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swift-mirror/lib/LLVMPasses/LLVMARCOpts.cpp
Michael Gottesman 9326ef4d73 [upstream-update] Update LLVMPasses for new objc arc intrinsics. 
We used to represent these just as normal LLVM functions, e.x.:

  declare objc_object* @objc_retain(objc_object*)
  declare void @objc_release(objc_object*)

Recently, special objc intrinsics were added to LLVM. This pass updates these
small (old) passes to use the new intrinsics.

This turned out to not be too difficult since we never create these
instructions. We only analyze them, move them, and delete them.

rdar://47852297
2019-02-08 14:22:57 -08:00

1033 lines
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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/ADT/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;
}
// 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;
}
// Have not encountered a strong retain/release. keep it in the
// unknown retain/release list for now. It might get replaced
// later.
if (NativeRefs.find(ArgVal) == NativeRefs.end()) {
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.find(ArgVal) == NativeRefs.end()) {
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_DEPENDENCY(SwiftRCIdentity)
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.addRequired<SwiftRCIdentity>();
AU.setPreservesCFG();
}
bool SwiftARCOpt::runOnFunction(Function &F) {
if (DisableARCOpts)
return false;
bool Changed = false;
ARCEntryPointBuilder B(F);
RC = &getAnalysis<SwiftRCIdentity>();
// 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;
}
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