Re: nvptx multilib setup

[Thomas Schwinge <thomas@codesourcery.com>]

Hi Tom!

On 2022-02-04T13:09:29+0100, Tom de Vries via Gcc <gcc@gcc.gnu.org> wrote:
> On 2/4/22 08:21, Thomas Schwinge wrote:
>> On 2022-02-03T13:35:55+0000, "vries at gcc dot gnu.org via Gcc-bugs" <gcc-bugs@gcc.gnu.org> wrote:
>>> I've tested this using (recommended) driver 470.94 on boards:

>>> while iterating over dimensions { -mptx=3.1 , -mptx=6.3 } x { GOMP_NVPTX_JIT=-O0, <default> }.
>>
>> Do you use separate (nvptx-none offload target only?) builds for
>> different '-mptx' variants (likewise: '-misa'), or have you hacked up the
>> multilib configuration?
>
> Neither, I'm using --target_board=unix/foffload= for that.

ACK, I see.  So these flags then only affect GCC/nvptx code generation
for the actual user code (here: GCC libgomp test cases), but for the
GCC/nvptx target libraries (such as: libc, libm, libgfortran, libgomp --
the latter especially relevant for OpenMP), it uses PTX code from one of
the two "pre-compiled" GCC/nvptx multilibs: default or '-mptx=3.1'.

Meaning, one can't just use such a flag for "completely building code"
for a specific configuration.  Random example,
'-foffload-options=nvptx-none=-march=sm_75': as GCC/nvptx target
libraries aren't being built for '-march=sm_75' multilib,
'-foffload-options=nvptx-none=-march=sm_75' uses the default multilib,
which isn't '-march=sm_75'.


>   ('gcc/config/nvptx/t-nvptx:MULTILIB_OPTIONS'
>> etc., I suppose?)  Should we add a few representative configurations to
>> be built by default?  And/or, should we have a way to 'configure' per
>> user needs (I suppose: '--with-multilib-list=[...]', as supported for a
>> few other targets?)?  (I see there's also a new
>> '--with-multilib-generator=[...]', haven't looked in detail.)  No matter
>> which way: again, combinatorial explosion is a problem, of course...
>
> As far as I know, the gcc build doesn't finish when switching default to
> higher than sm_35, so there's little point to go to a multilib setup at
> this point.  But once we fix that, we could reconsider, otherwise,
> things are likely to regress again.

As far as I remember, several issues have been fixed.  Still waiting for
Roger's "middle-end: Support ABIs that pass FP values as wider integers"
or something similar, but that PR104489 issue is being worked around by
"Limit HFmode support to mexperimental", if I got that right.

Now I'm not suggesting we should now enable all or any random GCC/nvptx
multilibs, to get all these variants of GCC/nvptx target libraries built;
especially also given that GCC/nvptx code generation currently doesn't
make too much use of the new capabilities.

However, we do have a specific request that a customer would like to be
able to change at GCC 'configure' time the GCC/nvptx default multilib
(including that being used for building corresponding GCC/nvptx target
libraries).

Per 'gcc/doc/install.texi', I do see that some GCC targets allow for
GCC 'configure'-time '--with-multilib-list=[...]', or
'--with-multilib-generator=[...]', and I suppose we could be doing
something similar?  But before starting implementing, I'd like your
input, as you'll be the one to approve in the end.  And/or, maybe you've
already made up your own ideas about that?


Grüße
 Thomas
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