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Subject: [Bug target/101544] [OpenMP][AMDGCN][nvptx] C++ offloading:
 unresolved _Znwm = "operator new(unsigned long)"
Date: Wed, 21 Jul 2021 10:03:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101544

--- Comment #5 from Andrew Stubbs <ams at gcc dot gnu.org> ---
[Note: all of my comments refer to the amdgcn case. nvptx has somewhat
different support in this area.]

(In reply to Jonathan Wakely from comment #4)
> But it's a waste of space in the .so to build lots of symbols that use the
> stubs.

DSOs are not supported. This is strictly for static linking only.

> There are other reasons it might be nice to be able to configure libstdc++
> for something in between a full hosted environment and a minimal
> freestanding one.

If it isn't a horrible hack, like libgfortran minimal mode, then fine.

> > I believe static constructors work (libgfortran uses some), but excepti=
on
> > handling does not. I'm not sure what other exotica C++ might need?
>=20
> Ideally, __cxa_atexit and __cxa_thread_atexit for static and thread-local
> destructors, but we can survive without them (and have not-fully-conformi=
ng
> destruction ordering).

Offload kernels are just fragments of programs, so this is tricky in those
cases. Libgomp explicitly calls _init_array and _fini_array as single-threa=
ded
kernel launches. Actually, it's not clear that deconstruction is in any way
interesting, given that code running on the GPU has no external access and =
the
resources are all released when the host program exits.

Similarly, C++ threads are not interesting in the GPU-offload case. There a=
re a
fixed number or threads launched on entry and they are managed by libgomp. =
In
theory it would be possible to code gthreads/libstdc++ to use them in
standalone mode, but really that mode only exists to facilitate compiler
testing.=