[build] JSG: store struct field wrappers in a flat array - #7249
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Now that all of a struct's field wrappers hold nothing but an interned name, they no longer need to be distinct types. Move the field type from FieldWrapper's template arguments onto wrap()/unwrap() and keep the wrappers in a kj::Array instead of a kj::Tuple. Using an array allows us to avoid templating based on the list of nested ExpandAndApplyFunc types seen so far. Field access stays direct: the `fields` pack still holds pointers to members, so `in.*fields` is a constant member offset, and the field type still comes from the pack via FieldType, so no dispatch becomes indirect.
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LGTM |
jasnell
approved these changes
Sep 4, 2026
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Now that all of a struct's field wrappers hold nothing but an interned name, they no longer need to be distinct types. Move the field type from FieldWrapper's template arguments onto wrap()/unwrap() and keep the wrappers in a kj::Array instead of a kj::Tuple. Using an array allows us to avoid templating based on the list of nested ExpandAndApplyFunc types seen so far.
Field access stays direct: the
fieldspack still holds pointers to members, soin.*fieldsis a constant member offset, and the field type still comes from the pack via FieldType, so no dispatch becomes indirect.================
Follow-up to #7248. This further reduces code size by reducing excessive template instantiations. Compile time (which I think people care about more) also improves. Part of a 3-part series to debloat workerd-api. Code changes made using an LLM, fully reviewed and edited by me.
No optimization, limited debug info, before change (i.e. with just #7248):
No optimization, limited debug info, after:
When compiling server directory with -O3, before:
When compiling server directory with -O3, after: