Date: Thu, 7 Aug 2003 11:11:07 -0400 From: "Portante, Peter" <peter.portante@hp.com> To: <deischen@freebsd.org> Cc: alpha@freebsd.org Subject: RE: Atomic swap Message-ID: <B24FABB430F7C94D942D6386447C93DC0512B55D@tayexc17.americas.cpqcorp.net>
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Dan,
I don't think you want to do the stq_c if the location already holds the =
same value. Instead, check the loaded value to see if it is the same as =
the value to be stored, and branch out of the loop returning the result =
if it is they are the same. And starting with EV56, the need to do the =
branch forward/branch back logic has been removed. And EV6 and later =
CPUs do such a good job predicting the branching that it is not worth =
the instruction stream space when that space can be used to avoid a =
stq_c.
Additionally, the stq_c destroys the contents of %2, so you need to move =
the value in %2 into another register for use in the stq_c. I don't =
know how to do that in the ASM, so I just used raw register names below, =
highlighted in red.
-Peter
> /*
> * Atomic swap:
> * Atomic (tmp =3D *dst, *dst =3D val), then *res =3D tmp
> *
> * void atomic_swap_long(long *dst, long val, long *res);
> */
> static __inline=20
> void atomic_swap_long(volatile long *dst, long val, long *res)
> {
> u_int64_t result;
>=20
> __asm __volatile (
> "1:\tldq_l %0,%1\n\t"
"mov %2,r1\n\t" /* Hide under the load latency */
"cmpeq %0,%2,r0\n\t"
"bne r0,3f\n\t" /* Branches out are ok, but not to targets within =
ld_l/st_c */
> "stq_c r1,%1\n\t"
> "beq r1,1b\n\t"
> "3:\n"
> : "=3D&r" (result)
> : "m" (*dst), "r" (val)
> : "memory");
>=20
> *res =3D result;
> }
>=20
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