[Issue 20112] __vector casts don't do type conversions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20112

--- Comment #3 from thomas.bockman at gmail.com ---
> Semantically, this can only be done by unrolling the assignment

I've found that this is very unreliable. Sometimes the optimizer correctly
replaces the individual component casts with the SIMD conversion instructions,
and sometimes it doesn't. On LLVM, at least, inlining sometimes undoes the
optimization.

I haven't been able to get this working reliably without resorting to inline
assembly language.

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