[Issue 13742] undefined reference to __coverage

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=13742

--- Comment #8 from github-bugzilla at puremagic.com ---
Commits pushed to master at https://github.com/dlang/dmd

https://github.com/dlang/dmd/commit/69e7bd18892df18f682906916bf8c8ad87fb30e5
fix Issue 13742 - undefined reference to __coverage

- Nested template functions may reference coverage symbols of other modules.
  Those coverage symbols are either still `null` or already `symbol_reset`, in
  the former case coverage instrumentation is silently skipped, in the later
  case the linker will cause an error. This is because the current object file
  ends up with two `__coverage` symbols, one local BSS symbols and an undefined
  external symbol (for the other module's coverage).

- Fixed by mangling the symbols as _D3pkg3mod__coverageZ (similar to
  __ModuleInfoZ) and making them global.

- Ideally the symbols would use -visibility=hidden, but that ELF backend
feature
  isn't yet accessible from glue.d.

- This does not fix missing coverage when the referenced module hasn't yet
  been codegen'ed (e.g. because it follows on the command line or is part of
  another separate compilation). Using weak linkage to make this heuristically
  work would incurr some overhead for the coverage instrumentation.
  Unconditionally referencing coverage info OTOH might be too excessive,
  e.g. when the template function is in phobos, which would require to
recompile
  phobos with -cov for successful linking.

https://github.com/dlang/dmd/commit/0cb16110c307b608034d49289e2a650e7d7d7dd8
Merge pull request #7654 from MartinNowak/fix13742

fix Issue 13742 - undefined reference to __coverage

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