Add local label support within preproc.

[luckytyphlosion]

Description

The prefix chosen was $ since it's a valid gnu assembler name character and isn't used by other directives. . would have been preferable but it would require extra coding.

Semantics:

• Local labels are scoped by a non-local (or global) label. That is, local labels will become undefined when a non-local label is used, which means that the names of local labels can be reused as long as a non-local label is defined.
• Local labels can be referenced before they are declared, like normal labels. If a local label is referenced but not defined within its scope, then preproc will report an error.
• Since this is a preproc extension, local labels will not work in macros. If you need this advanced support, I would suggest looking at my fork of PikalaxALT/easyaspi's agbcc + binutils, which implements true local label support within gnu assembler.
• Internally, preproc will transform a local label into the concatenation of the last non-local label and the local label. E.g. for this input:

  NonlocalLabel1:
      .byte 10
      .word $localLabel @ forward reference
  $localLabel:
      .byte 20
      .word $localLabel @ backwards reference
  
  NonlocalLabel2:
      .hword 30
      .word $localLabel @ forward reference
  $localLabel:
      .hword 40
      .word $localLabel @ backwards reference
  

  preproc will transform it to:

  NonlocalLabel:
      .byte 10
      .word NonlocalLabel$localLabel @ forward reference
  NonlocalLabel$localLabel:
      .byte 20
      .word NonlocalLabel$localLabel @ backwards reference
  
  NonlocalLabel2:
      .hword 30
      .word NonlocalLabel2$localLabel @ forward reference
  NonlocalLabel2$localLabel:
      .hword 40
      .word NonlocalLabel2$localLabel @ backwards reference
  

  This means that you can also reference a local label outside of its scope, but this hasn't been tested. What is guaranteed is that the local label substitution (e.g. .word $localLabel -> .word NonlocalLabel2$localLabel) will only happen if the $ is directly after a non-word character (anything that isn't alphanumeric, _, ., or $).
• Per file, local labels cannot be defined before a non-local label is defined. Part of this behaviour is a bit weird and not really designed for since it represents an edge case. For example:

  NonlocalLabel1:
      .byte 10
  $localLabel1:
      .byte 20
  
      .include "some_data.inc"
  
  @ in file "some_data.inc"
      .word $localLabel2
  

  would not be valid, however:

  @ in file 
  NonlocalLabel1:
      .byte 10
      .word $localLabel1
  $localLabel2:
      .include "more_data.inc"
  @ in file "more_data.inc"
  NonlocalLabel2:
      .byte 20
  
  @ back to the base file
  $localLabel1:
      .byte 30
  

  is (probably) valid. However, most users will probably never feel the need for cross-file local labels.
• Tests were done in another, private repository, but if needed I could refactor some scripts to use local labels to show their correctness. The following tests were done:
  • defining a label before it's referenced, then defining the label
  • defining a label before it's referenced, and not defining it (should error)
  • defining multiple of the same label before it's referenced, and not defining it (should error)
  • defining multiple different labels before they're referenced, and not defining them (should error)
  • defining multiple different labels multiple times before they're referenced, and not defining them (should error)
  • defining local label as global (should error)
  • defining local label before a non-local label is defined (should error)
  • defining just the local label prefix (should error)
  • referencing a local label at the end of a file without a newline
  • validating local labels at the end of the file

Discord contact info

luckytyphlosion#1166

[GriffinRichards]

When you say it will not work in macros do you mean a label in a macro definition or a label given as an argument to a macro? If the latter I'm struggling to see a use case for this given that the repo's scripts are almost all macro

[luckytyphlosion]

I mean label in a macro definition. So:

  .macro my_macro
$localLabel:
  .byte 10
  .endm

would not work when the macro is actually used.

[rawr51919]

So you're saying with this it's possible to add labels in macro definitions within the macros themselves? Would this PR still be relevant in today's pokeemerald?

[rawr51919]

Either way though it still needs a rebase to fix merge conflicts

[GriffinRichards]

The above merge conflict was trivial and unrelated; seems like this doesn't build on current pokeemerald. Do you still have any interest in this?

[tustin2121]

What purpose is this for that isn't already possible with GAS's built-in local labels 0 through 9, and referencing them via 0b (backward reference) or 0f (forward reference)? https://ftp.gnu.org/old-gnu/Manuals/gas-2.9.1/html_chapter/as_5.html#SEC48

[luckytyphlosion]

What purpose is this for that isn't already possible with GAS's built-in local labels 0 through 9, and referencing them via 0b (backward reference) or 0f (forward reference)? https://ftp.gnu.org/old-gnu/Manuals/gas-2.9.1/html_chapter/as_5.html#SEC48

gas local labels suck because they can't be named