arm runtime: attempt to fix build by adding casp (same as cas)

untested.

Fixes #1523.

R=rsc
CC=golang-dev
https://golang.org/cl/4171057

src/pkg/runtime/arm/cas5.s

@@ -20,7 +20,6 @@ TEXT runtime·cas(SB),7,$0
 	MOVW	8(FP), R2	// new
 	MOVW	$1, R3
 	MOVW	$runtime·cas_mutex(SB), R4
-l:
 	SWPW	(R4), R3	// acquire mutex
 	CMP		$0, R3
 	BNE		fail0
@@ -38,6 +37,38 @@ fail1:
 fail0:
 	MOVW	$0, R0
 	RET
- 
+
+// bool casp(void **p, void *old, void *new)
+// Atomically:
+//	if(*p == old){
+//		*p = new;
+//		return 1;
+//	}else
+//		return 0;
+
+TEXT runtime·casp(SB),7,$0
+	MOVW	0(FP), R0	// *p
+	MOVW	4(FP), R1	// old
+	MOVW	8(FP), R2	// new
+	MOVW	$1, R3
+	MOVW	$runtime·cas_mutex(SB), R4
+	SWPW	(R4), R3	// acquire mutex
+	CMP		$0, R3
+	BNE		failp0
+	
+	MOVW	(R0), R5
+	CMP		R1, R5
+	BNE		failp1
+	
+	MOVW	R2, (R0)	
+	MOVW	R3, (R4)	// release mutex
+	MOVW	$1, R0
+	RET
+failp1:	
+	MOVW	R3, (R4)	// release mutex
+failp0:
+	MOVW	$0, R0
+	RET
+
 DATA runtime·cas_mutex(SB)/4, $0
 GLOBL runtime·cas_mutex(SB), $4

src/pkg/runtime/arm/cas6.s

@@ -27,3 +27,26 @@ fail:
 	MOVW	$0, R0
 	RET
 
+// bool casp(void **p, void *old, void *new)
+// Atomically:
+//	if(*p == old){
+//		*p = new;
+//		return 1;
+//	}else
+//		return 0;
+TEXT runtime·casp(SB), 7, $0
+	MOVW	0(FP), R1	// *p
+	MOVW	4(FP), R2	// old
+	MOVW	8(FP), R3	// new
+lp:
+	LDREX	(R1), R0
+	CMP		R0, R2
+	BNE		failp
+	STREX	R3, (R1), R0
+	CMP		$0, R0
+	BNE		lp
+	MOVW	$1, R0
+	RET
+failp:
+	MOVW	$0, R0
+	RET