Date: Thu, 7 Aug 2003 15:05:28 -0400 (EDT) From: Daniel Eischen <eischen@vigrid.com> To: Marcel Moolenaar <marcel@xcllnt.net> Cc: alpha@freebsd.org Subject: Re: Atomic swap Message-ID: <Pine.GSO.4.10.10308071500390.29641-100000@pcnet5.pcnet.com> In-Reply-To: <20030807185507.GA559@athlon.pn.xcllnt.net>
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On Thu, 7 Aug 2003, Marcel Moolenaar wrote:
> On Thu, Aug 07, 2003 at 01:44:18PM -0400, Daniel Eischen wrote:
> > How about this?
> >
> > static __inline void
> > atomic_swap_long(volatile long *dst, long val, long *res)
> > {
> > u_int64_t result, temp;
> >
> > __asm __volatile (
> > "1:\tldq %1, %3\n\t" /* load value to store */
> > "ldq_l %0, %2\n\t" /* load current value, asserting lock */
> > "stq_c %1, %2\n\t" /* attempt to store */
> > "beq %1, 2f\n\t" /* if the store failed, spin */
> > "br 3f\n" /* it worked, exit */
> > "2:\tbr 1b\n" /* *dst not updated, loop */
> > "3:\n" /* it worked */
> > : "=&r" (result), "=&r" (temp)
> > : "m" (*dst), "m" (val)
> > : "memory");
> >
> > *res = result;
> > }
>
> The first instruction is wrong. "val" isn't memory. Also,
> forget about the branch prediction optimization. It just
> makes the code unreadable and we don't even know if it
> makes a difference.
>
> The following has been written down without testing (I
> dropped the cosmetic \t and instead indented by hand to
> make the source code readable, not what is given to the
> assembler (per se):
>
> static __inline void
> atomic_swap_long(volatile long *dst, long val, long *res)
> {
> __asm ( "1: ldq_l t0,%0\n"
> " mov %1,t1\n"
If I swap the first 2 instructions:
__asm ( "1: mov %1,t1\n"
ldq_l t0,%0\n"
that eliminates 1 instruction from between the locked
instructions. Is there anything wrong with doing that?
> " stq_c t1,%0\n"
> " beq t1,1b\n"
> " stq t0,%3\n"
> :: "m"(*dst), "r"(val), "m"(*res) : "memory");
> }
>
> In words:
> o Read the current value at *dst in register t0 and lock the address.
> o Since stq_c clobbers it's input and we may need to loop, save val
> in temporary register t1.
> o Store val at *dst (by means of t1) and if the lock failed, retry.
> o Write the old value in register t0 to *res.
So it's OK to hardcode registers (t0, t1) in inline assembly?
--
Dan Eischen
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