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
