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[heap] String::MakeThin can get away without NotifyObjectLayoutChange

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String::MakeThin doesn't need to invoke NotifyObjectLayoutChange because
ThinString will only introduce tagged values and hence will not
overwrite recorded slots with untagged values.

Bug: v8:10315
Change-Id: Iaff9c06cef763462eb57bf3debc5183ae8db6fa0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2448792
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70321}
This commit is contained in:
Dominik Inführ 2020-10-05 16:33:52 +02:00 committed by Commit Bot
parent a024ea4ba7
commit 6e621f84ba
2 changed files with 7 additions and 5 deletions
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8
src/heap/heap.cc
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@ -3518,13 +3518,19 @@ class SlotCollectingVisitor final : public ObjectVisitor {
void Heap::VerifyObjectLayoutChange(HeapObject object, Map new_map) {
if (!FLAG_verify_heap) return;
// Check that Heap::NotifyObjectLayout was called for object transitions
// Check that Heap::NotifyObjectLayoutChange was called for object transitions
// that are not safe for concurrent marking.
// If you see this check triggering for a freshly allocated object,
// use object->set_map_after_allocation() to initialize its map.
if (pending_layout_change_object_.is_null()) {
if (object.IsJSObject()) {
DCHECK(!object.map().TransitionRequiresSynchronizationWithGC(new_map));
} else if (object.IsString() &&
(new_map == ReadOnlyRoots(this).thin_string_map() ||
new_map == ReadOnlyRoots(this).thin_one_byte_string_map())) {
// When transitioning a string to ThinString,
// Heap::NotifyObjectLayoutChange doesn't need to be invoked because only
// tagged fields are introduced.
} else {
// Check that the set of slots before and after the transition match.
SlotCollectingVisitor old_visitor;

4
src/objects/string.cc
View File

@ -117,10 +117,6 @@ void String::MakeThin(Isolate* isolate, String internalized) {
bool has_pointers = StringShape(*this).IsIndirect();
int old_size = this->Size();
// Slot invalidation is not necessary here: ThinString only stores tagged
// value, so it can't store an untagged value in a recorded slot.
isolate->heap()->NotifyObjectLayoutChange(*this, no_gc,
InvalidateRecordedSlots::kNo);
bool one_byte = internalized.IsOneByteRepresentation();
Handle<Map> map = one_byte ? isolate->factory()->thin_one_byte_string_map()
: isolate->factory()->thin_string_map();