Not much help or documentation from Google here. I suppose they expect
everybody just to Google it. Anyway, just in case if anyone is interested,
here's what's going on here.
The method `v8::internal::GlobalHandles::Node::CollectPhantomCallbackData`
packs internal fields in this loop:
if (weakness_type() != PHANTOM_WEAK && object()->IsJSObject()) {
auto jsobject = JSObject::cast(object());
int field_count = jsobject->GetEmbedderFieldCount();
for (int i = 0; i < v8::kEmbedderFieldsInWeakCallback; ++i) {
if (field_count == i) break;
auto field = jsobject->GetEmbedderField(i);
if (field->IsSmi()) embedder_fields[i] = field;
}
}
The call `IsSmi` checks whether the least significant bit is zero or not,
as follows:
const int kSmiTag = 0;
const int kSmiTagSize = 1;
const intptr_t kSmiTagMask = (1 << kSmiTagSize) - 1;
...
#define HAS_SMI_TAG(value) \
((reinterpret_cast<intptr_t>(value) & ::i::kSmiTagMask) == ::i::kSmiTag)
So, internal fields in weak handle callbacks only passed in as non-NULL
values for v8 integer object type, which has this bit set, and not for
arbitrary object types.
A comment about this around where `kInternalFields` is defined would be
nice.
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