Win64 ABI: Pass/return delegates like slices - in (up to) 2 GP registers #3609
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kinke
commented
Nov 7, 2020
| // LDC-specific exceptions: slices and delegates are left alone (as non- | ||
| // rewritten IR structs) and passed/returned as 2 separate args => passed in | ||
| // up to 2 GP registers and returned in RAX & RDX. | ||
| return isAggregate(t) && !canRewriteAsInt(t) && t->ty != Tarray && |
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[isAggregate(t) already excludes Tarray, so no change for slices. - That's a general limitation, as we cannot ABI-rewrite slices because we don't ABI-rewrite the druntime hooks.]
I expect this to be slightly more performant than the previous behavior, where a delegate was treated like a corresponding struct, passed via hidden pointer and returned via sret. The primary motivation is a smooth preparation for PR ldc-developers#3578 - in order to allow people to experiment with `-preview=in` without recompiling druntime and Phobos, `in` slices and delegates must not be passed by-ref with `-preview=in` (see dlang/dmd#11828). This would have required a special case for delegates on Win64, which is IMO better handled this way.
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I expect this to be slightly more performant than the previous behavior, where a delegate was treated like a corresponding struct, passed via hidden pointer and returned via sret.
The primary motivation is a smooth preparation for PR #3578 - in order to allow people to experiment with
-preview=inwithout recompiling druntime and Phobos,inslices and delegates must not be passed by-ref with-preview=in(see dlang/dmd#11828). This would have required a special case for delegates on Win64, which is IMO better handled this way.