popcnt usage

[Kai Nacke]

On Monday, 23 December 2013 at 16:47:32 UTC, Todd VanderVeen 
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
>    }
> }
>
> But the miscellaneous features of core.cpuid are not available 
> for conditional compilation. Is there an undocumented version 
> label that could be used to this end? Is my only option to pass 
> a version flag on the command line?
>
> version(X86_64) {
>    version(Has_Popcnt) {
>       asm {
>          ... performant assembly version
>       }
>    }
>    else {
>       ... slower procedural version
>    }
> }
>
> This is workable, but it would be nice if these finer 
> architectural distinctions were available for conditional 
> compilation without the need for the extra external 
> configuration.

With ldc2 you can use -mattr=+popcnt to use popcnt instruction 
and -mattr=-popcnt to use the emulation.

Regards,
Kai