[Bug objc/102537] New: Objective-C: can't use >= USE_FIXUP_BEFORE paths for non-Darwin

["mhjacobson at me dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102537

            Bug ID: 102537
           Summary: Objective-C: can't use >= USE_FIXUP_BEFORE paths for
                    non-Darwin
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: objc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mhjacobson at me dot com
  Target Milestone: ---

I am working on a NeXTv2-ABI-compatible Objective-C runtime for a non-Darwin
platform (AVR micros).  I'd like my Objective-C code to make use of the most
modern ABI features, namely those guarded in the code by `flag_next_runtime >=
USE_FIXUP_BEFORE`.

However, there appears to be no way to control `flag_next_runtime` (short of
modifying the compiler source).  I can set it to zero with `-fgnu-runtime` or
one with `-fnext-runtime`, but `USE_FIXUP_BEFORE` is an encoded Mac OS X
version (namely 100600, referring to Snow Leopard).

There is Darwin-specific option parsing code (`darwin_override_options()`) that
appears to set `flag_next_runtime` based on `-mmacosx-version-min`, but
obviously that doesn't run for non-Darwin targets.

I could imagine a few approaches to fixing this:

1. Parse `-mmacosx-version-min` even when the target is not Darwin, whenever
we're compiling Objective-C.  On non-Darwin, this would be interpreted as
requesting Objective-C codegen compatible with the Objective-C ABI of the
specified release of Mac OS X.

2. Allow an argument to `-fnext-runtime`, with the meaning approximately the
same as in #1.

3. Instead of using `flag_next_runtime` as a version number, switch it back to
being zero or one, and use a separate flag (perhaps the existing
`-fobjc-abi-version`?) to differentiate ABIs.