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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/97203] [nvptx] 'illegal memory access was encountered'
 with 'omp simd'/SIMT and cexpf call
Date: Fri, 09 Oct 2020 08:36:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97203

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amonakov at gcc dot gnu.org

--- Comment #6 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #4)
> So, I think calling functions from simd code is atm not supported for nvp=
tx.
>=20
> Stack variables in simd code are mapped on a per-thread stack rather than=
 on
> the
> usual per-warp stack.
>=20
> The functions are compiled with the usual per-warp stack, so calling those
> functions from simd might mean the different lanes are gonna disagree abo=
ut
> what the value in a stack variable should be.

This is inaccurate. In -msoft-stack mode there's no baked-in assumption that
stacks are always per-warp. The "soft stack" pointer can point either to gl=
obal
memory (outside of SIMD regions), or to local memory (inside SIMD regions).=
 The
pointer is switched between per-warp global memory and per-lane local memor=
y by
nvptx.c:nvptx_output_softstack_switch.

The main requirement is that functions callable from OpenMP offloaded code =
are
compiled for -mgomp multilib variant. The design allows calling functions e=
ven
from inside SIMD regions, and it should be supported.

It is very disappointing that the first reaction was "I think ... is not
supported" without reaching out and asking questions. Lack of efficient
communication was a huge issue when OpenMP offloading support was contribut=
ed,
and it's disappointing to see it again years later.=