Atomic swap

Portante, Peter peter.portante at hp.com
Thu Aug 7 12:10:22 PDT 2003


Dan,

> ----------
> From: 	Daniel Eischen
> Reply To: 	deischen at freebsd.org
> Sent: 	Thursday, August 7, 2003 3:05 PM
> To: 	Marcel Moolenaar
> Cc: 	Portante, Peter; alpha at freebsd.org; deischen at freebsd.org
> Subject: 	Re: Atomic swap
> 
> 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?
> 
Actually, the processor has a chance to do something while waiting for memory, so it does not hurt to have the mov inside the ldq_l/stq_c pair.

-Peter


More information about the freebsd-alpha mailing list