[swift-dev] Value-result ABI for small trivial inouts

[Joe Groff]

We currently always pass inout parameters indirectly, but it seems to me that for inout parameters that are of small trivial types like Int or CGSize, a value-result calling convention might always be preferable, and we might want to codify that in the stable ABI. Values of these types are likely to be SSAed, and the indirect convention forces memory traffic that would otherwise be unnecessary. On ARMv7 and ARM64, the argument and return register sets are the same, so a value-result convention is likely to be able to compile down to in-place operations on those registers, so it's a potential code size win too.

(Value-result is not a clear win for nontrivial types, since it forces a load and retain onto the outermost caller that inouts a value in memory, since we can't invalidate the memory during the inout call. The extra retain would potentially defeat COW optimization. We have primitives like `isUniquelyReferenced` that depend on mutable references being in memory too.)

-Joe