Re: [PATCH] x86: Add optimized popcnt variants

[Borislav Petkov]

From: "H. Peter Anvin" <hpa@xxxxxxxxx>
Date: Mon, Feb 22, 2010 at 11:55:48AM -0800

> On 02/22/2010 10:49 AM, Borislav Petkov wrote:
> >>
> >> Just a note: this still means rdi is clobbered on x86-64, which is
> >> probably fine, but needs to be recorded as such.  Since gcc doesn't
> >> support clobbers for registers used as operands (sigh), you have to
> >> create a dummy output and assign it a "=D" constraint.
> >>
> >> I don't know if gcc would handle -fcall-saved-rdi here... and if so, how
> >> reliably.
> > 
> > Ok, from looking at kernel/sched.s output it looks like it saves rdi
> > content over the alternative where needed. I'll do some more testing
> > just to make sure.
> > 
> 
> No, you can't rely on behavioral observation.  A different version of
> gcc could behave differently.  We need to make sure we tell gcc what the
> requirements actually are, as opposed to thinking we can just fix them.

Ok, I've added the dummy "=D" constraint since it sounded like the more
stable thing to do WRT gcc versions. BTW, I left the machine overnight
and it is still alive cheerfully building randconfigs.

I'll try the -fcall-saved-rdi thing also later today.

> +#define POPCNT ".byte 0xf3\n\t.byte 0x48\n\t.byte 0x0f\n\t.byte
> 0xb8\n\t.byte 0xc7"
> 
> BTW, this can be written:
> 
> #define POPCNT ".byte 0xf3,0x48,0x0f,0xb8,0xc7"

done, updated version coming up.

-- 
Regards/Gruss,
    Boris.
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