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atomic: return a boolean from the cas variants

while it's sometimes useful to know the old value in the
atomic location, it's harder to implement on some cpu
architectures like arm.

--- a/atomic-amd64.s

+++ b/atomic-amd64.s

@@ -34,6 +34,8 @@

 	MOVL	c+8(FP), AX

 	MOVL	v+16(FP), BX

 	LOCK; CMPXCHGL	BX, (RARG)

+	SETEQ	AX

+	MOVBLZX	AX, AX

 	RET

 TEXT acasv+0(SB),1,$0

@@ -41,8 +43,11 @@

 	MOVQ	c+8(FP), AX

 	MOVQ	v+16(FP), BX

 	LOCK; CMPXCHGQ BX, (RARG)

+	SETEQ	AX

+	MOVBLZX	AX, AX

 	RET

 /* barriers (do we want to distinguish types?) */

 TEXT coherence+0(SB),1,$0

 	MFENCE

+	RET

--- a/atomic.h

+++ b/atomic.h

@@ -10,8 +10,8 @@

 vlong	aincv(vlong*, vlong);

 void*	aincp(void**, void*);

-long	acasl(long*, long, long);

-vlong	acasv(vlong*, vlong, vlong);

-void*	acasp(void**, void*, void*);

+int	acasl(long*, long, long);

+int	acasv(vlong*, vlong, vlong);

+int	acasp(void**, void*, void*);

-void	coherence(void);

\ No newline at end of file

+void	coherence(void);

--- a/blk.c

+++ b/blk.c

@@ -29,7 +29,7 @@

 	while(1){

 		ov = agetl(&b->flag);

 		nv = ov | f;

-		if(acasl(&b->flag, ov, nv) == ov)

+		if(acasl(&b->flag, ov, nv))

 			break;

 	}

 }

@@ -42,7 +42,7 @@

 	while(1){

 		ov = agetl(&b->flag);

 		nv = ov & ~f;

-		if(acasl(&b->flag, ov, nv) == ov)

+		if(acasl(&b->flag, ov, nv))

 			break;

 	}

 }

--- a/fs.c

+++ b/fs.c

@@ -149,7 +149,7 @@

 	long v;

 	v = agetl(&c->count);

-	if(v == 0 || acasl(&c->count, v, v-1) != v)

+	if(v == 0 || !acasl(&c->count, v, v-1))

 		semacquire(&c->count, 1);

 	lock(&c->rl);

 	a = *c->rp;

@@ -166,7 +166,7 @@

 	long v;

 	v = agetl(&c->avail);

-	if(v == 0 || acasl(&c->avail, v, v-1) != v)

+	if(v == 0 || !acasl(&c->avail, v, v-1))

 		semacquire(&c->avail, 1);

 	lock(&c->wl);

 	*c->wp = m;