The XL toolchain supports shared object files stored in archives. Since
CMake lists libraries on link lines by full path it is common for a
shared library link line to contain the path to an archive file.
When linking a shared library the compiler front-end by default runs
CreateExportList to construct the list of symbols to be exported.
Unfortunately it passes all files found on the command line to the tool
so archive and library files get processed along with the object files.
The tool returns a list of all symbols in all objects, archives, and
libraries on the command line. This causes the linker to copy every
object file out of every archive into the shared library whether they
are dependencies of the original object files or not.
Work around this problem by running CreateExportList ourselves with just
the original object files intended for inclusion in the shared library.
Then pass the list it produces on the link line to prevent the compiler
front-end from constructing its own. This tells the linker to export
only the symbols provided by the original source files of the shared
library.
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