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SubjectRe: [PATCH 7/10] local_t : powerpc
On Wed, Jan 24, 2007 at 08:08:12PM +1100, Paul Mackerras wrote:
> Mathieu Desnoyers writes:
>
> > +static __inline__ int local_dec_if_positive(local_t *l)
> > +{
> > + int t;
> > +
> > + __asm__ __volatile__(
> > +"1: lwarx %0,0,%1 # local_dec_if_positive\n\
> > + addic. %0,%0,-1\n\
> > + blt- 2f\n"
> > + PPC405_ERR77(0,%1)
> > +" stwcx. %0,0,%1\n\
> > + bne- 1b"
>
> This has the same bugs that we fixed recently in atomic_dec_if_positive;
> first, on 64-bit machines, the lwarx will zero-extend the word loaded
> from memory, and so the result of the addic will be negative only if
> the word was originally 0. Secondly, even on 32-bit machines,
> 0x80000000 will be considered positive since decrementing it gives
> 0x7fffffff, which is positive.
>
> > +/* Use these for per-cpu local_t variables: on some archs they are
> > + * much more efficient than these naive implementations. Note they take
> > + * a variable, not an address.
> > + *
> > + * This could be done better if we moved the per cpu data directly
> > + * after GS.
> > + */
>
> What's "GS"? Does this comment really apply on powerpc?
>

1) It's an (application visible) i386/x86_64 segment register used
to make memory addressing more confusing.
2) Because of 1), obviously not ;-)

Gabriel
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