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webkit / WebKit / 41419438b681f832b2c71541303b7213480702be / . / Source / JavaScriptCore / dfg / DFGObjectAllocationSinkingPhase.cpp
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/*
* Copyright (C) 2015-2018 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include "config.h"
#include "DFGObjectAllocationSinkingPhase.h"
#if ENABLE(DFG_JIT)
#include "DFGBlockMapInlines.h"
#include "DFGClobbersExitState.h"
#include "DFGCombinedLiveness.h"
#include "DFGGraph.h"
#include "DFGInsertionSet.h"
#include "DFGLazyNode.h"
#include "DFGLivenessAnalysisPhase.h"
#include "DFGOSRAvailabilityAnalysisPhase.h"
#include "DFGPhase.h"
#include "DFGPromotedHeapLocation.h"
#include "DFGSSACalculator.h"
#include "DFGValidate.h"
#include "JSCInlines.h"
#include <wtf/StdList.h>
namespace JSC { namespace DFG {
namespace {
namespace DFGObjectAllocationSinkingPhaseInternal {
static const bool verbose = false;
}
// In order to sink object cycles, we use a points-to analysis coupled
// with an escape analysis. This analysis is actually similar to an
// abstract interpreter focused on local allocations and ignoring
// everything else.
//
// We represent the local heap using two mappings:
//
// - A set of the local allocations present in the function, where
// each of those have a further mapping from
// PromotedLocationDescriptor to local allocations they must point
// to.
//
// - A "pointer" mapping from nodes to local allocations, if they must
// be equal to said local allocation and are currently live. This
// can be because the node is the actual node that created the
// allocation, or any other node that must currently point to it -
// we don't make a difference.
//
// The following graph is a motivation for why we separate allocations
// from pointers:
//
// Block #0
// 0: NewObject({})
// 1: NewObject({})
// -: PutByOffset(@0, @1, x)
// -: PutStructure(@0, {x:0})
// 2: GetByOffset(@0, x)
// -: Jump(#1)
//
// Block #1
// -: Return(@2)
//
// Here, we need to remember in block #1 that @2 points to a local
// allocation with appropriate fields and structures information
// (because we should be able to place a materialization on top of
// block #1 here), even though @1 is dead. We *could* just keep @1
// artificially alive here, but there is no real reason to do it:
// after all, by the end of block #0, @1 and @2 should be completely
// interchangeable, and there is no reason for us to artificially make
// @1 more important.
//
// An important point to consider to understand this separation is
// that we should think of the local heap as follow: we have a
// bunch of nodes that are pointers to "allocations" that live
// someplace on the heap, and those allocations can have pointers in
// between themselves as well. We shouldn't care about whatever
// names we give to the allocations ; what matters when
// comparing/merging two heaps is the isomorphism/comparison between
// the allocation graphs as seen by the nodes.
//
// For instance, in the following graph:
//
// Block #0
// 0: NewObject({})
// -: Branch(#1, #2)
//
// Block #1
// 1: NewObject({})
// -: PutByOffset(@0, @1, x)
// -: PutStructure(@0, {x:0})
// -: Jump(#3)
//
// Block #2
// 2: NewObject({})
// -: PutByOffset(@2, undefined, x)
// -: PutStructure(@2, {x:0})
// -: PutByOffset(@0, @2, x)
// -: PutStructure(@0, {x:0})
// -: Jump(#3)
//
// Block #3
// -: Return(@0)
//
// we should think of the heaps at tail of blocks #1 and #2 as being
// exactly the same, even though one has @0.x pointing to @1 and the
// other has @0.x pointing to @2, because in essence this should not
// be different from the graph where we hoisted @1 and @2 into a
// single allocation in block #0. We currently will not handle this
// case, because we merge allocations based on the node they are
// coming from, but this is only a technicality for the sake of
// simplicity that shouldn't hide the deeper idea outlined here.
class Allocation {
public:
// We use Escaped as a special allocation kind because when we
// decide to sink an allocation, we still need to keep track of it
// once it is escaped if it still has pointers to it in order to
// replace any use of those pointers by the corresponding
// materialization
enum class Kind { Escaped, Object, Activation, Function, GeneratorFunction, AsyncFunction, AsyncGeneratorFunction, RegExpObject };
using Fields = HashMap<PromotedLocationDescriptor, Node*>;
explicit Allocation(Node* identifier = nullptr, Kind kind = Kind::Escaped)
: m_identifier(identifier)
, m_kind(kind)
{
}
const Fields& fields() const
{
return m_fields;
}
Fields& fields()
{
return m_fields;
}
Node* get(PromotedLocationDescriptor descriptor)
{
return m_fields.get(descriptor);
}
Allocation& set(PromotedLocationDescriptor descriptor, Node* value)
{
// Pointing to anything else than an unescaped local
// allocation is represented by simply not having the
// field
if (value)
m_fields.set(descriptor, value);
else
m_fields.remove(descriptor);
return *this;
}
void remove(PromotedLocationDescriptor descriptor)
{
set(descriptor, nullptr);
}
bool hasStructures() const
{
switch (kind()) {
case Kind::Object:
return true;
default:
return false;
}
}
Allocation& setStructures(const RegisteredStructureSet& structures)
{
ASSERT(hasStructures() && !structures.isEmpty());
m_structures = structures;
return *this;
}
Allocation& mergeStructures(const RegisteredStructureSet& structures)
{
ASSERT(hasStructures() || structures.isEmpty());
m_structures.merge(structures);
return *this;
}
Allocation& filterStructures(const RegisteredStructureSet& structures)
{
ASSERT(hasStructures());
m_structures.filter(structures);
RELEASE_ASSERT(!m_structures.isEmpty());
return *this;
}
const RegisteredStructureSet& structures() const
{
return m_structures;
}
Node* identifier() const { return m_identifier; }
Kind kind() const { return m_kind; }
bool isEscapedAllocation() const
{
return kind() == Kind::Escaped;
}
bool isObjectAllocation() const
{
return m_kind == Kind::Object;
}
bool isActivationAllocation() const
{
return m_kind == Kind::Activation;
}
bool isFunctionAllocation() const
{
return m_kind == Kind::Function || m_kind == Kind::GeneratorFunction || m_kind == Kind::AsyncFunction;
}
bool isRegExpObjectAllocation() const
{
return m_kind == Kind::RegExpObject;
}
bool operator==(const Allocation& other) const
{
return m_identifier == other.m_identifier
&& m_kind == other.m_kind
&& m_fields == other.m_fields
&& m_structures == other.m_structures;
}
bool operator!=(const Allocation& other) const
{
return !(*this == other);
}
void dump(PrintStream& out) const
{
dumpInContext(out, nullptr);
}
void dumpInContext(PrintStream& out, DumpContext* context) const
{
switch (m_kind) {
case Kind::Escaped:
out.print("Escaped");
break;
case Kind::Object:
out.print("Object");
break;
case Kind::Function:
out.print("Function");
break;
case Kind::GeneratorFunction:
out.print("GeneratorFunction");
break;
case Kind::AsyncFunction:
out.print("AsyncFunction");
break;
case Kind::AsyncGeneratorFunction:
out.print("AsyncGeneratorFunction");
break;
case Kind::Activation:
out.print("Activation");
break;
case Kind::RegExpObject:
out.print("RegExpObject");
break;
}
out.print("Allocation(");
if (!m_structures.isEmpty())
out.print(inContext(m_structures.toStructureSet(), context));
if (!m_fields.isEmpty()) {
if (!m_structures.isEmpty())
out.print(", ");
out.print(mapDump(m_fields, " => #", ", "));
}
out.print(")");
}
private:
Node* m_identifier; // This is the actual node that created the allocation
Kind m_kind;
Fields m_fields;
RegisteredStructureSet m_structures;
};
class LocalHeap {
public:
Allocation& newAllocation(Node* node, Allocation::Kind kind)
{
ASSERT(!m_pointers.contains(node) && !isAllocation(node));
m_pointers.add(node, node);
return m_allocations.set(node, Allocation(node, kind)).iterator->value;
}
bool isAllocation(Node* identifier) const
{
return m_allocations.contains(identifier);
}
// Note that this is fundamentally different from
// onlyLocalAllocation() below. getAllocation() takes as argument
// a node-as-identifier, that is, an allocation node. This
// allocation node doesn't have to be alive; it may only be
// pointed to by other nodes or allocation fields.
// For instance, in the following graph:
//
// Block #0
// 0: NewObject({})
// 1: NewObject({})
// -: PutByOffset(@0, @1, x)
// -: PutStructure(@0, {x:0})
// 2: GetByOffset(@0, x)
// -: Jump(#1)
//
// Block #1
// -: Return(@2)
//
// At head of block #1, the only reachable allocation is #@1,
// which can be reached through node @2. Thus, getAllocation(#@1)
// contains the appropriate metadata for this allocation, but
// onlyLocalAllocation(@1) is null, as @1 is no longer a pointer
// to #@1 (since it is dead). Conversely, onlyLocalAllocation(@2)
// is the same as getAllocation(#@1), while getAllocation(#@2)
// does not make sense since @2 is not an allocation node.
//
// This is meant to be used when the node is already known to be
// an identifier (i.e. an allocation) - probably because it was
// found as value of a field or pointer in the current heap, or
// was the result of a call to follow(). In any other cases (such
// as when doing anything while traversing the graph), the
// appropriate function to call is probably onlyLocalAllocation.
Allocation& getAllocation(Node* identifier)
{
auto iter = m_allocations.find(identifier);
ASSERT(iter != m_allocations.end());
return iter->value;
}
void newPointer(Node* node, Node* identifier)
{
ASSERT(!m_allocations.contains(node) && !m_pointers.contains(node));
ASSERT(isAllocation(identifier));
m_pointers.add(node, identifier);
}
// follow solves the points-to problem. Given a live node, which
// may be either an allocation itself or a heap read (e.g. a
// GetByOffset node), it returns the corresponding allocation
// node, if there is one. If the argument node is neither an
// allocation or a heap read, or may point to different nodes,
// nullptr will be returned. Note that a node that points to
// different nodes can never point to an unescaped local
// allocation.
Node* follow(Node* node) const
{
auto iter = m_pointers.find(node);
ASSERT(iter == m_pointers.end() || m_allocations.contains(iter->value));
return iter == m_pointers.end() ? nullptr : iter->value;
}
Node* follow(PromotedHeapLocation location) const
{
const Allocation& base = m_allocations.find(location.base())->value;
auto iter = base.fields().find(location.descriptor());
if (iter == base.fields().end())
return nullptr;
return iter->value;
}
// onlyLocalAllocation find the corresponding allocation metadata
// for any live node. onlyLocalAllocation(node) is essentially
// getAllocation(follow(node)), with appropriate null handling.
Allocation* onlyLocalAllocation(Node* node)
{
Node* identifier = follow(node);
if (!identifier)
return nullptr;
return &getAllocation(identifier);
}
Allocation* onlyLocalAllocation(PromotedHeapLocation location)
{
Node* identifier = follow(location);
if (!identifier)
return nullptr;
return &getAllocation(identifier);
}
// This allows us to store the escapees only when necessary. If
// set, the current escapees can be retrieved at any time using
// takeEscapees(), which will clear the cached set of escapees;
// otherwise the heap won't remember escaping allocations.
void setWantEscapees()
{
m_wantEscapees = true;
}
HashMap<Node*, Allocation> takeEscapees()
{
return WTFMove(m_escapees);
}
void escape(Node* node)
{
Node* identifier = follow(node);
if (!identifier)
return;
escapeAllocation(identifier);
}
void merge(const LocalHeap& other)
{
assertIsValid();
other.assertIsValid();
ASSERT(!m_wantEscapees);
if (!reached()) {
ASSERT(other.reached());
*this = other;
return;
}
NodeSet toEscape;
for (auto& allocationEntry : other.m_allocations)
m_allocations.add(allocationEntry.key, allocationEntry.value);
for (auto& allocationEntry : m_allocations) {
auto allocationIter = other.m_allocations.find(allocationEntry.key);
// If we have it and they don't, it died for them but we
// are keeping it alive from another field somewhere.
// There is nothing to do - we will be escaped
// automatically when we handle that other field.
// This will also happen for allocation that we have and
// they don't, and all of those will get pruned.
if (allocationIter == other.m_allocations.end())
continue;
if (allocationEntry.value.kind() != allocationIter->value.kind()) {
toEscape.addVoid(allocationEntry.key);
for (const auto& fieldEntry : allocationIter->value.fields())
toEscape.addVoid(fieldEntry.value);
} else {
mergePointerSets(allocationEntry.value.fields(), allocationIter->value.fields(), toEscape);
allocationEntry.value.mergeStructures(allocationIter->value.structures());
}
}
mergePointerSets(m_pointers, other.m_pointers, toEscape);
for (Node* identifier : toEscape)
escapeAllocation(identifier);
if (!ASSERT_DISABLED) {
for (const auto& entry : m_allocations)
ASSERT_UNUSED(entry, entry.value.isEscapedAllocation() || other.m_allocations.contains(entry.key));
}
// If there is no remaining pointer to an allocation, we can
// remove it. This should only happen for escaped allocations,
// because we only merge liveness-pruned heaps in the first
// place.
prune();
assertIsValid();
}
void pruneByLiveness(const NodeSet& live)
{
m_pointers.removeIf(
[&] (const auto& entry) {
return !live.contains(entry.key);
});
prune();
}
void assertIsValid() const
{
if (ASSERT_DISABLED)
return;
// Pointers should point to an actual allocation
for (const auto& entry : m_pointers) {
ASSERT_UNUSED(entry, entry.value);
ASSERT(m_allocations.contains(entry.value));
}
for (const auto& allocationEntry : m_allocations) {
// Fields should point to an actual allocation
for (const auto& fieldEntry : allocationEntry.value.fields()) {
ASSERT_UNUSED(fieldEntry, fieldEntry.value);
ASSERT(m_allocations.contains(fieldEntry.value));
}
}
}
bool operator==(const LocalHeap& other) const
{
assertIsValid();
other.assertIsValid();
return m_allocations == other.m_allocations
&& m_pointers == other.m_pointers;
}
bool operator!=(const LocalHeap& other) const
{
return !(*this == other);
}
const HashMap<Node*, Allocation>& allocations() const
{
return m_allocations;
}
const HashMap<Node*, Node*>& pointers() const
{
return m_pointers;
}
void dump(PrintStream& out) const
{
out.print(" Allocations:\n");
for (const auto& entry : m_allocations)
out.print(" #", entry.key, ": ", entry.value, "\n");
out.print(" Pointers:\n");
for (const auto& entry : m_pointers)
out.print(" ", entry.key, " => #", entry.value, "\n");
}
bool reached() const
{
return m_reached;
}
void setReached()
{
m_reached = true;
}
private:
// When we merge two heaps, we escape all fields of allocations,
// unless they point to the same thing in both heaps.
// The reason for this is that it allows us not to do extra work
// for diamond graphs where we would otherwise have to check
// whether we have a single definition or not, which would be
// cumbersome.
//
// Note that we should try to unify nodes even when they are not
// from the same allocation; for instance we should be able to
// completely eliminate all allocations from the following graph:
//
// Block #0
// 0: NewObject({})
// -: Branch(#1, #2)
//
// Block #1
// 1: NewObject({})
// -: PutByOffset(@1, "left", val)
// -: PutStructure(@1, {val:0})
// -: PutByOffset(@0, @1, x)
// -: PutStructure(@0, {x:0})
// -: Jump(#3)
//
// Block #2
// 2: NewObject({})
// -: PutByOffset(@2, "right", val)
// -: PutStructure(@2, {val:0})
// -: PutByOffset(@0, @2, x)
// -: PutStructure(@0, {x:0})
// -: Jump(#3)
//
// Block #3:
// 3: GetByOffset(@0, x)
// 4: GetByOffset(@3, val)
// -: Return(@4)
template<typename Key>
static void mergePointerSets(HashMap<Key, Node*>& my, const HashMap<Key, Node*>& their, NodeSet& toEscape)
{
auto escape = [&] (Node* identifier) {
toEscape.addVoid(identifier);
};
for (const auto& entry : their) {
if (!my.contains(entry.key))
escape(entry.value);
}
my.removeIf([&] (const auto& entry) {
auto iter = their.find(entry.key);
if (iter == their.end()) {
escape(entry.value);
return true;
}
if (iter->value != entry.value) {
escape(entry.value);
escape(iter->value);
return true;
}
return false;
});
}
void escapeAllocation(Node* identifier)
{
Allocation& allocation = getAllocation(identifier);
if (allocation.isEscapedAllocation())
return;
Allocation unescaped = WTFMove(allocation);
allocation = Allocation(unescaped.identifier(), Allocation::Kind::Escaped);
for (const auto& entry : unescaped.fields())
escapeAllocation(entry.value);
if (m_wantEscapees)
m_escapees.add(unescaped.identifier(), WTFMove(unescaped));
}
void prune()
{
NodeSet reachable;
for (const auto& entry : m_pointers)
reachable.addVoid(entry.value);
// Repeatedly mark as reachable allocations in fields of other
// reachable allocations
{
Vector<Node*> worklist;
worklist.appendRange(reachable.begin(), reachable.end());
while (!worklist.isEmpty()) {
Node* identifier = worklist.takeLast();
Allocation& allocation = m_allocations.find(identifier)->value;
for (const auto& entry : allocation.fields()) {
if (reachable.add(entry.value).isNewEntry)
worklist.append(entry.value);
}
}
}
// Remove unreachable allocations
m_allocations.removeIf(
[&] (const auto& entry) {
return !reachable.contains(entry.key);
});
}
bool m_reached = false;
HashMap<Node*, Node*> m_pointers;
HashMap<Node*, Allocation> m_allocations;
bool m_wantEscapees = false;
HashMap<Node*, Allocation> m_escapees;
};
class ObjectAllocationSinkingPhase : public Phase {
public:
ObjectAllocationSinkingPhase(Graph& graph)
: Phase(graph, "object allocation elimination")
, m_pointerSSA(graph)
, m_allocationSSA(graph)
, m_insertionSet(graph)
{
}
bool run()
{
ASSERT(m_graph.m_form == SSA);
ASSERT(m_graph.m_fixpointState == FixpointNotConverged);
if (!performSinking())
return false;
if (DFGObjectAllocationSinkingPhaseInternal::verbose) {
dataLog("Graph after elimination:\n");
m_graph.dump();
}
return true;
}
private:
bool performSinking()
{
m_graph.computeRefCounts();
m_graph.initializeNodeOwners();
m_graph.ensureSSADominators();
performLivenessAnalysis(m_graph);
performOSRAvailabilityAnalysis(m_graph);
m_combinedLiveness = CombinedLiveness(m_graph);
CString graphBeforeSinking;
if (Options::verboseValidationFailure() && Options::validateGraphAtEachPhase()) {
StringPrintStream out;
m_graph.dump(out);
graphBeforeSinking = out.toCString();
}
if (DFGObjectAllocationSinkingPhaseInternal::verbose) {
dataLog("Graph before elimination:\n");
m_graph.dump();
}
performAnalysis();
if (!determineSinkCandidates())
return false;
if (DFGObjectAllocationSinkingPhaseInternal::verbose) {
for (BasicBlock* block : m_graph.blocksInNaturalOrder()) {
dataLog("Heap at head of ", *block, ": \n", m_heapAtHead[block]);
dataLog("Heap at tail of ", *block, ": \n", m_heapAtTail[block]);
}
}
promoteLocalHeap();
if (Options::validateGraphAtEachPhase())
DFG::validate(m_graph, DumpGraph, graphBeforeSinking);
return true;
}
void performAnalysis()
{
m_heapAtHead = BlockMap<LocalHeap>(m_graph);
m_heapAtTail = BlockMap<LocalHeap>(m_graph);
bool changed;
do {
if (DFGObjectAllocationSinkingPhaseInternal::verbose)
dataLog("Doing iteration of escape analysis.\n");
changed = false;
for (BasicBlock* block : m_graph.blocksInPreOrder()) {
m_heapAtHead[block].setReached();
m_heap = m_heapAtHead[block];
for (Node* node : *block) {
handleNode(
node,
[] (PromotedHeapLocation, LazyNode) { },
[&] (PromotedHeapLocation) -> Node* {
return nullptr;
});
}
if (m_heap == m_heapAtTail[block])
continue;
m_heapAtTail[block] = m_heap;
changed = true;
m_heap.assertIsValid();
// We keep only pointers that are live, and only
// allocations that are either live, pointed to by a
// live pointer, or (recursively) stored in a field of
// a live allocation.
//
// This means we can accidentaly leak non-dominating
// nodes into the successor. However, due to the
// non-dominance property, we are guaranteed that the
// successor has at least one predecessor that is not
// dominated either: this means any reference to a
// non-dominating allocation in the successor will
// trigger an escape and get pruned during the merge.
m_heap.pruneByLiveness(m_combinedLiveness.liveAtTail[block]);
for (BasicBlock* successorBlock : block->successors())
m_heapAtHead[successorBlock].merge(m_heap);
}
} while (changed);
}
template<typename WriteFunctor, typename ResolveFunctor>
void handleNode(
Node* node,
const WriteFunctor& heapWrite,
const ResolveFunctor& heapResolve)
{
m_heap.assertIsValid();
ASSERT(m_heap.takeEscapees().isEmpty());
Allocation* target = nullptr;
HashMap<PromotedLocationDescriptor, LazyNode> writes;
PromotedLocationDescriptor exactRead;
switch (node->op()) {
case NewObject:
target = &m_heap.newAllocation(node, Allocation::Kind::Object);
target->setStructures(node->structure());
writes.add(
StructurePLoc, LazyNode(m_graph.freeze(node->structure().get())));
break;
case NewFunction:
case NewGeneratorFunction:
case NewAsyncGeneratorFunction:
case NewAsyncFunction: {
if (isStillValid(node->castOperand<FunctionExecutable*>()->singletonFunction())) {
m_heap.escape(node->child1().node());
break;
}
if (node->op() == NewGeneratorFunction)
target = &m_heap.newAllocation(node, Allocation::Kind::GeneratorFunction);
else if (node->op() == NewAsyncFunction)
target = &m_heap.newAllocation(node, Allocation::Kind::AsyncFunction);
else if (node->op() == NewAsyncGeneratorFunction)
target = &m_heap.newAllocation(node, Allocation::Kind::AsyncGeneratorFunction);
else
target = &m_heap.newAllocation(node, Allocation::Kind::Function);
writes.add(FunctionExecutablePLoc, LazyNode(node->cellOperand()));
writes.add(FunctionActivationPLoc, LazyNode(node->child1().node()));
break;
}
case NewRegexp: {
target = &m_heap.newAllocation(node, Allocation::Kind::RegExpObject);
writes.add(RegExpObjectRegExpPLoc, LazyNode(node->cellOperand()));
writes.add(RegExpObjectLastIndexPLoc, LazyNode(node->child1().node()));
break;
}
case CreateActivation: {
if (isStillValid(node->castOperand<SymbolTable*>()->singletonScope())) {
m_heap.escape(node->child1().node());
break;
}
target = &m_heap.newAllocation(node, Allocation::Kind::Activation);
writes.add(ActivationSymbolTablePLoc, LazyNode(node->cellOperand()));
writes.add(ActivationScopePLoc, LazyNode(node->child1().node()));
{
SymbolTable* symbolTable = node->castOperand<SymbolTable*>();
ConcurrentJSLocker locker(symbolTable->m_lock);
LazyNode initialValue(m_graph.freeze(node->initializationValueForActivation()));
for (auto iter = symbolTable->begin(locker), end = symbolTable->end(locker); iter != end; ++iter) {
writes.add(
PromotedLocationDescriptor(ClosureVarPLoc, iter->value.scopeOffset().offset()),
initialValue);
}
}
break;
}
case PutStructure:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isObjectAllocation()) {
writes.add(StructurePLoc, LazyNode(m_graph.freeze(JSValue(node->transition()->next.get()))));
target->setStructures(node->transition()->next);
} else
m_heap.escape(node->child1().node());
break;
case CheckStructureOrEmpty:
case CheckStructure: {
Allocation* allocation = m_heap.onlyLocalAllocation(node->child1().node());
if (allocation && allocation->isObjectAllocation()) {
RegisteredStructureSet filteredStructures = allocation->structures();
filteredStructures.filter(node->structureSet());
if (filteredStructures.isEmpty()) {
// FIXME: Write a test for this:
// https://bugs.webkit.org/show_bug.cgi?id=174322
m_heap.escape(node->child1().node());
break;
}
allocation->setStructures(filteredStructures);
if (Node* value = heapResolve(PromotedHeapLocation(allocation->identifier(), StructurePLoc)))
node->convertToCheckStructureImmediate(value);
} else
m_heap.escape(node->child1().node());
break;
}
case GetByOffset:
case GetGetterSetterByOffset:
target = m_heap.onlyLocalAllocation(node->child2().node());
if (target && target->isObjectAllocation()) {
unsigned identifierNumber = node->storageAccessData().identifierNumber;
exactRead = PromotedLocationDescriptor(NamedPropertyPLoc, identifierNumber);
} else {
m_heap.escape(node->child1().node());
m_heap.escape(node->child2().node());
}
break;
case MultiGetByOffset: {
Allocation* allocation = m_heap.onlyLocalAllocation(node->child1().node());
if (allocation && allocation->isObjectAllocation()) {
MultiGetByOffsetData& data = node->multiGetByOffsetData();
RegisteredStructureSet validStructures;
bool hasInvalidStructures = false;
for (const auto& multiGetByOffsetCase : data.cases) {
if (!allocation->structures().overlaps(multiGetByOffsetCase.set()))
continue;
switch (multiGetByOffsetCase.method().kind()) {
case GetByOffsetMethod::LoadFromPrototype: // We need to escape those
case GetByOffsetMethod::Constant: // We don't really have a way of expressing this
hasInvalidStructures = true;
break;
case GetByOffsetMethod::Load: // We're good
validStructures.merge(multiGetByOffsetCase.set());
break;
default:
RELEASE_ASSERT_NOT_REACHED();
}
}
if (hasInvalidStructures || validStructures.isEmpty()) {
m_heap.escape(node->child1().node());
break;
}
unsigned identifierNumber = data.identifierNumber;
PromotedHeapLocation location(NamedPropertyPLoc, allocation->identifier(), identifierNumber);
if (Node* value = heapResolve(location)) {
if (allocation->structures().isSubsetOf(validStructures))
node->replaceWithWithoutChecks(value);
else {
Node* structure = heapResolve(PromotedHeapLocation(allocation->identifier(), StructurePLoc));
ASSERT(structure);
allocation->filterStructures(validStructures);
node->convertToCheckStructure(m_graph.addStructureSet(allocation->structures()));
node->convertToCheckStructureImmediate(structure);
node->setReplacement(value);
}
} else if (!allocation->structures().isSubsetOf(validStructures)) {
// Even though we don't need the result here, we still need
// to make the call to tell our caller that we could need
// the StructurePLoc.
// The reason for this is that when we decide not to sink a
// node, we will still lower any read to its fields before
// it escapes (which are usually reads across a function
// call that DFGClobberize can't handle) - but we only do
// this for PromotedHeapLocations that we have seen read
// during the analysis!
heapResolve(PromotedHeapLocation(allocation->identifier(), StructurePLoc));
allocation->filterStructures(validStructures);
}
Node* identifier = allocation->get(location.descriptor());
if (identifier)
m_heap.newPointer(node, identifier);
} else
m_heap.escape(node->child1().node());
break;
}
case PutByOffset:
target = m_heap.onlyLocalAllocation(node->child2().node());
if (target && target->isObjectAllocation()) {
unsigned identifierNumber = node->storageAccessData().identifierNumber;
writes.add(
PromotedLocationDescriptor(NamedPropertyPLoc, identifierNumber),
LazyNode(node->child3().node()));
} else {
m_heap.escape(node->child1().node());
m_heap.escape(node->child2().node());
m_heap.escape(node->child3().node());
}
break;
case GetClosureVar:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isActivationAllocation()) {
exactRead =
PromotedLocationDescriptor(ClosureVarPLoc, node->scopeOffset().offset());
} else
m_heap.escape(node->child1().node());
break;
case PutClosureVar:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isActivationAllocation()) {
writes.add(
PromotedLocationDescriptor(ClosureVarPLoc, node->scopeOffset().offset()),
LazyNode(node->child2().node()));
} else {
m_heap.escape(node->child1().node());
m_heap.escape(node->child2().node());
}
break;
case SkipScope:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isActivationAllocation())
exactRead = ActivationScopePLoc;
else
m_heap.escape(node->child1().node());
break;
case GetExecutable:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isFunctionAllocation())
exactRead = FunctionExecutablePLoc;
else
m_heap.escape(node->child1().node());
break;
case GetScope:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isFunctionAllocation())
exactRead = FunctionActivationPLoc;
else
m_heap.escape(node->child1().node());
break;
case GetRegExpObjectLastIndex:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isRegExpObjectAllocation())
exactRead = RegExpObjectLastIndexPLoc;
else
m_heap.escape(node->child1().node());
break;
case SetRegExpObjectLastIndex:
target = m_heap.onlyLocalAllocation(node->child1().node());
if (target && target->isRegExpObjectAllocation()) {
writes.add(
PromotedLocationDescriptor(RegExpObjectLastIndexPLoc),
LazyNode(node->child2().node()));
} else {
m_heap.escape(node->child1().node());
m_heap.escape(node->child2().node());
}
break;
case Check:
case CheckVarargs:
m_graph.doToChildren(
node,
[&] (Edge edge) {
if (edge.willNotHaveCheck())
return;
if (alreadyChecked(edge.useKind(), SpecObject))
return;
m_heap.escape(edge.node());
});
break;
case MovHint:
case PutHint:
// Handled by OSR availability analysis
break;
default:
m_graph.doToChildren(
node,
[&] (Edge edge) {
m_heap.escape(edge.node());
});
break;
}
if (exactRead) {
ASSERT(target);
ASSERT(writes.isEmpty());
if (Node* value = heapResolve(PromotedHeapLocation(target->identifier(), exactRead))) {
ASSERT(!value->replacement());
node->replaceWith(m_graph, value);
}
Node* identifier = target->get(exactRead);
if (identifier)
m_heap.newPointer(node, identifier);
}
for (auto entry : writes) {
ASSERT(target);
if (entry.value.isNode())
target->set(entry.key, m_heap.follow(entry.value.asNode()));
else
target->remove(entry.key);
heapWrite(PromotedHeapLocation(target->identifier(), entry.key), entry.value);
}
m_heap.assertIsValid();
}
bool determineSinkCandidates()
{
m_sinkCandidates.clear();
m_materializationToEscapee.clear();
m_materializationSiteToMaterializations.clear();
m_materializationSiteToRecoveries.clear();
m_materializationSiteToHints.clear();
// Logically we wish to consider every allocation and sink
// it. However, it is probably not profitable to sink an
// allocation that will always escape. So, we only sink an
// allocation if one of the following is true:
//
// 1) There exists a basic block with only backwards outgoing
// edges (or no outgoing edges) in which the node wasn't
// materialized. This is meant to catch
// effectively-infinite loops in which we don't need to
// have allocated the object.
//
// 2) There exists a basic block at the tail of which the node
// is dead and not materialized.
//
// 3) The sum of execution counts of the materializations is
// less than the sum of execution counts of the original
// node.
//
// We currently implement only rule #2.
// FIXME: Implement the two other rules.
// https://bugs.webkit.org/show_bug.cgi?id=137073 (rule #1)
// https://bugs.webkit.org/show_bug.cgi?id=137074 (rule #3)
//
// However, these rules allow for a sunk object to be put into
// a non-sunk one, which we don't support. We could solve this
// by supporting PutHints on local allocations, making these
// objects only partially correct, and we would need to adapt
// the OSR availability analysis and OSR exit to handle
// this. This would be totally doable, but would create a
// super rare, and thus bug-prone, code path.
// So, instead, we need to implement one of the following
// closure rules:
//
// 1) If we put a sink candidate into a local allocation that
// is not a sink candidate, change our minds and don't
// actually sink the sink candidate.
//
// 2) If we put a sink candidate into a local allocation, that
// allocation becomes a sink candidate as well.
//
// We currently choose to implement closure rule #2.
HashMap<Node*, Vector<Node*>> dependencies;
bool hasUnescapedReads = false;
for (BasicBlock* block : m_graph.blocksInPreOrder()) {
m_heap = m_heapAtHead[block];
for (Node* node : *block) {
handleNode(
node,
[&] (PromotedHeapLocation location, LazyNode value) {
if (!value.isNode())
return;
Allocation* allocation = m_heap.onlyLocalAllocation(value.asNode());
if (allocation && !allocation->isEscapedAllocation())
dependencies.add(allocation->identifier(), Vector<Node*>()).iterator->value.append(location.base());
},
[&] (PromotedHeapLocation) -> Node* {
hasUnescapedReads = true;
return nullptr;
});
}
// The sink candidates are initially the unescaped
// allocations dying at tail of blocks
NodeSet allocations;
for (const auto& entry : m_heap.allocations()) {
if (!entry.value.isEscapedAllocation())
allocations.addVoid(entry.key);
}
m_heap.pruneByLiveness(m_combinedLiveness.liveAtTail[block]);
for (Node* identifier : allocations) {
if (!m_heap.isAllocation(identifier))
m_sinkCandidates.addVoid(identifier);
}
}
// Ensure that the set of sink candidates is closed for put operations
Vector<Node*> worklist;
worklist.appendRange(m_sinkCandidates.begin(), m_sinkCandidates.end());
while (!worklist.isEmpty()) {
for (Node* identifier : dependencies.get(worklist.takeLast())) {
if (m_sinkCandidates.add(identifier).isNewEntry)
worklist.append(identifier);
}
}
if (m_sinkCandidates.isEmpty())
return hasUnescapedReads;
if (DFGObjectAllocationSinkingPhaseInternal::verbose)
dataLog("Candidates: ", listDump(m_sinkCandidates), "\n");
// Create the materialization nodes
for (BasicBlock* block : m_graph.blocksInNaturalOrder()) {
m_heap = m_heapAtHead[block];
m_heap.setWantEscapees();
for (Node* node : *block) {
handleNode(
node,
[] (PromotedHeapLocation, LazyNode) { },
[] (PromotedHeapLocation) -> Node* {
return nullptr;
});
auto escapees = m_heap.takeEscapees();
if (!escapees.isEmpty())
placeMaterializations(escapees, node);
}
m_heap.pruneByLiveness(m_combinedLiveness.liveAtTail[block]);
{
HashMap<Node*, Allocation> escapingOnEdge;
for (const auto& entry : m_heap.allocations()) {
if (entry.value.isEscapedAllocation())
continue;
bool mustEscape = false;
for (BasicBlock* successorBlock : block->successors()) {
if (!m_heapAtHead[successorBlock].isAllocation(entry.key)
|| m_heapAtHead[successorBlock].getAllocation(entry.key).isEscapedAllocation())
mustEscape = true;
}
if (mustEscape)
escapingOnEdge.add(entry.key, entry.value);
}
placeMaterializations(WTFMove(escapingOnEdge), block->terminal());
}
}
return hasUnescapedReads || !m_sinkCandidates.isEmpty();
}
void placeMaterializations(HashMap<Node*, Allocation> escapees, Node* where)
{
// We don't create materializations if the escapee is not a
// sink candidate
escapees.removeIf(
[&] (const auto& entry) {
return !m_sinkCandidates.contains(entry.key);
});
if (escapees.isEmpty())
return;
// First collect the hints that will be needed when the node
// we materialize is still stored into other unescaped sink candidates.
// The way to interpret this vector is:
//
// PromotedHeapLocation(NotEscapedAllocation, field) = identifierAllocation
//
// e.g:
// PromotedHeapLocation(@PhantomNewFunction, FunctionActivationPLoc) = IdentifierOf(@MaterializeCreateActivation)
// or:
// PromotedHeapLocation(@PhantomCreateActivation, ClosureVarPLoc(x)) = IdentifierOf(@NewFunction)
//
// When the rhs of the `=` is to be materialized at this `where` point in the program
// and IdentifierOf(Materialization) is the original sunken allocation of the materialization.
//
// The reason we need to collect all the `identifiers` here is that
// we may materialize multiple versions of the allocation along control
// flow edges. We will PutHint these values along those edges. However,
// we also need to PutHint them when we join and have a Phi of the allocations.
Vector<std::pair<PromotedHeapLocation, Node*>> hints;
for (const auto& entry : m_heap.allocations()) {
if (escapees.contains(entry.key))
continue;
for (const auto& field : entry.value.fields()) {
ASSERT(m_sinkCandidates.contains(entry.key) || !escapees.contains(field.value));
auto iter = escapees.find(field.value);
if (iter != escapees.end()) {
ASSERT(m_sinkCandidates.contains(field.value));
hints.append(std::make_pair(PromotedHeapLocation(entry.key, field.key), field.value));
}
}
}
// Now we need to order the materialization. Any order is
// valid (as long as we materialize a node first if it is
// needed for the materialization of another node, e.g. a
// function's activation must be materialized before the
// function itself), but we want to try minimizing the number
// of times we have to place Puts to close cycles after a
// materialization. In other words, we are trying to find the
// minimum number of materializations to remove from the
// materialization graph to make it a DAG, known as the
// (vertex) feedback set problem. Unfortunately, this is a
// NP-hard problem, which we don't want to solve exactly.
//
// Instead, we use a simple greedy procedure, that procedes as
// follow:
// - While there is at least one node with no outgoing edge
// amongst the remaining materializations, materialize it
// first
//
// - Similarily, while there is at least one node with no
// incoming edge amongst the remaining materializations,
// materialize it last.
//
// - When both previous conditions are false, we have an
// actual cycle, and we need to pick a node to
// materialize. We try greedily to remove the "pressure" on
// the remaining nodes by choosing the node with maximum
// |incoming edges| * |outgoing edges| as a measure of how
// "central" to the graph it is. We materialize it first,
// so that all the recoveries will be Puts of things into
// it (rather than Puts of the materialization into other
// objects), which means we will have a single
// StoreBarrier.
// Compute dependencies between materializations
HashMap<Node*, NodeSet> dependencies;
HashMap<Node*, NodeSet> reverseDependencies;
HashMap<Node*, NodeSet> forMaterialization;
for (const auto& entry : escapees) {
auto& myDependencies = dependencies.add(entry.key, NodeSet()).iterator->value;
auto& myDependenciesForMaterialization = forMaterialization.add(entry.key, NodeSet()).iterator->value;
reverseDependencies.add(entry.key, NodeSet());
for (const auto& field : entry.value.fields()) {
if (escapees.contains(field.value) && field.value != entry.key) {
myDependencies.addVoid(field.value);
reverseDependencies.add(field.value, NodeSet()).iterator->value.addVoid(entry.key);
if (field.key.neededForMaterialization())
myDependenciesForMaterialization.addVoid(field.value);
}
}
}
// Helper function to update the materialized set and the
// dependencies
NodeSet materialized;
auto materialize = [&] (Node* identifier) {
materialized.addVoid(identifier);
for (Node* dep : dependencies.get(identifier))
reverseDependencies.find(dep)->value.remove(identifier);
for (Node* rdep : reverseDependencies.get(identifier)) {
dependencies.find(rdep)->value.remove(identifier);
forMaterialization.find(rdep)->value.remove(identifier);
}
dependencies.remove(identifier);
reverseDependencies.remove(identifier);
forMaterialization.remove(identifier);
};
// Nodes without remaining unmaterialized fields will be
// materialized first - amongst the remaining unmaterialized
// nodes
StdList<Allocation> toMaterialize;
auto firstPos = toMaterialize.begin();
auto materializeFirst = [&] (Allocation&& allocation) {
materialize(allocation.identifier());
// We need to insert *after* the current position
if (firstPos != toMaterialize.end())
++firstPos;
firstPos = toMaterialize.insert(firstPos, WTFMove(allocation));
};
// Nodes that no other unmaterialized node points to will be
// materialized last - amongst the remaining unmaterialized
// nodes
auto lastPos = toMaterialize.end();
auto materializeLast = [&] (Allocation&& allocation) {
materialize(allocation.identifier());
lastPos = toMaterialize.insert(lastPos, WTFMove(allocation));
};
// These are the promoted locations that contains some of the
// allocations we are currently escaping. If they are a location on
// some other allocation we are currently materializing, we will need
// to "recover" their value with a real put once the corresponding
// allocation is materialized; if they are a location on some other
// not-yet-materialized allocation, we will need a PutHint.
Vector<PromotedHeapLocation> toRecover;
// This loop does the actual cycle breaking
while (!escapees.isEmpty()) {
materialized.clear();
// Materialize nodes that won't require recoveries if we can
for (auto& entry : escapees) {
if (!forMaterialization.find(entry.key)->value.isEmpty())
continue;
if (dependencies.find(entry.key)->value.isEmpty()) {
materializeFirst(WTFMove(entry.value));
continue;
}
if (reverseDependencies.find(entry.key)->value.isEmpty()) {
materializeLast(WTFMove(entry.value));
continue;
}
}
// We reach this only if there is an actual cycle that needs
// breaking. Because we do not want to solve a NP-hard problem
// here, we just heuristically pick a node and materialize it
// first.
if (materialized.isEmpty()) {
uint64_t maxEvaluation = 0;
Allocation* bestAllocation = nullptr;
for (auto& entry : escapees) {
if (!forMaterialization.find(entry.key)->value.isEmpty())
continue;
uint64_t evaluation =
static_cast<uint64_t>(dependencies.get(entry.key).size()) * reverseDependencies.get(entry.key).size();
if (evaluation > maxEvaluation) {
maxEvaluation = evaluation;
bestAllocation = &entry.value;
}
}
RELEASE_ASSERT(maxEvaluation > 0);
materializeFirst(WTFMove(*bestAllocation));
}
RELEASE_ASSERT(!materialized.isEmpty());
for (Node* identifier : materialized)
escapees.remove(identifier);
}
materialized.clear();
NodeSet escaped;
for (const Allocation& allocation : toMaterialize)
escaped.addVoid(allocation.identifier());
for (const Allocation& allocation : toMaterialize) {
for (const auto& field : allocation.fields()) {
if (escaped.contains(field.value) && !materialized.contains(field.value))
toRecover.append(PromotedHeapLocation(allocation.identifier(), field.key));
}
materialized.addVoid(allocation.identifier());
}
Vector<Node*>& materializations = m_materializationSiteToMaterializations.add(
where, Vector<Node*>()).iterator->value;
for (const Allocation& allocation : toMaterialize) {
Node* materialization = createMaterialization(allocation, where);
materializations.append(materialization);
m_materializationToEscapee.add(materialization, allocation.identifier());
}
if (!toRecover.isEmpty()) {
m_materializationSiteToRecoveries.add(
where, Vector<PromotedHeapLocation>()).iterator->value.appendVector(toRecover);
}
// The hints need to be after the "real" recoveries so that we
// don't hint not-yet-complete objects
m_materializationSiteToHints.add(
where, Vector<std::pair<PromotedHeapLocation, Node*>>()).iterator->value.appendVector(hints);
}
Node* createMaterialization(const Allocation& allocation, Node* where)
{
// FIXME: This is the only place where we actually use the
// fact that an allocation's identifier is indeed the node
// that created the allocation.
switch (allocation.kind()) {
case Allocation::Kind::Object: {
ObjectMaterializationData* data = m_graph.m_objectMaterializationData.add();
return m_graph.addNode(
allocation.identifier()->prediction(), Node::VarArg, MaterializeNewObject,
where->origin.withSemantic(allocation.identifier()->origin.semantic),
OpInfo(m_graph.addStructureSet(allocation.structures())), OpInfo(data), 0, 0);
}
case Allocation::Kind::AsyncGeneratorFunction:
case Allocation::Kind::AsyncFunction:
case Allocation::Kind::GeneratorFunction:
case Allocation::Kind::Function: {
FrozenValue* executable = allocation.identifier()->cellOperand();
NodeType nodeType;
switch (allocation.kind()) {
case Allocation::Kind::GeneratorFunction:
nodeType = NewGeneratorFunction;
break;
case Allocation::Kind::AsyncGeneratorFunction:
nodeType = NewAsyncGeneratorFunction;
break;
case Allocation::Kind::AsyncFunction:
nodeType = NewAsyncFunction;
break;
default:
nodeType = NewFunction;
}
return m_graph.addNode(
allocation.identifier()->prediction(), nodeType,
where->origin.withSemantic(
allocation.identifier()->origin.semantic),
OpInfo(executable));
}
case Allocation::Kind::Activation: {
ObjectMaterializationData* data = m_graph.m_objectMaterializationData.add();
FrozenValue* symbolTable = allocation.identifier()->cellOperand();
return m_graph.addNode(
allocation.identifier()->prediction(), Node::VarArg, MaterializeCreateActivation,
where->origin.withSemantic(
allocation.identifier()->origin.semantic),
OpInfo(symbolTable), OpInfo(data), 0, 0);
}
case Allocation::Kind::RegExpObject: {
FrozenValue* regExp = allocation.identifier()->cellOperand();
return m_graph.addNode(
allocation.identifier()->prediction(), NewRegexp,
where->origin.withSemantic(
allocation.identifier()->origin.semantic),
OpInfo(regExp));
}
default:
DFG_CRASH(m_graph, allocation.identifier(), "Bad allocation kind");
}
}
void promoteLocalHeap()
{
// Collect the set of heap locations that we will be operating
// over.
HashSet<PromotedHeapLocation> locations;
for (BasicBlock* block : m_graph.blocksInNaturalOrder()) {
m_heap = m_heapAtHead[block];
for (Node* node : *block) {
handleNode(
node,
[&] (PromotedHeapLocation location, LazyNode) {
// If the location is not on a sink candidate,
// we only sink it if it is read
if (m_sinkCandidates.contains(location.base()))
locations.addVoid(location);
},
[&] (PromotedHeapLocation location) -> Node* {
locations.addVoid(location);
return nullptr;
});
}
}
// Figure out which locations belong to which allocations.
m_locationsForAllocation.clear();
for (PromotedHeapLocation location : locations) {
auto result = m_locationsForAllocation.add(
location.base(),
Vector<PromotedHeapLocation>());
ASSERT(!result.iterator->value.contains(location));
result.iterator->value.append(location);
}
m_pointerSSA.reset();
m_allocationSSA.reset();
// Collect the set of "variables" that we will be sinking.
m_locationToVariable.clear();
m_nodeToVariable.clear();
Vector<Node*> indexToNode;
Vector<PromotedHeapLocation> indexToLocation;
for (Node* index : m_sinkCandidates) {
SSACalculator::Variable* variable = m_allocationSSA.newVariable();
m_nodeToVariable.add(index, variable);
ASSERT(indexToNode.size() == variable->index());
indexToNode.append(index);
}
for (PromotedHeapLocation location : locations) {
SSACalculator::Variable* variable = m_pointerSSA.newVariable();
m_locationToVariable.add(location, variable);
ASSERT(indexToLocation.size() == variable->index());
indexToLocation.append(location);
}
// We insert all required constants at top of block 0 so that
// they are inserted only once and we don't clutter the graph
// with useless constants everywhere
HashMap<FrozenValue*, Node*> lazyMapping;
if (!m_bottom)
m_bottom = m_insertionSet.insertConstant(0, m_graph.block(0)->at(0)->origin, jsNumber(1927));
Vector<HashSet<PromotedHeapLocation>> hintsForPhi(m_sinkCandidates.size());
for (BasicBlock* block : m_graph.blocksInNaturalOrder()) {
m_heap = m_heapAtHead[block];
for (unsigned nodeIndex = 0; nodeIndex < block->size(); ++nodeIndex) {
Node* node = block->at(nodeIndex);
// Some named properties can be added conditionally,
// and that would necessitate bottoms
for (PromotedHeapLocation location : m_locationsForAllocation.get(node)) {
if (location.kind() != NamedPropertyPLoc)
continue;
SSACalculator::Variable* variable = m_locationToVariable.get(location);
m_pointerSSA.newDef(variable, block, m_bottom);
}
for (Node* materialization : m_materializationSiteToMaterializations.get(node)) {
Node* escapee = m_materializationToEscapee.get(materialization);
m_allocationSSA.newDef(m_nodeToVariable.get(escapee), block, materialization);
}
for (std::pair<PromotedHeapLocation, Node*> pair : m_materializationSiteToHints.get(node)) {
PromotedHeapLocation location = pair.first;
Node* identifier = pair.second;
// We're materializing `identifier` at this point, and the unmaterialized
// version is inside `location`. We track which SSA variable this belongs
// to in case we also need a PutHint for the Phi.
if (UNLIKELY(validationEnabled())) {
RELEASE_ASSERT(m_sinkCandidates.contains(location.base()));
RELEASE_ASSERT(m_sinkCandidates.contains(identifier));
bool found = false;
for (Node* materialization : m_materializationSiteToMaterializations.get(node)) {
// We're materializing `identifier` here. This asserts that this is indeed the case.
if (m_materializationToEscapee.get(materialization) == identifier) {
found = true;
break;
}
}
RELEASE_ASSERT(found);
}
SSACalculator::Variable* variable = m_nodeToVariable.get(identifier);
hintsForPhi[variable->index()].addVoid(location);
}
if (m_sinkCandidates.contains(node))
m_allocationSSA.newDef(m_nodeToVariable.get(node), block, node);
handleNode(
node,
[&] (PromotedHeapLocation location, LazyNode value) {
if (!locations.contains(location))
return;
Node* nodeValue;
if (value.isNode())
nodeValue = value.asNode();
else {
auto iter = lazyMapping.find(value.asValue());
if (iter != lazyMapping.end())
nodeValue = iter->value;
else {
nodeValue = value.ensureIsNode(
m_insertionSet, m_graph.block(0), 0);
lazyMapping.add(value.asValue(), nodeValue);
}
}
SSACalculator::Variable* variable = m_locationToVariable.get(location);
m_pointerSSA.newDef(variable, block, nodeValue);
},
[] (PromotedHeapLocation) -> Node* {
return nullptr;
});
}
}
m_insertionSet.execute(m_graph.block(0));
// Run the SSA calculators to create Phis
m_pointerSSA.computePhis(
[&] (SSACalculator::Variable* variable, BasicBlock* block) -> Node* {
PromotedHeapLocation location = indexToLocation[variable->index()];
// Don't create Phi nodes for fields of dead allocations
if (!m_heapAtHead[block].isAllocation(location.base()))
return nullptr;
// Don't create Phi nodes once we are escaped
if (m_heapAtHead[block].getAllocation(location.base()).isEscapedAllocation())
return nullptr;
// If we point to a single allocation, we will
// directly use its materialization
if (m_heapAtHead[block].follow(location))
return nullptr;
Node* phiNode = m_graph.addNode(SpecHeapTop, Phi, block->at(0)->origin.withInvalidExit());
phiNode->mergeFlags(NodeResultJS);
return phiNode;
});
m_allocationSSA.computePhis(
[&] (SSACalculator::Variable* variable, BasicBlock* block) -> Node* {
Node* identifier = indexToNode[variable->index()];
// Don't create Phi nodes for dead allocations
if (!m_heapAtHead[block].isAllocation(identifier))
return nullptr;
// Don't create Phi nodes until we are escaped
if (!m_heapAtHead[block].getAllocation(identifier).isEscapedAllocation())
return nullptr;
Node* phiNode = m_graph.addNode(SpecHeapTop, Phi, block->at(0)->origin.withInvalidExit());
phiNode->mergeFlags(NodeResultJS);
return phiNode;
});
// Place Phis in the right places, replace all uses of any load with the appropriate
// value, and create the materialization nodes.
LocalOSRAvailabilityCalculator availabilityCalculator(m_graph);
m_graph.clearReplacements();
for (BasicBlock* block : m_graph.blocksInPreOrder()) {
m_heap = m_heapAtHead[block];
availabilityCalculator.beginBlock(block);
// These mapping tables are intended to be lazy. If
// something is omitted from the table, it means that
// there haven't been any local stores to the promoted
// heap location (or any local materialization).
m_localMapping.clear();
m_escapeeToMaterialization.clear();
// Insert the Phi functions that we had previously
// created.
for (SSACalculator::Def* phiDef : m_pointerSSA.phisForBlock(block)) {
SSACalculator::Variable* variable = phiDef->variable();
m_insertionSet.insert(0, phiDef->value());
PromotedHeapLocation location = indexToLocation[variable->index()];
m_localMapping.set(location, phiDef->value());
if (m_sinkCandidates.contains(location.base())) {
m_insertionSet.insert(
0,
location.createHint(
m_graph, block->at(0)->origin.withInvalidExit(), phiDef->value()));
}
}
for (SSACalculator::Def* phiDef : m_allocationSSA.phisForBlock(block)) {
SSACalculator::Variable* variable = phiDef->variable();
m_insertionSet.insert(0, phiDef->value());
Node* identifier = indexToNode[variable->index()];
m_escapeeToMaterialization.add(identifier, phiDef->value());
bool canExit = false;
insertOSRHintsForUpdate(
0, block->at(0)->origin, canExit,
availabilityCalculator.m_availability, identifier, phiDef->value());
for (PromotedHeapLocation location : hintsForPhi[variable->index()]) {
m_insertionSet.insert(0,
location.createHint(m_graph, block->at(0)->origin.withInvalidExit(), phiDef->value()));
m_localMapping.set(location, phiDef->value());
}
}
if (DFGObjectAllocationSinkingPhaseInternal::verbose) {
dataLog("Local mapping at ", pointerDump(block), ": ", mapDump(m_localMapping), "\n");
dataLog("Local materializations at ", pointerDump(block), ": ", mapDump(m_escapeeToMaterialization), "\n");
}
for (unsigned nodeIndex = 0; nodeIndex < block->size(); ++nodeIndex) {
Node* node = block->at(nodeIndex);
bool canExit = true;
bool nextCanExit = node->origin.exitOK;
for (PromotedHeapLocation location : m_locationsForAllocation.get(node)) {
if (location.kind() != NamedPropertyPLoc)
continue;
m_localMapping.set(location, m_bottom);
if (m_sinkCandidates.contains(node)) {
if (DFGObjectAllocationSinkingPhaseInternal::verbose)
dataLog("For sink candidate ", node, " found location ", location, "\n");
m_insertionSet.insert(
nodeIndex + 1,
location.createHint(
m_graph, node->origin.takeValidExit(nextCanExit), m_bottom));
}
}
for (Node* materialization : m_materializationSiteToMaterializations.get(node)) {
materialization->origin.exitOK &= canExit;
Node* escapee = m_materializationToEscapee.get(materialization);
populateMaterialization(block, materialization, escapee);
m_escapeeToMaterialization.set(escapee, materialization);
m_insertionSet.insert(nodeIndex, materialization);
if (DFGObjectAllocationSinkingPhaseInternal::verbose)
dataLog("Materializing ", escapee, " => ", materialization, " at ", node, "\n");
}
for (PromotedHeapLocation location : m_materializationSiteToRecoveries.get(node))
m_insertionSet.insert(nodeIndex, createRecovery(block, location, node, canExit));
for (std::pair<PromotedHeapLocation, Node*> pair : m_materializationSiteToHints.get(node))
m_insertionSet.insert(nodeIndex, createRecovery(block, pair.first, node, canExit));
// We need to put the OSR hints after the recoveries,
// because we only want the hints once the object is
// complete
for (Node* materialization : m_materializationSiteToMaterializations.get(node)) {
Node* escapee = m_materializationToEscapee.get(materialization);
insertOSRHintsForUpdate(
nodeIndex, node->origin, canExit,
availabilityCalculator.m_availability, escapee, materialization);
}
if (node->origin.exitOK && !canExit) {
// We indicate that the exit state is fine now. It is OK because we updated the
// state above. We need to indicate this manually because the validation doesn't
// have enough information to infer that the exit state is fine.
m_insertionSet.insertNode(nodeIndex, SpecNone, ExitOK, node->origin);
}
if (m_sinkCandidates.contains(node))
m_escapeeToMaterialization.set(node, node);
availabilityCalculator.executeNode(node);
bool desiredNextExitOK = node->origin.exitOK && !clobbersExitState(m_graph, node);
bool doLower = false;
handleNode(
node,
[&] (PromotedHeapLocation location, LazyNode value) {
if (!locations.contains(location))
return;
Node* nodeValue;
if (value.isNode())
nodeValue = value.asNode();
else
nodeValue = lazyMapping.get(value.asValue());
nodeValue = resolve(block, nodeValue);
m_localMapping.set(location, nodeValue);
if (!m_sinkCandidates.contains(location.base()))
return;
doLower = true;
if (DFGObjectAllocationSinkingPhaseInternal::verbose)
dataLog("Creating hint with value ", nodeValue, " before ", node, "\n");
m_insertionSet.insert(
nodeIndex + 1,
location.createHint(
m_graph, node->origin.takeValidExit(nextCanExit), nodeValue));
},
[&] (PromotedHeapLocation location) -> Node* {
return resolve(block, location);
});
if (!nextCanExit && desiredNextExitOK) {
// We indicate that the exit state is fine now. We need to do this because we
// emitted hints that appear to invalidate the exit state.
m_insertionSet.insertNode(nodeIndex + 1, SpecNone, ExitOK, node->origin);
}
if (m_sinkCandidates.contains(node) || doLower) {
switch (node->op()) {
case NewObject:
node->convertToPhantomNewObject();
break;
case NewFunction:
node->convertToPhantomNewFunction();
break;
case NewGeneratorFunction:
node->convertToPhantomNewGeneratorFunction();
break;
case NewAsyncGeneratorFunction:
node->convertToPhantomNewAsyncGeneratorFunction();
break;
case NewAsyncFunction:
node->convertToPhantomNewAsyncFunction();
break;
case CreateActivation:
node->convertToPhantomCreateActivation();
break;
case NewRegexp:
node->convertToPhantomNewRegexp();
break;
default:
node->remove(m_graph);
break;
}
}
m_graph.doToChildren(
node,
[&] (Edge& edge) {
edge.setNode(resolve(block, edge.node()));
});
}
// Gotta drop some Upsilons.
NodeAndIndex terminal = block->findTerminal();
size_t upsilonInsertionPoint = terminal.index;
NodeOrigin upsilonOrigin = terminal.node->origin;
for (BasicBlock* successorBlock : block->successors()) {
for (SSACalculator::Def* phiDef : m_pointerSSA.phisForBlock(successorBlock)) {
Node* phiNode = phiDef->value();
SSACalculator::Variable* variable = phiDef->variable();
PromotedHeapLocation location = indexToLocation[variable->index()];
Node* incoming = resolve(block, location);
m_insertionSet.insertNode(
upsilonInsertionPoint, SpecNone, Upsilon, upsilonOrigin,
OpInfo(phiNode), incoming->defaultEdge());
}
for (SSACalculator::Def* phiDef : m_allocationSSA.phisForBlock(successorBlock)) {
Node* phiNode = phiDef->value();
SSACalculator::Variable* variable = phiDef->variable();
Node* incoming = getMaterialization(block, indexToNode[variable->index()]);
m_insertionSet.insertNode(
upsilonInsertionPoint, SpecNone, Upsilon, upsilonOrigin,
OpInfo(phiNode), incoming->defaultEdge());
}
}
m_insertionSet.execute(block);
}
}
NEVER_INLINE Node* resolve(BasicBlock* block, PromotedHeapLocation location)
{
// If we are currently pointing to a single local allocation,
// simply return the associated materialization.
if (Node* identifier = m_heap.follow(location))
return getMaterialization(block, identifier);
if (Node* result = m_localMapping.get(location))
return result;
// This implies that there is no local mapping. Find a non-local mapping.
SSACalculator::Def* def = m_pointerSSA.nonLocalReachingDef(
block, m_locationToVariable.get(location));
ASSERT(def);
ASSERT(def->value());
Node* result = def->value();
if (result->replacement())
result = result->replacement();
ASSERT(!result->replacement());
m_localMapping.add(location, result);
return result;
}
NEVER_INLINE Node* resolve(BasicBlock* block, Node* node)
{
// If we are currently pointing to a single local allocation,
// simply return the associated materialization.
if (Node* identifier = m_heap.follow(node))
return getMaterialization(block, identifier);
if (node->replacement())
node = node->replacement();
ASSERT(!node->replacement());
return node;
}
NEVER_INLINE Node* getMaterialization(BasicBlock* block, Node* identifier)
{
ASSERT(m_heap.isAllocation(identifier));
if (!m_sinkCandidates.contains(identifier))
return identifier;
if (Node* materialization = m_escapeeToMaterialization.get(identifier))
return materialization;
SSACalculator::Def* def = m_allocationSSA.nonLocalReachingDef(
block, m_nodeToVariable.get(identifier));
ASSERT(def && def->value());
m_escapeeToMaterialization.add(identifier, def->value());
ASSERT(!def->value()->replacement());
return def->value();
}
void insertOSRHintsForUpdate(unsigned nodeIndex, NodeOrigin origin, bool& canExit, AvailabilityMap& availability, Node* escapee, Node* materialization)
{
if (DFGObjectAllocationSinkingPhaseInternal::verbose) {
dataLog("Inserting OSR hints at ", origin, ":\n");
dataLog(" Escapee: ", escapee, "\n");
dataLog(" Materialization: ", materialization, "\n");
dataLog(" Availability: ", availability, "\n");
}
// We need to follow() the value in the heap.
// Consider the following graph:
//
// Block #0
// 0: NewObject({})
// 1: NewObject({})
// -: PutByOffset(@0, @1, x:0)
// -: PutStructure(@0, {x:0})
// 2: GetByOffset(@0, x:0)
// -: MovHint(@2, loc1)
// -: Branch(#1, #2)
//
// Block #1
// 3: Call(f, @1)
// 4: Return(@0)
//
// Block #2
// -: Return(undefined)
//
// We need to materialize @1 at @3, and when doing so we need
// to insert a MovHint for the materialization into loc1 as
// well.
// In order to do this, we say that we need to insert an
// update hint for any availability whose node resolve()s to
// the materialization.
for (auto entry : availability.m_heap) {
if (!entry.value.hasNode())
continue;
if (m_heap.follow(entry.value.node()) != escapee)
continue;
m_insertionSet.insert(
nodeIndex,
entry.key.createHint(m_graph, origin.takeValidExit(canExit), materialization));
}
for (unsigned i = availability.m_locals.size(); i--;) {
if (!availability.m_locals[i].hasNode())
continue;
if (m_heap.follow(availability.m_locals[i].node()) != escapee)
continue;
int operand = availability.m_locals.operandForIndex(i);
m_insertionSet.insertNode(
nodeIndex, SpecNone, MovHint, origin.takeValidExit(canExit), OpInfo(operand),
materialization->defaultEdge());
}
}
void populateMaterialization(BasicBlock* block, Node* node, Node* escapee)
{
Allocation& allocation = m_heap.getAllocation(escapee);
switch (node->op()) {
case MaterializeNewObject: {
ObjectMaterializationData& data = node->objectMaterializationData();
unsigned firstChild = m_graph.m_varArgChildren.size();
Vector<PromotedHeapLocation> locations = m_locationsForAllocation.get(escapee);
PromotedHeapLocation structure(StructurePLoc, allocation.identifier());
ASSERT(locations.contains(structure));
m_graph.m_varArgChildren.append(Edge(resolve(block, structure), KnownCellUse));
for (PromotedHeapLocation location : locations) {
switch (location.kind()) {
case StructurePLoc:
ASSERT(location == structure);
break;
case NamedPropertyPLoc: {
ASSERT(location.base() == allocation.identifier());
data.m_properties.append(location.descriptor());
Node* value = resolve(block, location);
if (m_sinkCandidates.contains(value))
m_graph.m_varArgChildren.append(m_bottom);
else
m_graph.m_varArgChildren.append(value);
break;
}
default:
DFG_CRASH(m_graph, node, "Bad location kind");
}
}
node->children = AdjacencyList(
AdjacencyList::Variable,
firstChild, m_graph.m_varArgChildren.size() - firstChild);
break;
}
case MaterializeCreateActivation: {
ObjectMaterializationData& data = node->objectMaterializationData();
unsigned firstChild = m_graph.m_varArgChildren.size();
Vector<PromotedHeapLocation> locations = m_locationsForAllocation.get(escapee);
PromotedHeapLocation symbolTable(ActivationSymbolTablePLoc, allocation.identifier());
ASSERT(locations.contains(symbolTable));
ASSERT(node->cellOperand() == resolve(block, symbolTable)->constant());
m_graph.m_varArgChildren.append(Edge(resolve(block, symbolTable), KnownCellUse));
PromotedHeapLocation scope(ActivationScopePLoc, allocation.identifier());
ASSERT(locations.contains(scope));
m_graph.m_varArgChildren.append(Edge(resolve(block, scope), KnownCellUse));
for (PromotedHeapLocation location : locations) {
switch (location.kind()) {
case ActivationScopePLoc: {
ASSERT(location == scope);
break;
}
case ActivationSymbolTablePLoc: {
ASSERT(location == symbolTable);
break;
}
case ClosureVarPLoc: {
ASSERT(location.base() == allocation.identifier());
data.m_properties.append(location.descriptor());
Node* value = resolve(block, location);
if (m_sinkCandidates.contains(value))
m_graph.m_varArgChildren.append(m_bottom);
else
m_graph.m_varArgChildren.append(value);
break;
}
default:
DFG_CRASH(m_graph, node, "Bad location kind");
}
}
node->children = AdjacencyList(
AdjacencyList::Variable,
firstChild, m_graph.m_varArgChildren.size() - firstChild);
break;
}
case NewFunction:
case NewGeneratorFunction:
case NewAsyncGeneratorFunction:
case NewAsyncFunction: {
Vector<PromotedHeapLocation> locations = m_locationsForAllocation.get(escapee);
ASSERT(locations.size() == 2);
PromotedHeapLocation executable(FunctionExecutablePLoc, allocation.identifier());
ASSERT_UNUSED(executable, locations.contains(executable));
PromotedHeapLocation activation(FunctionActivationPLoc, allocation.identifier());
ASSERT(locations.contains(activation));
node->child1() = Edge(resolve(block, activation), KnownCellUse);
break;
}
case NewRegexp: {
Vector<PromotedHeapLocation> locations = m_locationsForAllocation.get(escapee);
ASSERT(locations.size() == 2);
PromotedHeapLocation regExp(RegExpObjectRegExpPLoc, allocation.identifier());
ASSERT_UNUSED(regExp, locations.contains(regExp));
PromotedHeapLocation lastIndex(RegExpObjectLastIndexPLoc, allocation.identifier());
ASSERT(locations.contains(lastIndex));
Node* value = resolve(block, lastIndex);
if (m_sinkCandidates.contains(value))
node->child1() = Edge(m_bottom);
else
node->child1() = Edge(value);
break;
}
default:
DFG_CRASH(m_graph, node, "Bad materialize op");
}
}
Node* createRecovery(BasicBlock* block, PromotedHeapLocation location, Node* where, bool& canExit)
{
if (DFGObjectAllocationSinkingPhaseInternal::verbose)
dataLog("Recovering ", location, " at ", where, "\n");
ASSERT(location.base()->isPhantomAllocation());
Node* base = getMaterialization(block, location.base());
Node* value = resolve(block, location);
NodeOrigin origin = where->origin.withSemantic(base->origin.semantic);
if (DFGObjectAllocationSinkingPhaseInternal::verbose)
dataLog("Base is ", base, " and value is ", value, "\n");
if (base->isPhantomAllocation()) {
return PromotedHeapLocation(base, location.descriptor()).createHint(
m_graph, origin.takeValidExit(canExit), value);
}
switch (location.kind()) {
case NamedPropertyPLoc: {
Allocation& allocation = m_heap.getAllocation(location.base());
Vector<RegisteredStructure> structures;
structures.appendRange(allocation.structures().begin(), allocation.structures().end());
unsigned identifierNumber = location.info();
UniquedStringImpl* uid = m_graph.identifiers()[identifierNumber];
std::sort(
structures.begin(),
structures.end(),
[uid] (RegisteredStructure a, RegisteredStructure b) -> bool {
return a->getConcurrently(uid) < b->getConcurrently(uid);
});
RELEASE_ASSERT(structures.size());
PropertyOffset firstOffset = structures[0]->getConcurrently(uid);
if (firstOffset == structures.last()->getConcurrently(uid)) {
Node* storage = base;
// FIXME: When we decide to sink objects with a
// property storage, we should handle non-inline offsets.
RELEASE_ASSERT(isInlineOffset(firstOffset));
StorageAccessData* data = m_graph.m_storageAccessData.add();
data->offset = firstOffset;
data->identifierNumber = identifierNumber;
return m_graph.addNode(
PutByOffset,
origin.takeValidExit(canExit),
OpInfo(data),
Edge(storage, KnownCellUse),
Edge(base, KnownCellUse),
value->defaultEdge());
}
MultiPutByOffsetData* data = m_graph.m_multiPutByOffsetData.add();
data->identifierNumber = identifierNumber;
{
PropertyOffset currentOffset = firstOffset;
StructureSet currentSet;
for (RegisteredStructure structure : structures) {
PropertyOffset offset = structure->getConcurrently(uid);
if (offset != currentOffset) {
// Because our analysis treats MultiPutByOffset like an escape, we only have to
// deal with storing results that would have been previously stored by PutByOffset
// nodes. Those nodes were guarded by the appropriate type checks. This means that
// at this point, we can simply trust that the incoming value has the right type
// for whatever structure we are using.
data->variants.append(
PutByIdVariant::replace(currentSet, currentOffset, InferredType::Top));
currentOffset = offset;
currentSet.clear();
}
currentSet.add(structure.get());
}
data->variants.append(
PutByIdVariant::replace(currentSet, currentOffset, InferredType::Top));
}
return m_graph.addNode(
MultiPutByOffset,
origin.takeValidExit(canExit),
OpInfo(data),
Edge(base, KnownCellUse),
value->defaultEdge());
}
case ClosureVarPLoc: {
return m_graph.addNode(
PutClosureVar,
origin.takeValidExit(canExit),
OpInfo(location.info()),
Edge(base, KnownCellUse),
value->defaultEdge());
}
case RegExpObjectLastIndexPLoc: {
return m_graph.addNode(
SetRegExpObjectLastIndex,
origin.takeValidExit(canExit),
OpInfo(true),
Edge(base, KnownCellUse),
value->defaultEdge());
}
default:
DFG_CRASH(m_graph, base, "Bad location kind");
break;
}
RELEASE_ASSERT_NOT_REACHED();
}
// This is a great way of asking value->isStillValid() without having to worry about getting
// different answers. It turns out that this analysis works OK regardless of what this
// returns but breaks badly if this changes its mind for any particular InferredValue. This
// method protects us from that.
bool isStillValid(InferredValue* value)
{
return m_validInferredValues.add(value, value->isStillValid()).iterator->value;
}
SSACalculator m_pointerSSA;
SSACalculator m_allocationSSA;
NodeSet m_sinkCandidates;
HashMap<PromotedHeapLocation, SSACalculator::Variable*> m_locationToVariable;
HashMap<Node*, SSACalculator::Variable*> m_nodeToVariable;
HashMap<PromotedHeapLocation, Node*> m_localMapping;
HashMap<Node*, Node*> m_escapeeToMaterialization;
InsertionSet m_insertionSet;
CombinedLiveness m_combinedLiveness;
HashMap<InferredValue*, bool> m_validInferredValues;
HashMap<Node*, Node*> m_materializationToEscapee;
HashMap<Node*, Vector<Node*>> m_materializationSiteToMaterializations;
HashMap<Node*, Vector<PromotedHeapLocation>> m_materializationSiteToRecoveries;
HashMap<Node*, Vector<std::pair<PromotedHeapLocation, Node*>>> m_materializationSiteToHints;
HashMap<Node*, Vector<PromotedHeapLocation>> m_locationsForAllocation;
BlockMap<LocalHeap> m_heapAtHead;
BlockMap<LocalHeap> m_heapAtTail;
LocalHeap m_heap;
Node* m_bottom = nullptr;
};
}
bool performObjectAllocationSinking(Graph& graph)
{
return runPhase<ObjectAllocationSinkingPhase>(graph);
}
} } // namespace JSC::DFG
#endif // ENABLE(DFG_JIT)