[5.0] Remove the extra-inhabitant value witnesses #21240
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This is essentially a long-belated follow-up to Arnold's swiftlang#12606. The key observation here is that the enum-tag-single-payload witnesses are strictly more powerful than the XI witnesses: you can simulate the XI witnesses by using an extra case count that's <= the XI count. Of course the result is less efficient than the XI witnesses, but that's less important than overall code size, and we can work on fast-paths for that. The extra inhabitant count is stored in a 32-bit field (always present) following the ValueWitnessFlags, which now occupy a fixed 32 bits. This inflates non-XI VWTs on 32-bit targets by a word, but the net effect on XI VWTs is to shrink them by two words, which is likely to be the more important change. Also, being able to access the XI count directly should be a nice win.
This allows callers to avoid needing to reload these tags in common cases.
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@swift-ci Please test |
AnnaZaks
approved these changes
Dec 12, 2018
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This is essentially a long-belated follow-up to Arnold's #12606. The key observation here is that the enum-tag-single-payload witnesses are strictly more powerful than the XI witnesses: you can simulate
the XI witnesses by using an extra case count that's <= the XI count. Of course the result is less efficient than the XI witnesses when actually running generic code, but that's less important than overall code size, and we can work on fast-paths in the future.
The extra inhabitant count is stored in a 32-bit field (always present) following the
ValueWitnessFlags, which now occupy a fixed 32 bits. This inflates non-XI VWTs on 32-bit targets by a word, but the net effect on XI VWTs is to shrink them by two words, which is likely to be the more important change. Also, being able to access the XI count directly should be a nice win.This is the 5.0 version of #21204. It is ABI-changing and important for convergence to take early.