Opt-in warnings when ld can be ldh, or jp can be jr

[Rangi42]

Issue #986 plans to deprecate and remove the -L flag, which stops rgbasm from optimizing ld to ldh. It would still be convenient to know when ld can manually be turned into ldh, even if the target address is only known at link time (which rgbasm cannot handle). It would also be convenient to know which jp instructions could be jr.

Once -L is removed, we could reintroduce it to warn when any ld can be ldh. Likewise, -J can warn when jp can be jr.

Edit: actually, more -W flags would make more sense for these.

[aaaaaa123456789]

I'd say that all warnings should be enabled by -W flags. Other than that, good idea.

[Rangi42]

Hm, so rgbasm and rgblink would both need flags for this.

[Rangi42]

As pointed out in #1114, it would be useful to allow each warning for all occurrences of a pattern, or just the first occurrence, or not warn for that pattern. ("A pattern" might be "ld which can be ldh", or "jp which can be jr", or "ld X, Y after ld Y, X", or "call+ret which can be jp", or so on.)

Inline comments or pragmas to suppress warnings at particular sites would also be useful (e.g. maybe this ld [$ff00], a is in a LOAD section and will be modifiable to non-HRAM addresses).

[im-mi]

Checking if a jump target is close enough to use jr when the target label exists in a different section is bound to cause all sorts of unwanted warnings as sections get shifted around by the linker. Perhaps that feature should be limited to cases where the jump target is in the same section (or section fragment, for fragmented sections).

On a similar note, a warning could also be produced when performing a suspicious call or jump. For example: if code in a non-fixed ROM bank is calling a subroutine in a different non-fixed ROM bank, then it's probably a bug.

Of course many of these features can be implemented using macros, but having them built-in would be pretty convenient.