Add runtime functions to compute tuple layouts from element layouts #18342
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rjmccall
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Previously, when a tuple type had non-fixed layout, we would compute a layout by building the metadata for that tuple type and then extracting the layout from the VWT. This can be quite expensive because it involves constructing the exact metadata for types like arrays and functions despite those types being fixed-layout across all instantiations. It also tends to cause unnecessary recursive-type issues, especially with enums where tuples are currently used to model cases with mutliple payloads. Since we just need a layout, computing it directly from element layouts instead of constructing metadata for the formal type lets us take advantage of all the other fast paths for layout construction, e.g. for fixed types and single-field aggregates. This is a good improvement overall, but it also serves to alleviate some of the problems of rdar://40810002 / SR-7876 in a way that might be suitable for integration to 4.2.
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Previously, when a tuple type had non-fixed layout, we would compute a layout by building the metadata for that tuple type and then extracting the layout from the VWT. This can be quite expensive because it involves constructing the exact metadata for types like arrays and functions despite those types being fixed-layout across all instantiations. It also tends to cause unnecessary recursive-type issues, especially with enums where tuples are currently used to model cases with mutliple payloads. Since we just need a layout, computing it directly from element layouts instead of constructing metadata for the formal type lets us take advantage of all the other fast paths for layout construction, e.g. for fixed types and single-field aggregates.
This is a good improvement overall, but it also serves to alleviate some of the problems of rdar://40810002 / SR-7876 in a way that might be suitable for integration to 4.2.