[RFC] Offloading and automatic linking of libraries

[Tom de Vries]

On 9/24/20 8:32 AM, Tobias Burnus wrote:
> Hi all,
> 
> we got the user comment that it is far from obvious to
> use  -foffload=-latomic if the following error shows up:
> 
> unresolved symbol __atomic_compare_exchange_16
> collect2: error: ld returned 1 exit status
> mkoffload: fatal error:
> ..../powerpc64le-none-linux-gnu-accel-nvptx-none-gcc returned 1 exit status
> 
> In principle, the same issue pops up with -lm and -lgfortran,
> which on the host are automatically linked for g++ (-lm) and
> for gfortran (both) but not gcc.
> 
> Atomic is a bit more special as '-lm' and its functions
> are better known and when compiling with gfortran and
> getting 'unresolved symbol _gfortran_stop_numeric' is
> also a bit more obvious.
> 
> Thoughts? Or just better educating our users in the
> documentation? — Currently, -foffload= is only documented
> in the wiki ...
> 
> For nvptx-none and libatomic, I was thinking of adding
>    -foffload=nvptx-none=-latomic
> to the host's
>    libgomp/libgomp.spec(.in)
> if configured with --enable-offload-targets=nvptx-none
> but that assumes that nvptx-none has been configured
> without --disable-atomic. – And I fail how the host
> configure can see how the device configure was invoked.

Maybe we could require building libatomic for nvptx.

Thanks,
- Tom