add sys.BUG_intereq to compare interfaces for equality

R=r
OCL=16929
CL=16929

src/cmd/gc/sys.go

@@ -80,3 +80,4 @@ export func	stringtorune(string, int32) (int32, int32);	// convert bytes to rune
 
 export func	exit(int32);
 
+export func	BUG_intereq(a, b interface{}) bool;	// should not be necessary

src/cmd/gc/sysimport.c

@@ -68,5 +68,6 @@ char *sysimport =
 	"export func sys.bytestorune (? *sys.uint8, ? sys.int32, ? sys.int32) (? sys.int32, ? sys.int32)\n"
 	"export func sys.stringtorune (? sys.string, ? sys.int32) (? sys.int32, ? sys.int32)\n"
 	"export func sys.exit (? sys.int32)\n"
+	"export func sys.BUG_intereq (a interface { }, b interface { }) (? sys.bool)\n"
 	"\n"
 	"$$\n";

src/runtime/rt0_amd64.s

@@ -133,3 +133,21 @@ TEXT cas(SB), 7, $0
 	MOVL	$1, AX
 	RET
 
+// func BUG_intereq(a, b interface{}) bool {
+//	return a == b
+// }
+// TODO: delete once 6g can compile a == b on interfaces.
+TEXT sys·BUG_intereq(SB),7,$0
+	// First interface at 8(SP) and 16(SP)
+	// Second interface at 24(SP) and 32(SP)
+	// Answer at 40(SP)
+	MOVQ 8(SP), AX
+	CMPQ AX, 24(SP)
+	JNE 6(PC)
+	MOVQ 16(SP), AX
+	CMPQ AX, 32(SP)
+	JNE 3(PC)
+	MOVQ $1, 40(SP)
+	RET
+	MOVQ $0, 40(SP)
+	RET