Messages in this thread | | | Date | Tue, 23 Feb 2010 07:37:34 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86: Add optimized popcnt variants |
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From: "H. Peter Anvin" <hpa@zytor.com> 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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