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Date: Wed, 18 Jan 2006 15:05:25 -0600
From: Joel Schopp <jschopp@austin.ibm.com>
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To: Nick Piggin <npiggin@suse.de>
CC: Linus Torvalds <torvalds@osdl.org>, Andrew Morton <akpm@osdl.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [patch 2/2] powerpc: native atomic_add_unless
References: <20060118063636.GA14608@wotan.suse.de> <20060118063921.GB14608@wotan.suse.de> <43CE76B8.1000905@austin.ibm.com> <20060118172822.GG28418@wotan.suse.de>
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>>Why use a separate register here? Why not reuse %0 instead of using %1? 
>>Registers are valuable.
>>
> You still need to get the output (t) because you need to return
> whether the operation met with the "unless" case or not. If there is
> a better way to do this then I'd like to know.

I was thinking something like this would do:

static __inline__ int atomic_add_unless(atomic_t *v, int a, int u)
{
	int t;

	__asm__ __volatile__ (
	LWSYNC_ON_SMP
"1:	lwarx	%0,0,%1		# atomic_add_unless\n\
	cmpw	%0,%3 \n\
	beq-	2f \n\
	add	%0,%2,%0 \n"
	PPC405_ERR77(0,%1)
"	stwcx.	%0,0,%1 \n\
	bne-	1b \n"
	ISYNC_ON_SMP
	"subf	%0,%2,%0 \n\
2:"
	: "=&r" (t)
	: "r" (&v->counter), "r" (a), "r" (u)
	: "cc", "memory");

	return likely(t != u);
}

Though if I could figure out how to get gcc to do it I'd much rather do 
something like this (which won't compile but I think illustrates the concept):

static __inline__ int atomic_add_unless(atomic_t *v, int a, int u)
{
	int t;

	__asm__ __volatile__ (
	LWSYNC_ON_SMP
"1:	lwarx	%0,0,%1		# atomic_add_unless\n\
	cmpw	%0,%3 \n\
	beq-	3f \n\
	add	%0,%2,%0 \n"
	PPC405_ERR77(0,%1)
"	stwcx.	%0,0,%1 \n\
	bne-	1b \n"
	ISYNC_ON_SMP
2:"
	: "=&r" (t)
	: "r" (&v->counter), "r" (a), "r" (u)
	: "cc", "memory");

	return 1;
3:
	return 0;
}