Followup to: <20020318232740.39289.qmail@web13305.mail.yahoo.com>
By author: Carl Spalletta <cspalletta@yahoo.com>
In newsgroup: linux.dev.kernel
>
> I have read all the docs and I still can't clearly understand when it
> is required to specify a clobberlist - a register or memory that will
> be modified and must be preserved by gcc.
>
> In searching the kernel source there were very few clobbers given
> and most of those were for memory.
>
> For example in arch/i386/lib/delay.c:
>
> 71 __asm__("mull %0"
> 72 :"=d" (xloops), "=&a" (d0)
> 73 :"1" (xloops),"" (current_cpu_data.loops_per_jiffy));
> 74 __delay(xloops * HZ);
>
> It's pretty obvious that eax gets clobbered, as I know clobber. Why is
> it not listed as such? Does the answer to this question apply in all cases?
> What about memory clobbers - how do they happen?
>
It's not listed as such because it's an output (the second output),
not a clobber. This is particularly common on x86 where registers are
so specialized that they all have their own constraint letters.
Incidentally, the above asm statement is technically incorrect, "=&a"
(d0) means eax is an "early clobber", however, the instruction doesn't
require it to be early clobbered.
-hpa
-- <hpa@transmeta.com> at work, <hpa@zytor.com> in private! "Unix gives you enough rope to shoot yourself in the foot." http://www.zytor.com/~hpa/puzzle.txt <amsp@zytor.com> - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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