Messages in this thread | | | Date | Mon, 22 Feb 2010 11:55:48 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86: Add optimized popcnt variants |
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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.
+#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"
-hpa
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