVal]) {
B.setInsertPoint(X);
B.createRetain(ArgVal, cast<CallInst>(X));
X->eraseFromParent();
++NumUnknownObjectRetainReleaseSRed;
Changed = true;
}
UnknownObjectRetains[ArgVal].clear();
break;
}
case RT_UnknownObjectRetain: {
CallInst &CI = cast<CallInst>(Inst);
Value *ArgVal = RC.getSwiftRCIdentityRoot(CI.getArgOperand(0));
// unknownObjectRetain(null) is a no-op.
if (isa<ConstantPointerNull>(ArgVal)) {
CI.eraseFromParent();
Changed = true;
++NumNoopDeleted;
continue;
}
if (!CI.use_empty()) {
// Do not get RC identical value here, could end up with a
// crash in replaceAllUsesWith as the type maybe different.
CI.replaceAllUsesWith(CI.getArgOperand(0));
Changed = true;
}
// Have not encountered a strong retain/release. keep it in the
// unknown retain/release list for now. It might get replaced
// later.
if (!NativeRefs.contains(ArgVal)) {
UnknownObjectRetains[ArgVal].push_back(&CI);
} else {
B.setInsertPoint(&CI);
B.createRetain(ArgVal, &CI);
CI.eraseFromParent();
++NumUnknownObjectRetainReleaseSRed;
Changed = true;
}
break;
}
case RT_Release: {
CallInst &CI = cast<CallInst>(Inst);
Value *ArgVal = RC.getSwiftRCIdentityRoot(CI.getArgOperand(0));
// release(null) is a no-op.
if (isa<ConstantPointerNull>(ArgVal)) {
CI.eraseFromParent();
Changed = true;
++NumNoopDeleted;
continue;
}
// Rewrite unknown releases into swift_releases.
NativeRefs.insert(ArgVal);
for (auto &X : UnknownObjectReleases[ArgVal]) {
B.setInsertPoint(X);
B.createRelease(ArgVal, cast<CallInst>(X));
X->eraseFromParent();
++NumUnknownObjectRetainReleaseSRed;
Changed = true;
}
UnknownObjectReleases[ArgVal].clear();
break;
}
case RT_UnknownObjectRelease: {
CallInst &CI = cast<CallInst>(Inst);
Value *ArgVal = RC.getSwiftRCIdentityRoot(CI.getArgOperand(0));
// unknownObjectRelease(null) is a no-op.
if (isa<ConstantPointerNull>(ArgVal)) {
CI.eraseFromParent();
Changed = true;
++NumNoopDeleted;
continue;
}
// Have not encountered a strong retain/release. keep it in the
// unknown retain/release list for now. It might get replaced
// later.
if (!NativeRefs.contains(ArgVal)) {
UnknownObjectReleases[ArgVal].push_back(&CI);
} else {
B.setInsertPoint(&CI);
B.createRelease(ArgVal, &CI);
CI.eraseFromParent();
++NumUnknownObjectRetainReleaseSRed;
Changed = true;
}
break;
}
case RT_ObjCRelease: {
CallInst &CI = cast<CallInst>(Inst);
Value *ArgVal = RC.getSwiftRCIdentityRoot(CI.getArgOperand(0));
// objc_release(null) is a noop, zap it.
if (isa<ConstantPointerNull>(ArgVal)) {
CI.eraseFromParent();
Changed = true;
++NumNoopDeleted;
continue;
}
break;
}
// These retain instructions return their argument so must be processed
// specially.
case RT_BridgeRetain:
case RT_ObjCRetain: {
// Canonicalize the retain so that nothing uses its result.
CallInst &CI = cast<CallInst>(Inst);
// Do not get RC identical value here, could end up with a
// crash in replaceAllUsesWith as the type maybe different.
Value *ArgVal = CI.getArgOperand(0);
if (!CI.use_empty()) {
CI.replaceAllUsesWith(ArgVal);
Changed = true;
}
// {objc_retain,swift_unknownObjectRetain}(null) is a noop, delete it.
if (isa<ConstantPointerNull>(ArgVal)) {
CI.eraseFromParent();
Changed = true;
++NumNoopDeleted;
continue;
}
break;
}
}
}
}
return Changed;
}
//===----------------------------------------------------------------------===//
// Release() Motion
//===----------------------------------------------------------------------===//
/// performLocalReleaseMotion - Scan backwards from the specified release,
/// moving it earlier in the function if possible, over instructions that do not
/// access the released object. If we get to a retain or allocation of the
/// object, zap both.
static bool performLocalReleaseMotion(CallInst &Release, BasicBlock &BB,
SwiftRCIdentity &RC) {
// FIXME: Call classifier should identify the object for us. Too bad C++
// doesn't have nice Swift-style enums.
Value *ReleasedObject = RC.getSwiftRCIdentityRoot(Release.getArgOperand(0));
BasicBlock::iterator BBI = Release.getIterator();
// Scan until we get to the top of the block.
while (BBI != BB.begin()) {
--BBI;
// Don't analyze PHI nodes. We can't move retains before them and they
// aren't "interesting".
if (isa<PHINode>(BBI) ||
// If we found the instruction that defines the value we're releasing,
// don't push the release past it.
&*BBI == Release.getArgOperand(0)) {
++BBI;
goto OutOfLoop;
}
switch (classifyInstruction(*BBI)) {
// These instructions should not reach here based on the pass ordering.
// i.e. LLVMARCOpt -> LLVMContractOpt.
case RT_UnknownObjectRetainN:
case RT_BridgeRetainN:
case RT_RetainN:
case RT_UnknownObjectReleaseN:
case RT_BridgeReleaseN:
case RT_ReleaseN:
llvm_unreachable("These are only created by LLVMARCContract !");
case RT_NoMemoryAccessed:
// Skip over random instructions that don't touch memory. They don't need
// protection by retain/release.
continue;
case RT_UnknownObjectRelease:
case RT_BridgeRelease:
case RT_ObjCRelease:
case RT_Release: {
// If we get to a release, we can generally ignore it and scan past it.
// However, if we get to a release of obviously the same object, we stop
// scanning here because it should have already be moved as early as
// possible, so there is no reason to move its friend to the same place.
//
// NOTE: If this occurs frequently, maybe we can have a release(Obj, N)
// API to drop multiple retain counts at once.
CallInst &ThisRelease = cast<CallInst>(*BBI);
Value *ThisReleasedObject = ThisRelease.getArgOperand(0);
ThisReleasedObject = RC.getSwiftRCIdentityRoot(ThisReleasedObject);
if (ThisReleasedObject == ReleasedObject) {
//Release.dump(); ThisRelease.dump(); BB.getParent()->dump();
++BBI;
goto OutOfLoop;
}
continue;
}
case RT_UnknownObjectRetain:
case RT_BridgeRetain:
case RT_ObjCRetain:
case RT_Retain: { // swift_retain(obj)
CallInst &Retain = cast<CallInst>(*BBI);
Value *RetainedObject = Retain.getArgOperand(0);
RetainedObject = RC.getSwiftRCIdentityRoot(RetainedObject);
// Since we canonicalized earlier, we know that if our retain has any
// uses, they were replaced already. This assertion documents this
// assumption.
assert(Retain.use_empty() && "Retain should have been canonicalized to "
"have no uses.");
// If the retain and release are to obviously pointer-equal objects, then
// we can delete both of them. We have proven that they do not protect
// anything of value.
if (RetainedObject == ReleasedObject) {
Retain.eraseFromParent();
Release.eraseFromParent();
++NumRetainReleasePairs;
return true;
}
// Otherwise, this is a retain of an object that is not statically known
// to be the same object. It may still be dynamically the same object
// though. In this case, we can't move the release past it.
// TODO: Strengthen analysis.
//Release.dump(); ThisRelease.dump(); BB.getParent()->dump();
++BBI;
goto OutOfLoop;
}
case RT_AllocObject: { // %obj = swift_alloc(...)
CallInst &Allocation = cast<CallInst>(*BBI);
// If this is an allocation of an unrelated object, just ignore it.
// TODO: This is not safe without proving the object being released is not
// related to the allocated object. Consider something silly like this:
// A = allocate()
// B = bitcast A to object
// release(B)
if (ReleasedObject != &Allocation) {
// Release.dump(); BB.getParent()->dump();
++BBI;
goto OutOfLoop;
}
// If this is a release right after an allocation of the object, then we
// can zap both.
Allocation.replaceAllUsesWith(UndefValue::get(Allocation.getType()));
Allocation.eraseFromParent();
Release.eraseFromParent();
++NumAllocateReleasePairs;
return true;
}
case RT_FixLifetime:
case RT_EndBorrow:
case RT_RetainUnowned:
case RT_CheckUnowned:
case RT_Unknown:
// Otherwise, we have reached something that we do not understand. Do not
// attempt to shorten the lifetime of this object beyond this point so we
// are conservative.
++BBI;
goto OutOfLoop;
}
}
OutOfLoop:
// If we got to the top of the block, (and if the instruction didn't start
// there) move the release to the top of the block.
// TODO: This is where we'd plug in some global algorithms someday.
if (&*BBI != &Release) {
Release.moveBefore(&*BBI);
return true;
}
return false;
}
//===----------------------------------------------------------------------===//
// Retain() Motion
//===----------------------------------------------------------------------===//
/// performLocalRetainMotion - Scan forward from the specified retain, moving it
/// later in the function if possible, over instructions that provably can't
/// release the object. If we get to a release of the object, zap both.
///
/// NOTE: this handles both objc_retain and swift_retain.
///
static bool performLocalRetainMotion(CallInst &Retain, BasicBlock &BB,
SwiftRCIdentity &RC) {
// FIXME: Call classifier should identify the object for us. Too bad C++
// doesn't have nice Swift-style enums.
Value *RetainedObject = RC.getSwiftRCIdentityRoot(Retain.getArgOperand(0));
BasicBlock::iterator BBI = Retain.getIterator(),
BBE = BB.getTerminator()->getIterator();
bool isObjCRetain = Retain.getIntrinsicID() == llvm::Intrinsic::objc_retain;
bool MadeProgress = false;
// Scan until we get to the end of the block.
for (++BBI; BBI != BBE; ++BBI) {
Instruction &CurInst = *BBI;
// Classify the instruction. This switch does a "break" when the instruction
// can be skipped and is interesting, and a "continue" when it is a retain
// of the same pointer.
switch (classifyInstruction(CurInst)) {
// These instructions should not reach here based on the pass ordering.
// i.e. LLVMARCOpt -> LLVMContractOpt.
case RT_RetainN:
case RT_UnknownObjectRetainN:
case RT_BridgeRetainN:
case RT_ReleaseN:
case RT_UnknownObjectReleaseN:
case RT_BridgeReleaseN:
llvm_unreachable("These are only created by LLVMARCContract !");
case RT_NoMemoryAccessed:
case RT_AllocObject:
case RT_CheckUnowned:
// Skip over random instructions that don't touch memory. They don't need
// protection by retain/release.
break;
case RT_FixLifetime: // This only stops release motion. Retains can move over it.
case RT_EndBorrow:
break;
case RT_Retain:
case RT_UnknownObjectRetain:
case RT_BridgeRetain:
case RT_RetainUnowned:
case RT_ObjCRetain: { // swift_retain(obj)
//CallInst &ThisRetain = cast<CallInst>(CurInst);
//Value *ThisRetainedObject = ThisRetain.getArgOperand(0);
// If we see a retain of the same object, we can skip over it, but we
// can't count it as progress. Just pushing a retain(x) past a retain(y)
// doesn't change the program.
continue;
}
case RT_UnknownObjectRelease:
case RT_BridgeRelease:
case RT_ObjCRelease:
case RT_Release: {
// If we get to a release that is provably to this object, then we can zap
// it and the retain.
CallInst &ThisRelease = cast<CallInst>(CurInst);
Value *ThisReleasedObject = ThisRelease.getArgOperand(0);
ThisReleasedObject = RC.getSwiftRCIdentityRoot(ThisReleasedObject);
if (ThisReleasedObject == RetainedObject) {
Retain.eraseFromParent();
ThisRelease.eraseFromParent();
if (isObjCRetain) {
++NumObjCRetainReleasePairs;
} else {
++NumRetainReleasePairs;
}
return true;
}
// Otherwise, if this is some other pointer, we can only ignore it if we
// can prove that the two objects don't alias.
// Retain.dump(); ThisRelease.dump(); BB.getParent()->dump();
goto OutOfLoop;
}
case RT_Unknown:
// Loads cannot affect the retain.
if (isa<LoadInst>(CurInst))
continue;
// Load, store, memcpy etc can't do a release.
if (isa<LoadInst>(CurInst) || isa<StoreInst>(CurInst) ||
isa<MemIntrinsic>(CurInst))
break;
// CurInst->dump(); BBI->dump();
// Otherwise, we get to something unknown/unhandled. Bail out for now.
goto OutOfLoop;
}
// If the switch did a break, we made some progress moving this retain.
MadeProgress = true;
}
OutOfLoop:
// If we were able to move the retain down, move it now.
// TODO: This is where we'd plug in some global algorithms someday.
if (MadeProgress) {
Retain.moveBefore(&*BBI);
return true;
}
return false;
}
//===----------------------------------------------------------------------===//
// Store-Only Object Elimination
//===----------------------------------------------------------------------===//
/// DT_Kind - Classification for destructor semantics.
enum class DtorKind {
/// NoSideEffects - The destructor does nothing, or just touches the local
/// object in a non-observable way after it is destroyed.
NoSideEffects,
/// NoEscape - The destructor potentially has some side effects, but the
/// address of the destroyed object never escapes (in the LLVM IR sense).
NoEscape,
/// Unknown - Something potentially crazy is going on here.
Unknown
};
/// analyzeDestructor - Given the heap.metadata argument to swift_allocObject,
/// take a look a the destructor and try to decide if it has side effects or any
/// other bad effects that can prevent it from being optimized.
static DtorKind analyzeDestructor(Value *P) {
// If we have a null pointer for the metadata info, the dtor has no side
// effects. Actually, the final release would crash. This is really only
// useful for writing testcases.
if (isa<ConstantPointerNull>(P->stripPointerCasts()))
return DtorKind::NoSideEffects;
// We have to have a known heap metadata value, reject dynamically computed
// ones, or places
// Also, make sure we have a definitive initializer for the global.
auto *GV = dyn_cast<GlobalVariable>(P->stripPointerCasts());
if (GV == nullptr || !GV->hasDefinitiveInitializer())
return DtorKind::Unknown;
ConstantStruct *CS = dyn_cast_or_null<ConstantStruct>(GV->getInitializer());
if (CS == nullptr || CS->getNumOperands() == 0)
return DtorKind::Unknown;
// FIXME: Would like to abstract the dtor slot (#0) out from this to somewhere
// unified.
enum { DTorSlotOfHeapMetadata = 0 };
auto *DtorFn = dyn_cast<Function>(CS->getOperand(DTorSlotOfHeapMetadata));
if (DtorFn == nullptr || DtorFn->isInterposable() ||
DtorFn->hasExternalLinkage())
return DtorKind::Unknown;
// Okay, we have a body, and we can trust it. If the function is marked
// readonly, then we know it can't have any interesting side effects, so we
// don't need to analyze it at all.
if (DtorFn->onlyReadsMemory())
return DtorKind::NoSideEffects;
// The first argument is the object being destroyed.
assert(DtorFn->arg_size() == 1 && !DtorFn->isVarArg() &&
"expected a single object argument to destructors");
Value *ThisObject = &*DtorFn->arg_begin();
// Scan the body of the function, looking for anything scary.
for (BasicBlock &BB : *DtorFn) {
for (Instruction &I : BB) {
// Note that the destructor may not be in any particular canonical form.
switch (classifyInstruction(I)) {
// These instructions should not reach here based on the pass ordering.
// i.e. LLVMARCOpt -> LLVMContractOpt.
case RT_RetainN:
case RT_UnknownObjectRetainN:
case RT_BridgeRetainN:
case RT_ReleaseN:
case RT_UnknownObjectReleaseN:
case RT_BridgeReleaseN:
llvm_unreachable("These are only created by LLVMARCContract !");
case RT_NoMemoryAccessed:
case RT_AllocObject:
case RT_FixLifetime:
case RT_EndBorrow:
case RT_CheckUnowned:
// Skip over random instructions that don't touch memory in the caller.
continue;
case RT_RetainUnowned:
case RT_BridgeRetain: // x = swift_bridgeRetain(y)
case RT_Retain: { // swift_retain(obj)
// Ignore retains of the "self" object, no resurrection is possible.
Value *ThisRetainedObject = cast<CallInst>(I).getArgOperand(0);
if (ThisRetainedObject->stripPointerCasts() ==
ThisObject->stripPointerCasts())
continue;
// Otherwise, we may be retaining something scary.
break;
}
case RT_Release: {
// If we get to a release that is provably to this object, then we can
// ignore it.
Value *ThisReleasedObject = cast<CallInst>(I).getArgOperand(0);
if (ThisReleasedObject->stripPointerCasts() ==
ThisObject->stripPointerCasts())
continue;
// Otherwise, we may be retaining something scary.
break;
}
case RT_ObjCRelease:
case RT_ObjCRetain:
case RT_UnknownObjectRetain:
case RT_UnknownObjectRelease:
case RT_BridgeRelease:
// Objective-C retain and release can have arbitrary side effects.
break;
case RT_Unknown:
// Ignore all instructions with no side effects.
if (!I.mayHaveSideEffects()) continue;
// store, memcpy, memmove *to* the object can be dropped.
if (auto *SI = dyn_cast<StoreInst>(&I)) {
if (SI->getPointerOperand()->stripInBoundsOffsets() == ThisObject)
continue;
}
if (auto *MI = dyn_cast<MemIntrinsic>(&I)) {
if (MI->getDest()->stripInBoundsOffsets() == ThisObject)
continue;
}
// Otherwise, we can't remove the deallocation completely.
break;
}
// Okay, the function has some side effects.
//
// TODO: We could in the future return more accurate information by
// checking if the function is able to capture the deinit parameter. We do
// not do that today.
return DtorKind::Unknown;
}
}
// If we didn't find any side effects, we win.
return DtorKind::NoSideEffects;
}
/// performStoreOnlyObjectElimination - Scan the graph of uses of the specified
/// object allocation. If the object does not escape and is only stored to
/// (this happens because GVN and other optimizations hoists forward substitutes
/// all stores to the object to eliminate all loads from it), then zap the
/// object and all accesses related to it.
static bool performStoreOnlyObjectElimination(CallInst &Allocation,
BasicBlock::iterator &BBI) {
DtorKind DtorInfo = analyzeDestructor(Allocation.getArgOperand(0));
// We can't delete the object if its destructor has side effects.
if (DtorInfo != DtorKind::NoSideEffects)
return false;
// Do a depth first search exploring all of the uses of the object pointer,
// following through casts, pointer adjustments etc. If we find any loads or
// any escape sites of the object, we give up. If we succeed in walking the
// entire graph of uses, we can remove the resultant set.
SmallSetVector<Instruction*, 16> InvolvedInstructions;
SmallVector<Instruction*, 16> Worklist;
Worklist.push_back(&Allocation);
// Stores - Keep track of all of the store instructions we see.
SmallVector<StoreInst*, 16> Stores;
while (!Worklist.empty()) {
Instruction *I = Worklist.pop_back_val();
// Insert the instruction into our InvolvedInstructions set. If we have
// already seen it, then don't reprocess all of the uses.
if (!InvolvedInstructions.insert(I)) continue;
// Okay, this is the first time we've seen this instruction, proceed.
switch (classifyInstruction(*I)) {
// These instructions should not reach here based on the pass ordering.
// i.e. LLVMARCOpt -> LLVMContractOpt.
case RT_RetainN:
case RT_UnknownObjectRetainN:
case RT_BridgeRetainN:
case RT_ReleaseN:
case RT_UnknownObjectReleaseN:
case RT_BridgeReleaseN:
llvm_unreachable("These are only created by LLVMARCContract !");
case RT_AllocObject:
// If this is a different swift_allocObject than we started with, then
// there is some computation feeding into a size or alignment computation
// that we have to keep... unless we can delete *that* entire object as
// well.
break;
case RT_NoMemoryAccessed:
// If no memory is accessed, then something is being done with the
// pointer: maybe it is bitcast or GEP'd. Since there are no side effects,
// it is perfectly fine to delete this instruction if all uses of the
// instruction are also eliminable.
if (I->mayHaveSideEffects() || I->isTerminator())
return false;
break;
case RT_Release:
case RT_Retain:
case RT_FixLifetime:
case RT_EndBorrow:
case RT_CheckUnowned:
// It is perfectly fine to eliminate various retains and releases of this
// object: we are zapping all accesses or none.
break;
// If this is an unknown instruction, we have more interesting things to
// consider.
case RT_Unknown:
case RT_ObjCRelease:
case RT_ObjCRetain:
case RT_UnknownObjectRetain:
case RT_UnknownObjectRelease:
case RT_BridgeRetain:
case RT_BridgeRelease:
case RT_RetainUnowned:
// Otherwise, this really is some unhandled instruction. Bail out.
return false;
}
// Okay, if we got here, the instruction can be eaten so-long as all of its
// uses can be. Scan through the uses and add them to the worklist for
// recursive processing.
for (auto UI = I->user_begin(), E = I->user_end(); UI != E; ++UI) {
Instruction *User = cast<Instruction>(*UI);
// Handle stores as a special case here: we want to make sure that the
// object is being stored *to*, not itself being stored (which would be an
// escape point). Since stores themselves don't have any uses, we can
// short-cut the classification scheme above.
if (auto *SI = dyn_cast<StoreInst>(User)) {
// If this is a store *to* the object, we can zap it.
if (UI.getUse().getOperandNo() == StoreInst::getPointerOperandIndex()) {
InvolvedInstructions.insert(SI);
continue;
}
// Otherwise, using the object as a source (or size) is an escape.
return false;
}
if (auto *MI = dyn_cast<MemIntrinsic>(User)) {
// If this is a memset/memcpy/memmove *to* the object, we can zap it.
if (UI.getUse().getOperandNo() == 0) {
InvolvedInstructions.insert(MI);
continue;
}
// Otherwise, using the object as a source (or size) is an escape.
return false;
}
// Otherwise, normal instructions just go on the worklist for processing.
Worklist.push_back(User);
}
}
// Ok, we succeeded! This means we can zap all of the instructions that use
// the object. One thing we have to be careful of is to make sure that we
// don't invalidate "BBI" (the iterator the outer walk of the optimization
// pass is using, and indicates the next instruction to process). This would
// happen if we delete the instruction it is pointing to. Advance the
// iterator if that would happen.
while (InvolvedInstructions.count(&*BBI))
++BBI;
// Zap all of the instructions.
for (auto I : InvolvedInstructions) {
if (!I->use_empty())
I->replaceAllUsesWith(UndefValue::get(I->getType()));
I->eraseFromParent();
}
++NumStoreOnlyObjectsEliminated;
return true;
}
/// Gets the underlying address of a load.
static Value *getBaseAddress(Value *val) {
for (;;) {
if (auto *GEP = dyn_cast<GetElementPtrInst>(val)) {
val = GEP->getPointerOperand();
continue;
}
if (auto *BC = dyn_cast<BitCastInst>(val)) {
val = BC->getOperand(0);
continue;
}
return val;
}
}
/// Replaces
///
/// strong_retain_unowned %x
/// ... // speculatively executable instructions, including loads from %x
/// strong_release %x
///
/// with
///
/// ... // speculatively executable instructions, including loads from %x
/// check_unowned %x
///
static bool performLocalRetainUnownedOpt(CallInst *Retain, BasicBlock &BB,
ARCEntryPointBuilder &B) {
Value *RetainedObject = Retain->getArgOperand(0);
Value *LoadBaseAddr = getBaseAddress(RetainedObject);
auto BBI = Retain->getIterator(), BBE = BB.getTerminator()->getIterator();
// Scan until we get to the end of the block.
for (++BBI; BBI != BBE; ++BBI) {
Instruction &I = *BBI;
if (classifyInstruction(I) == RT_Release) {
CallInst *ThisRelease = cast<CallInst>(&I);
// Is this the trailing release of the unowned-retained reference?
if (ThisRelease->getArgOperand(0) != RetainedObject)
return false;
// Replace the trailing release with a check_unowned.
B.setInsertPoint(ThisRelease);
B.createCheckUnowned(RetainedObject, ThisRelease);
Retain->eraseFromParent();
ThisRelease->eraseFromParent();
++NumRetainReleasePairs;
return true;
}
if (auto *LI = dyn_cast<LoadInst>(&I)) {
// Accept loads from the unowned-referenced object. This may load garbage
// values, but they are not used until the final check_unowned succeeds.
if (getBaseAddress(LI->getPointerOperand()) == LoadBaseAddr)
continue;
}
// Other than loads from the unowned-referenced object we only accept
// speculatively executable instructions.
if (!isSafeToSpeculativelyExecute(&I))
return false;
}
return false;
}
/// Removes redundant check_unowned calls if they check the same reference and
/// there is no instruction in between which could decrement the reference count.
static void performRedundantCheckUnownedRemoval(BasicBlock &BB) {
DenseSet<Value *> checkedValues;
for (BasicBlock::iterator BBI = BB.begin(), E = BB.end(); BBI != E; ) {
// Preincrement the iterator to avoid invalidation and out trouble.
Instruction &I = *BBI++;
switch (classifyInstruction(I)) {
case RT_NoMemoryAccessed:
case RT_AllocObject:
case RT_FixLifetime:
case RT_Retain:
case RT_UnknownObjectRetain:
case RT_BridgeRetain:
case RT_RetainUnowned:
case RT_ObjCRetain:
// All this cannot decrement reference counts.
continue;
case RT_CheckUnowned: {
Value *Arg = cast<CallInst>(&I)->getArgOperand(0);
if (checkedValues.count(Arg) != 0) {
// We checked this reference already -> delete the second check.
I.eraseFromParent();
} else {
// Record the check.
checkedValues.insert(Arg);
}
continue;
}
case RT_Unknown:
// Loads cannot affect the retain.
if (isa<LoadInst>(I) || isa<StoreInst>(I) || isa<MemIntrinsic>(I))
continue;
break;
default:
break;
}
// We found some potential reference decrementing instruction. Bail out.
checkedValues.clear();
}
}
/// performGeneralOptimizations - This does a forward scan over basic blocks,
/// looking for interesting local optimizations that can be done.
static bool performGeneralOptimizations(Function &F, ARCEntryPointBuilder &B,
SwiftRCIdentity &RC) {
bool Changed = false;
// TODO: This is a really trivial local algorithm. It could be much better.
for (BasicBlock &BB : F) {
SmallVector<CallInst *, 8> RetainUnownedInsts;
for (BasicBlock::iterator BBI = BB.begin(), E = BB.end(); BBI != E; ) {
// Preincrement the iterator to avoid invalidation and out trouble.
Instruction &I = *BBI++;
// Do various optimizations based on the instruction we find.
switch (classifyInstruction(I)) {
default: break;
case RT_AllocObject:
Changed |= performStoreOnlyObjectElimination(cast<CallInst>(I), BBI);
break;
case RT_BridgeRelease:
case RT_ObjCRelease:
case RT_UnknownObjectRelease:
case RT_Release:
Changed |= performLocalReleaseMotion(cast<CallInst>(I), BB, RC);
break;
case RT_BridgeRetain:
case RT_Retain:
case RT_UnknownObjectRetain:
case RT_ObjCRetain: {
// Retain motion is a forward pass over the block. Make sure we don't
// invalidate our iterators by parking it on the instruction before I.
BasicBlock::iterator Safe = I.getIterator();
Safe = Safe != BB.begin() ? std::prev(Safe) : BB.end();
if (performLocalRetainMotion(cast<CallInst>(I), BB, RC)) {
// If we zapped or moved the retain, reset the iterator on the
// instruction *newly* after the prev instruction.
BBI = Safe != BB.end() ? std::next(Safe) : BB.begin();
Changed = true;
}
break;
}
case RT_RetainUnowned:
RetainUnownedInsts.push_back(cast<CallInst>(&I));
break;
}
}
// Delay the retain-unowned optimization until we finished with all other
// optimizations in this block. The retain-unowned optimization will benefit
// from the release-motion.
bool CheckUnknownInserted = false;
for (auto *RetainUnowned : RetainUnownedInsts) {
if (performLocalRetainUnownedOpt(RetainUnowned, BB, B))
CheckUnknownInserted = true;
}
if (CheckUnknownInserted) {
Changed = true;
performRedundantCheckUnownedRemoval(BB);
}
}
return Changed;
}
//===----------------------------------------------------------------------===//
// SwiftARCOpt Pass
//===----------------------------------------------------------------------===//
char SwiftARCOpt::ID = 0;
INITIALIZE_PASS_BEGIN(SwiftARCOpt,
"swift-llvm-arc-optimize", "Swift LLVM ARC optimization",
false, false)
INITIALIZE_PASS_DEPENDENCY(SwiftAAWrapperPass)
INITIALIZE_PASS_END(SwiftARCOpt,
"swift-llvm-arc-optimize", "Swift LLVM ARC optimization",
false, false)
// Optimization passes.
llvm::FunctionPass *swift::createSwiftARCOptPass() {
initializeSwiftARCOptPass(*llvm::PassRegistry::getPassRegistry());
return new SwiftARCOpt();
}
SwiftARCOpt::SwiftARCOpt() : FunctionPass(ID) {
}
void SwiftARCOpt::getAnalysisUsage(llvm::AnalysisUsage &AU) const {
AU.addRequiredID(&SwiftAAWrapperPass::ID);
AU.setPreservesCFG();
}
static bool runSwiftARCOpts(Function &F, SwiftRCIdentity &RC) {
bool Changed = false;
ARCEntryPointBuilder B(F);
// First thing: canonicalize swift_retain and similar calls so that nothing
// uses their result. This exposes the copy that the function does to the
// optimizer.
Changed |= canonicalizeInputFunction(F, B, RC);
// Next, do a pass with a couple of optimizations:
// 1) release() motion, eliminating retain/release pairs when it turns out
// that a pair is not protecting anything that accesses the guarded heap
// object.
// 2) deletion of stored-only objects - objects that are allocated and
// potentially retained and released, but are only stored to and don't
// escape.
Changed |= performGeneralOptimizations(F, B, RC);
return Changed;
}
bool SwiftARCOpt::runOnFunction(Function &F) {
if (DisableARCOpts)
return false;
return runSwiftARCOpts(F, RC);
}
PreservedAnalyses SwiftARCOptPass::run(llvm::Function &F,
llvm::FunctionAnalysisManager &AM) {
bool changed = false;
if (!DisableARCOpts)
changed = runSwiftARCOpts(F, RC);
if (!changed) {
return PreservedAnalyses::all();
}
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
return PA;
}
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