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Thread-local variables are put in the tdata section on AIX / XCOFF [1]. Unfortunately
the ExportImportList script currently doesn't consider this section when looking for
symbols to export.
This results in linking failures in applications (such as LLVM) when building which expect these thread local variables to be exported from their shared libraries.
[1]: https://www.ibm.com/docs/en/aix/7.3?topic=formats-xcoff-object-file-format
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When interprocedural analysis is enabled on the IBMClang family of
compilers (via the `-flto` option) then the resulting object files
contain LLVM IR rather than XCOFF objects[1].
ExportImportList needs to detect LLVM IR objects and use the
`ibm-llvm-nm` tool that ships with the compiler to create the extract
the defined symbols.
Without this change, such objects result in an error message from
`dump` and no symbols being exported from the object file.
[1]: https://www.ibm.com/docs/en/openxl-c-and-cpp-aix/17.1.0?topic=compatibility-link-time-optimization-lto
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This will allow more steps to be added.
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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
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On AIX, symbols in shared objects must be exported in order to be
visible to dependents (similar to Windows). The AIX linker provides a
`-bE:...` option to specify a file listing symbols to be exported.
Compilers offer some features to help:
* When the XL compiler is invoked with its `-qmkshrobj`/`-G` options for
creating shared objects (without/with runtime linking), it recognizes
when no explicit `-bE:...` linker option is specified and runs a
`CreateExportList` tool provided with the compiler to compute one from
the object files. Since commit d468a2c2cb (XL: Avoid copying archives
into shared libraries that link them, 2011-04-07, v2.8.5~153^2) CMake
runs `CreateExportList` explicitly to ensure it only looks at the object
files and not any library files.
* When the GNU compiler is invoked with its `-shared` option for creating
shared objects, its internal `collect2` tool recognizes when no explicit
`-bE:...` linker option is specified and computes one itself from the
object files. However, it sometimes includes extra symbols such as
`.__init_aix_libgcc_cxa_atexit`.
Introduce our own internal `ExportImportList` script to compute symbol
export lists from object files. Use a basic implementation for now: it
can be extended as needed later. Update our shared library creation
rules to run the script explicitly for both the XL and GNU compilers.
Issue: #19163
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