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Date: Thu, 7 Aug 2003 09:43:18 -0400 (EDT)
From: Daniel Eischen <eischen@vigrid.com>
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Subject: Atomic swap
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[ 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.

Can someone look this over for me?  I really don't quite
know what I'm doing when it comes to inline assembly.

-- 
Dan Eischen