[Issue 21041] core.bitop.byteswap(ushort) should used ROL/ROR instead of XCHG

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21041

Bruce Carneal <bcarneal11 at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |bcarneal11 at gmail.com

--- Comment #1 from Bruce Carneal <bcarneal11 at gmail.com> ---
I didn't find a 'byteswap' in the core.bitop documentation.  There is a bswap
but only for uints and ulongs AFAICT.  Regardless, here's a byteswap
implementation for discussion:

auto byteswap(ushort x) { return cast(ushort)(x >> 8 | x << 8); }

For the above code ldc at -O or above generates:
  movl %edi, %eax
  rolw $8, %ax
  retq

With ldc you can also get the above sequence using core.bitop.rol!8 explicitly.

Current dmd -O emits 7 instructions to accomplish the rolw in the code body. 
The code emitted by dmd -O for the explicit call to core.bitop.rol is even
worse, which is strange.

So, yes, there's room here for DMD code gen improvement but ldc is right there.

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