[MrSapps]

VC6 can code a read of a global differently depending on whether the file defines it or only declares it extern. In gta2_re, a 2-byte global defined in the same file got 32-bit loads. The original's 16-bit loads came back once the definition moved to another file: Weapon_30::dual_pistol_5DDA70 matched, and four Wolfy_30 functions went from 181 and 226 diff lines to 32 and 20.

What --extern-globals does

• New regions: each global the target function names becomes a region holding its declaration line in the source file. Region::alt holds the other form.
  • A definition, DEFINE_GLOBAL...(type, name, ...), can become EXTERN_GLOBAL(type, name);, or EXTERN_GLOBAL_ARRAY(type, name, size); for arrays.
  • An extern can become its definition, copied from another .cpp in the source's directory.
• Other macros: --global-macros DEFINE=EXTERN names other macros with the same argument layout. The default is gta2_re's Function.hpp macros.
• Search:
  • Exhaustive mode adds one candidate per global: that global's declaration switched (extern_global in the stats).
  • Random mode switches a random global in one mutation in eight; the rest go to the function regions as before.
• Scoring: only the candidate's object is compiled, so a missing definition doesn't matter. A winning switch has to be applied by hand: move the definition to another .cpp, or into this one.

Tests

• New integration case (extern_global, --dry-run): the definition gets an extern candidate, the extern gets the definition from other.cpp, and a global the function doesn't read isn't offered. clang codes both forms the same, so the case checks the candidates, not a match.
• Unit and integration suites: they pass under ctest.
• With VC6 on gta2_re:
  • Setup: word_706D5E's definition was put back into Weapon_30.cpp, so dual_pistol_5DDA70 scored 245.
  • Run: -m exhaustive -p incdec --extern-globals.
  • Result: it found the match (score 0) at candidate 6: +EXTERN_GLOBAL(Ang16, word_706D5E);.

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