Gold readiness obstacle #5: libtool (part 2, OpenMP)

After some digression on the issues with the latest glibc version, it’s time for the fifth instalment of gold readiness for the Gentoo tree, which is completing the libtool issues that I noted a couple of days ago in part 1.

As I said, this relates to a known issue with libtool and OpenMP, to the point that there is a Gentoo bug open and upstream libtool package is already fixed to deal with this, it’s just not trickled down to a release and from there into Gentoo. Although I guess it might be a good idea to just apply this with elibtoolize as I’ve done for the configure fix for gold.

What is the problem? Well, the first problem is with the design of OpenMP language extensions, and with some other flags that implicitly enable those extensions. While these are flags that relate to the GCC frontend (gcc command), they not only change the semantics of the compiled code, they also change the semantics of the linking step, by adding a link to the OpenMP implementation library. This means that the frontend needs to know about this both at compile time as well as link time (where the frontend converts it to the proper linking flags for the link editor to pick up OpenMP).

Unfortunately, when using libtool, its script is parsing and mangling the options first (it’s for this reason that libtool had to be patched to get --as-needed working as intended). Up to now, it didn’t know that -fopenmp should have been passed to the linking frontend of gcc just the same.

Okay, in truth this is not much of an issue for gold only; it is just the same issue when using ld/bfd. But the switch to a link editor that has stricter underlinking rejection makes the issue much more apparent, in particular because, while libtool is usually involved in building the libraries, there is no reason (beside the slight chance of using static linking through libtool archives) to force its usage for building final executables, which means that a single -fopenmp at the final linking point would be quite enough.

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