popcnt usage

[Todd VanderVeen]

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.