runtime: use kernel-supplied cas on linux/arm
Using the kernel-supplied compare-and-swap code
on linux/arm means that runtime doesn't have to care
whether this is GOARM=5 or GOARM=6 anymore.
Fixes #1494.
R=r, r2
CC=golang-dev
https://golang.org/cl/4245043
diff --git a/src/pkg/runtime/linux/arm/sys.s b/src/pkg/runtime/linux/arm/sys.s
index 6c222fc..9daf9c2 100644
--- a/src/pkg/runtime/linux/arm/sys.s
+++ b/src/pkg/runtime/linux/arm/sys.s
@@ -230,3 +230,21 @@
MOVW $SYS_rt_sigreturn, R7
SWI $0
RET
+
+// Use kernel version instead of native armcas in ../../arm.s.
+// See ../../../sync/atomic/asm_linux_arm.s for details.
+TEXT cas<>(SB),7,$0
+ MOVW $0xffff0fc0, PC
+
+TEXT runtime·cas(SB),7,$0
+ MOVW valptr+0(FP), R2
+ MOVW old+4(FP), R0
+ MOVW new+8(FP), R1
+ BL cas<>(SB)
+ MOVW $0, R0
+ MOVW.CS $1, R0
+ RET
+
+TEXT runtime·casp(SB),7,$0
+ B runtime·cas(SB)
+
