Date: Thu, 7 Aug 2003 10:41:34 -0400 (EDT) From: Daniel Eischen <eischen@vigrid.com> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: alpha@freebsd.org Subject: Re: Atomic swap Message-ID: <Pine.GSO.4.10.10308071039270.12201-100000@pcnet5.pcnet.com> In-Reply-To: <16178.25370.731486.809755@grasshopper.cs.duke.edu>
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On Thu, 7 Aug 2003, Andrew Gallatin wrote:
>
> Daniel Eischen writes:
> > [ I'm not subscribed to alpha@; please keep me on the CC ]
> >
> > I need an atomic swap function for libpthread. Here's my hack
> > of an implementation:
> >
> > /*
> > * Atomic swap:
> > * Atomic (tmp = *dst, *dst = val), then *res = tmp
> > *
> > * void atomic_swap_long(long *dst, long val, long *res);
> > */
> > static __inline
> > void atomic_swap_long(volatile long *dst, long val, long *res)
> > {
> > u_int64_t result;
> >
> > __asm __volatile (
> > "1:\tldq_l %0,%1\n\t"
> > "stq_c %2,%1\n\t"
> > "beq %2,2f\n\t" /* Why is this beq instead of bne 1b? */
> > "br 3f\n"
> > "2:\tbr 1b\n"
> > "3:\n"
> > : "=&r" (result)
> > : "m" (*dst), "r" (val)
> > : "memory");
> >
> > *res = result;
> > }
> >
> > As annotated above, there seems to be one more branch than
> > necessary.
>
> Its actually an optimization. Alphas predict that backward branches
> will always be taken (think loops). If you were to branch directly
> back to 1:, then if the store succeeds (which it nearly always
> should), then the cpu would have been betting on taking the branch,
> and that would slow things down.
OK.
>
> > Can someone look this over for me? I really don't quite
> > know what I'm doing when it comes to inline assembly.
>
> I think it looks OK, but I'm also terrible at inline asm.
Yeah, me too. It took me quite a few tries to hit upon
something that seemed to work.
--
Dan Eischen
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