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

safety0ff.bugz <safety0ff.bugz@gmail.com> changed:

           What    |Removed                     |Added
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           Keywords|                            |performance

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

Bruce Carneal <bcarneal11@gmail.com> changed:

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                 CC|                            |bcarneal11@gmail.com

--- Comment #1 from Bruce Carneal <bcarneal11@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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https://issues.dlang.org/show_bug.cgi?id=21041

--- Comment #2 from safety0ff.bugz <safety0ff.bugz@gmail.com> ---
(In reply to Bruce Carneal from comment #1)
> I didn't find a 'byteswap' in the core.bitop documentation.  There is a bswap but only for uints and ulongs AFAICT.

The intrinsic in question was added in the master branch here: https://github.com/dlang/dmd/pull/11388

Also the 64 bit version is to be added here: https://github.com/dlang/dmd/pull/11408

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

I'd expect that since C/C++ clang emit that.

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

--- Comment #3 from safety0ff.bugz <safety0ff.bugz@gmail.com> ---
(In reply to Bruce Carneal from comment #1)
> Current dmd -O emits 7 instructions to accomplish the rolw in the code body.

D converts many operations on narrow types to int, which DMD's backend then fails to optimize away when it is possible/advantageous.

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

--- Comment #4 from safety0ff.bugz <safety0ff.bugz@gmail.com> ---
(In reply to safety0ff.bugz from comment #3)
> (In reply to Bruce Carneal from comment #1)
> > Current dmd -O emits 7 instructions to accomplish the rolw in the code body.
> 
> D converts many operations on narrow types to int, which DMD's backend then fails to optimize away when it is possible/advantageous.

Further investigation: dmd/backend/cod2.d function cdshift also converts rotates of 8 in upper/lower 8 of word into XCHG's

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

--- Comment #5 from Bruce Carneal <bcarneal11@gmail.com> ---
(In reply to safety0ff.bugz from comment #3)
> (In reply to Bruce Carneal from comment #1)
> > Current dmd -O emits 7 instructions to accomplish the rolw in the code body.
> 
> D converts many operations on narrow types to int, which DMD's backend then fails to optimize away when it is possible/advantageous.

Yes.  DMDs back end is quick, but the code it generates is not state-of-the-art.

That said, optimizing the DMD code gen for code.bitop rotations seems more useful than a ushort byteswap improvement.  The latter could be implemented as an "inline" of the former.

Recognizing the rotation patterns generally, ala LLVM, would be even better but quite a bit of work I'd imagine.  Probably not worth it given current resource constraints (Walter's time).  Lots of big front-end fish to fry.

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

Iain Buclaw <ibuclaw@gdcproject.org> changed:

           What    |Removed                     |Added
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           Keywords|                            |backend
                 CC|                            |ibuclaw@gdcproject.org

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

Iain Buclaw <ibuclaw@gdcproject.org> changed:

           What    |Removed                     |Added
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           Priority|P1                          |P4

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