cmd/internal/obj: add opcode space safety check

This CL adds a safety mechanism
for changing the number of opcodes
available per architecture.

A subsequent CL will actually make the change.

Change-Id: I6332ed5514f2f153c54d11b7da0cc8a6be1c8066
Reviewed-on: https://go-review.googlesource.com/24222
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/link.go

@@ -308,7 +308,8 @@ const (
 	ABaseMIPS64
 	ABaseS390X
 
-	AMask = 1<<12 - 1 // AND with this to use the opcode as an array index.
+	AllowedOpCodes = 1 << 12            // The number of opcodes available for any given architecture.
+	AMask          = AllowedOpCodes - 1 // AND with this to use the opcode as an array index.
 )
 
 // An LSym is the sort of symbol that is written to an object file.

src/cmd/internal/obj/util.go

@@ -462,6 +462,9 @@ var aSpace []opSet
 // RegisterOpcode binds a list of instruction names
 // to a given instruction number range.
 func RegisterOpcode(lo As, Anames []string) {
+	if len(Anames) > AllowedOpCodes {
+		panic(fmt.Sprintf("too many instructions, have %d max %d", len(Anames), AllowedOpCodes))
+	}
 	aSpace = append(aSpace, opSet{lo, Anames})
 }