[GPU Process] Encoding buffer for DisplayList items should be aligned to 8 bytes

[Said Abou-Hallawa]

ItemBuffer::append() allocates a static buffer whose size = sizeof(DisplayList::Item) + uint64_t for the DisplayList::ItemType. But this static buffer is of type uint8_t. So this array should be aligned to the alignment of a single uint8_t which is 1.

The expectation is to have this buffer aligned to 8 bytes so all each member in the encoded DisplayList::Item can be aligned to its alignment requirement.

This was caught on Apple Silicon when trying to enable GPU rendering for 2D canvas for layout tests. See https://build.webkit.org/#/builders/103/builds/88. But the same assertion fires if the same page is just opened in mini browser while GPU rendering for 2D canvas is enabled.

[Said Abou-Hallawa]

Created attachment 422983 [details]
Patch

[Geoffrey Garen]

Comment on attachment 422983 [details]
Patch

I think modern C++ has a nicer explicit feature to just say the alignment you want. alignas, maybe?

[Said Abou-Hallawa]

Created attachment 422987 [details]
Patch

[Said Abou-Hallawa]

Created attachment 422998 [details]
Patch

[Said Abou-Hallawa]

Created attachment 423002 [details]
Patch

[Said Abou-Hallawa]

Created attachment 423004 [details]
Patch

[EWS]

commit-queue failed to commit attachment 423004 [details] to WebKit repository. To retry, please set cq+ flag again.

[Said Abou-Hallawa]

Created attachment 423012 [details]
Patch

[EWS]

Committed r274326: <https://commits.webkit.org/r274326>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 423012 [details].

[Radar WebKit Bug Importer]

<rdar://problem/75348461>