Currently, in the following struct
struct LayoutObject : GarbageCollected<>, MixinA, MixinB {};
the subobject that corresponds to the first base GarbageCollected<>
always takes up some space (one word). The empty-base-optimization
doesn't happen because the second base (MixinA) has the same subobject
as the first base (GarbageCollected), which is the most parent class
GarbageCollectedBase. The compiler can't "merge" them because it must
guarantee that distinct objects of the same type have distinct
addresses.
The attribute [[no_unique_address]] doesn't work for base classes,
unfortunately (but is a good idea for a Standard proposal). As a
solution, the CL simply removes GarbageCollectedBase.
Bug: chromium:1260797
Change-Id: I415b10a5fbcebce3d6ee97b8870ea9ae90f383a8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3259654
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77693}
92eae6d126