295171 2025-06-28 23:30:28 -0700 Padding bits defeat intended HTMLEntityTableEntry size optimization 2025-07-05 23:31:13 -0700 1 1 1 Unclassified WebKit DOM WebKit Nightly Build Unspecified Unspecified ASSIGNED InRadar P2 Normal --- 1 squeek502 darin cdumez darin nathan_solomon rreno webkit-bug-importer oldest_to_newest 2126561 0 squeek502 2025-06-28 23:30:28 -0700 In https://github.com/WebKit/WebKit/pull/11089, HTMLEntityTableEntry was converted to a bit-field in order to decrease the size of kStaticEntityTable. HTMLEntityTableEntry theoretically uses 57 bits, but, due to padding bits, the size of HTMLEntityTableEntry is actually 12 bytes instead of 8 bytes (the same size it was before the bit-field change). Test code: struct HTMLEntityTableEntry { unsigned firstCharacter : 21; unsigned optionalSecondCharacter : 16; unsigned nameCharactersOffset : 14; unsigned nameLengthExcludingSemicolon : 5; unsigned nameIncludesTrailingSemicolon : 1; }; static_assert(sizeof(HTMLEntityTableEntry) == 8); Clang 20.1.3: HTMLEntityTableEntry.cpp:8:15: error: static assertion failed due to requirement 'sizeof(HTMLEntityTableEntry) == 8' 8 | static_assert(sizeof(HTMLEntityTableEntry) == 8); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ HTMLEntityTableEntry.cpp:8:44: note: expression evaluates to '12 == 8' 8 | static_assert(sizeof(HTMLEntityTableEntry) == 8); | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~ 1 error generated. GCC 11.4.0: HTMLEntityTableEntry.cpp:8:44: error: static assertion failed 8 | static_assert(sizeof(HTMLEntityTableEntry) == 8); | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~ (for context, this was found while I was writing https://www.ryanliptak.com/blog/better-named-character-reference-tokenization/) 2127128 1 darin 2025-07-01 14:50:14 -0700 I tried moving optionalSecondCharacter to be the last member of the HTMLEntityTableEntry structure and that worked; just have to revise the code that builds the table to write out the members in the right order. 2127998 2 webkit-bug-importer 2025-07-05 23:31:13 -0700 <rdar://problem/155138028>