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Reland "Document RootIndexMap's requirement for immovable roots."

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This is a reland of bb881c8880
Original change's description:
> Document RootIndexMap's requirement for immovable roots.
> 
> R=mstarzinger@chromium.org
> 
> Change-Id: I27cf279c77204becc449221e1bcebeacbc275792
> Reviewed-on: https://chromium-review.googlesource.com/565281
> Commit-Queue: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46517}

Change-Id: I6eeb68cee3bd2dff0eb6696bbe43a4d602ab960d
Reviewed-on: https://chromium-review.googlesource.com/566458
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46548}
This commit is contained in:
Yang Guo 2017-07-11 08:27:59 +02:00 committed by Commit Bot
parent bfba293c8d
commit 39bfd31588
2 changed files with 7 additions and 2 deletions
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2
src/address-map.cc
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@ -20,6 +20,8 @@ RootIndexMap::RootIndexMap(Isolate* isolate) {
if (!root->IsHeapObject()) continue;
// Omit root entries that can be written after initialization. They must
// not be referenced through the root list in the snapshot.
// Since we map the raw address of an root item to its root list index, the
// raw address must be constant, i.e. the object must be immovable.
if (isolate->heap()->RootCanBeTreatedAsConstant(root_index)) {
HeapObject* heap_object = HeapObject::cast(root);
Maybe<uint32_t> maybe_index = map_->Get(heap_object);

7
src/heap/heap.cc
View File

@ -2908,6 +2908,7 @@ bool Heap::RootCanBeWrittenAfterInitialization(Heap::RootListIndex root_index) {
case kPublicSymbolTableRootIndex:
case kApiSymbolTableRootIndex:
case kApiPrivateSymbolTableRootIndex:
case kMessageListenersRootIndex:
// Smi values
#define SMI_ENTRY(type, name, Name) case k##Name##RootIndex:
SMI_ROOT_LIST(SMI_ENTRY)
@ -2922,8 +2923,10 @@ bool Heap::RootCanBeWrittenAfterInitialization(Heap::RootListIndex root_index) {
}
bool Heap::RootCanBeTreatedAsConstant(RootListIndex root_index) {
return !RootCanBeWrittenAfterInitialization(root_index) &&
!InNewSpace(root(root_index));
bool can_be = !RootCanBeWrittenAfterInitialization(root_index) &&
!InNewSpace(root(root_index));
DCHECK_IMPLIES(can_be, IsImmovable(HeapObject::cast(root(root_index))));
return can_be;
}
int Heap::FullSizeNumberStringCacheLength() {