cmd/asm, cmd/internal/obj/mips: add an alias of RSB on mips64x

Change-Id: I724ce0a48c1aeed14267c049fa415a6fa2fffbcf
Reviewed-on: https://go-review.googlesource.com/19864Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/asm/internal/arch/arch.go

@@ -393,6 +393,9 @@ func archMips64() *Arch {
 	// Avoid unintentionally clobbering g using R30.
 	delete(register, "R30")
 	register["g"] = mips.REG_R30
+	// Avoid unintentionally clobbering RSB using R28.
+	delete(register, "R28")
+	register["RSB"] = mips.REG_R28
 	registerPrefix := map[string]bool{
 		"F":   true,
 		"FCR": true,

src/cmd/asm/internal/asm/operand_test.go

@@ -512,7 +512,6 @@ var mips64OperandTests = []operandTest{
 	{"R25", "R25"},
 	{"R26", "R26"},
 	{"R27", "R27"},
-	{"R28", "R28"},
 	{"R29", "R29"},
 	{"R3", "R3"},
 	{"R31", "R31"},
@@ -525,6 +524,7 @@ var mips64OperandTests = []operandTest{
 	{"LO", "LO"},
 	{"a(FP)", "a(FP)"},
 	{"g", "g"},
+	{"RSB", "RSB"},
 	{"ret+8(FP)", "ret+8(FP)"},
 	{"runtime·abort(SB)", "runtime.abort(SB)"},
 	{"·AddUint32(SB)", "\"\".AddUint32(SB)"},

src/cmd/internal/obj/mips/a.out.go

@@ -187,7 +187,7 @@ const (
 
 	REGZERO  = REG_R0 /* set to zero */
 	REGSP    = REG_R29
-	REGSB    = REG_R30
+	REGSB    = REG_R28
 	REGLINK  = REG_R31
 	REGRET   = REG_R1
 	REGARG   = -1      /* -1 disables passing the first argument in register */

src/cmd/internal/obj/mips/list0.go

@@ -47,6 +47,10 @@ func Rconv(r int) string {
 		// Special case.
 		return "g"
 	}
+	if r == REGSB {
+		// Special case.
+		return "RSB"
+	}
 	if REG_R0 <= r && r <= REG_R31 {
 		return fmt.Sprintf("R%d", r-REG_R0)
 	}