popcnt usage

[Marco Leise]

Am Mon, 23 Dec 2013 23:46:11 +0000
schrieb Iain Buclaw <ibuclaw at gdcproject.org>:

> On 23 December 2013 16:47, Todd VanderVeen <tdvanderveen at gmail.com> wrote:
> > First, let me say thanks for the addition of the popcnt inline assembler
> > opcode. I had placed a project on hold until it was available. I look
> > forward to using D again.
> >
> > I determined this instruction was available after some experimentation as
> > its not documented on the inline assembler page.
> >
> > uint popcnt (ulong bits) {
> >    asm {
> >       mov RAX, bits ;
> >       popcnt RAX, RAX ;
> >    }
> > }
> >
> > Mention is made in the documentation of SSE4.2 support but I understand
> > popcnt and lzcnt aren't really considered part of this instruction set as
> > they aren't register based. If I were to submit a pull request to address
> > the documentation, how would you prefer this is represented, simply as
> > additions to the opcode table or annotated that they were implemented
> > alongside SSE4.2? Both?
> >
> > A second concern is whether it is possible to determine the availability of
> > this instruction at compile time. I want to do something like the following
> > in a custom popcnt method:
> >
> > version(X86_64) {
> >    static if (hasPopcnt()) {
> >       asm {
> >          ... performant assembly version
> >       }
> >    } else {
> >       ... slower procedural version
> >    }
> > }
> >
> 
> There's no way to do this at compile time, other than assume that
> D_InlineAsm_X86_64 imples popcnt, or do a runtime check to determine
> the correct path to take.

Oh and if I remember correctly the popcnt intrinsic in GDC is
somewhat slow in emulation mode. No biggie, I just got
reminded.

-- 
Marco