add atomics to mips

Reference: /n/atom/patch/applied2013/libcmipsatom
Date: Wed Jun 19 06:12:13 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/mips/mkfile	Wed Jun 19 06:12:09 2013
+++ /sys/src/libc/mips/mkfile	Wed Jun 19 06:12:09 2013
@@ -4,6 +4,7 @@
 LIB=/$objtype/lib/libc.a
 SFILES=\
 	argv0.s\
+	atom.s\
 	getcallerpc.s\
 	getfcr.s\
 	main9.s\
--- /sys/src/libc/mips/atom.s	Thu Jan  1 00:00:00 1970
+++ /sys/src/libc/mips/atom.s	Wed Jun 19 06:12:10 2013
@@ -0,0 +1,57 @@
+/*
+ *	R4000 user-level atomic operations
+ */
+
+#define	LL(base, rt)	WORD	$((060<<26)|((base)<<21)|((rt)<<16))
+#define	SC(base, rt)	WORD	$((070<<26)|((base)<<21)|((rt)<<16))
+#define	NOOP		WORD	$0x27
+
+TEXT ainc(SB), 1, $-4			/* long ainc(long *); */
+TEXT _xinc(SB), 1, $-4			/* void _xinc(long *); */
+	MOVW	R1, R2			/* address of counter */
+loop:	MOVW	$1, R3
+	LL(2, 1)
+	NOOP
+	ADD	R1,R3,R3
+	SC(2, 3)
+	NOOP
+	BEQ	R3,loop
+	RET
+
+TEXT adec(SB), 1, $-4			/* long adec(long*); */
+TEXT _xdec(SB), 1, $-4			/* long _xdec(long *); */
+	MOVW	R1, R2			/* address of counter */
+loop1:	MOVW	$-1, R3
+	LL(2, 1)
+	NOOP
+	ADD	R1,R3,R3
+	MOVW	R3, R1
+	SC(2, 3)
+	NOOP
+	BEQ	R3,loop1
+	RET
+
+/*
+ * int cas(uint* p, int ov, int nv);
+ */
+TEXT cas(SB), 1, $-4
+	MOVW	ov+4(FP), R2
+	MOVW	nv+8(FP), R3
+spincas:
+	LL(1, 4)			/* R4 = *R1 */
+	NOOP
+	BNE	R2, R4, fail
+	SC(1, 3)			/* *R1 = R3 */
+	NOOP
+	BEQ	R3, spincas		/* R3 == 0 means store failed */
+	MOVW	$1, R1
+	RET
+fail:
+	MOVW	$0, R1
+	RET
+
+/* general-purpose abort */
+_trap:
+	MOVD	$0, R0
+	MOVD	0(R0), R0
+	RET