Re: [PATCH] 498+ days uptime

H. Peter Anvin (hpa@transmeta.com)
25 Aug 1998 23:10:10 GMT

Followup to: <19980825141946.A2198@tantalophile.demon.co.uk>
By author: Jamie Lokier <lkd@tantalophile.demon.co.uk>
In newsgroup: linux.dev.kernel
>
> Incrementing a 64-bit value on i386 is fast and simple. It's arithmetic
> done in GCC, especially with temporaries that you have to worry about.
>
> The instruction sequence is:
>
> incl counter
> adcl $0,counter+4
>
> This could be made into a kernel macro. Because the carry is so rare,
> you could safely use a lock prefix before the incl and put this in a
> macro called atomic_count or something.
>

This is wrong: INC doesn't update CF; one of the many quirks of the
x86 architecture. You need:

addl $1,counter
adcl $0,counter+4

-hpa

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