We've long created shared objects on AIX using the linker's `-G` option
(also offered by the XL front-end). The `-G` option implies `-brtl` and
enables runtime linking. This has been largely unnecessary because we
provide all dependencies on the link line and both XL and GNU compilers
offer builtin behavior to export symbols. Since commit 0f150b69d3 (AIX:
Explicitly compute shared object exports for both XL and GNU,
2019-07-11) we compute exports explicitly and consistently.
Therefore runtime linking is no longer necessary for shared objects.
We've also long created executables on AIX using the linker's `-brtl`
option to enable runtime linking in case they load plugins at runtime.
Since commit 9f5c2040bf (AIX: Explicitly compute executable exports for
both XL and GNU, 2019-07-12) and commit 2fa920c0cd (AIX: Create import
library for executables with exports, 2019-07-16) we now provide the
linker enough information to fully resolve symbols in plugins up front.
Therefore runtime linking is no longer necessary for executables.
Drop use of `-G` for creating shared objects and use the XL `-qmkshrobj`
and GCC `-shared` options instead. Both invoke the linker with the
`-bM:SRE -bnoentry` options to create a shared object without runtime
linking enabled. Also drop use of `-brtl` for creating executables.
Issue: #19163
On AIX, plugins meant to be loaded into executables via `dlopen` must be
linked with access to a list of symbols exported from the executable in
order to use them (when not using runtime linking). The AIX linker
supports specifying this list as an "import file" passed on the command
line either via the `-bI:...` option or (with a leading `#! .` line) as
a normal input file like any other library file.
The linker import file plays the same role on AIX as import libraries do
on Windows. Teach CMake to enable its import library abstraction on AIX
for executables with the `ENABLE_EXPORTS` target property set. Teach
our internal `ExportImportList` script to optionally generate a leading
`#! .` line at the top of the generated export/import list. Update our
rule for linking an executable with exports to generate a public-facing
"import library" implemented as an AIX linker import file.
With this approach, our existing infrastructure for handling import
libraries on Windows will now work for AIX linker import files too:
* Plugins that link to their executable's symbols will be automatically
linked using the import file on the command line.
* The executable's import file will be (optionally) installed and
exported for use in linking externally-built plugins.
This will allow executables and their plugins to build even if we later
turn off runtime linking.
Issue: #19163
