Re: [Patch] nvptx/mkoffload.cc: Add dummy proc for OpenMP rev-offload table [PR108098]

[Thomas Schwinge <thomas@codesourcery.com>]

Hi!

Are we going to install such a work-around?


Grüße
 Thomas


On 2022-12-19T13:04:43+0100, I wrote:
> Hi!
>
> On 2022-12-16T17:19:00+0100, Tobias Burnus <tobias@codesourcery.com> wrote:
>> Seems to be a CUDA JIT issue
>
> A Nvidia Driver JIT issue, more precisely.  ;-)
>
>> which is fixed by adding a dummy procedure.
>
> Gah...  :-|
>
>> Lightly tested with 4 systems at hand, where 2 failed before.
>
> I'm happy to confirm that indeed this does resolve the issue for all
> configurations that I reported in <https://gcc.gnu.org/PR108098>
> "OpenMP/nvptx reverse offload execution test FAILs".
>
>
> As I said on IRC, #gcc, 2022-12-16:
>
>> [...] we're unlikely to reverse-engineer the exact version/conditions
>> where this got fixed, so don't have a useful means for versioning the
>> workaround.  Fortunately, it doesn't "cost" anything really.  (In
>> constrast to some other GCC/nvptx back end workarounds, as I
>> understand.)
>
>
> Grüße
>  Thomas
>
>
>> One had 10.2 and
>> the other had some ancient CUDA where 'nvptx-smi' did not print a CUDA version
>> and requires -mptx=3.1.
>> (I did check that offloading indeed happened and no hostfallback was done.)
>>
>> OK for mainline?
>>
>> Tobias
>
>
>> nvptx/mkoffload.cc: Add dummy proc for OpenMP rev-offload table [PR108098]
>>
>> Seemingly, the ptx JIT of CUDA <= 10.2 replaces function pointers in global
>> variables by NULL if a translation does not contain any executable code. It
>> works with CUDA 11.1.  The code of this commit is about reverse offload;
>> having NULL values disables the side of reverse offload during image load.
>>
>> Solution is the same as found by Thomas for a related issue: Adding a dummy
>> procedure. Cf. the PR of this issue and Thomas' patch
>> "nvptx: Support global constructors/destructors via 'collect2'"
>> https://gcc.gnu.org/pipermail/gcc-patches/2022-December/607749.html
>>
>> As that approach also works here:
>>
>> Co-authored-by: Thomas Schwinge <thomas@codesourcery.com>
>>
>> gcc/
>>      PR libgomp/108098
>>
>>      * config/nvptx/mkoffload.cc (process): Emit dummy procedure
>>      alongside reverse-offload function table to prevent NULL values
>>      of the function addresses.
>>
>> ---
>>  gcc/config/nvptx/mkoffload.cc | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/gcc/config/nvptx/mkoffload.cc b/gcc/config/nvptx/mkoffload.cc
>> index 5d89ba8..8306aa0 100644
>> --- a/gcc/config/nvptx/mkoffload.cc
>> +++ b/gcc/config/nvptx/mkoffload.cc
>> @@ -357,6 +357,20 @@ process (FILE *in, FILE *out, uint32_t omp_requires)
>>      fputc (sm_ver2[i], out);
>>        fprintf (out, "\"\n\t\".file 1 \\\"<dummy>\\\"\"\n");
>>
>> +      /* WORKAROUND - see PR 108098
>> +     It seems as if older CUDA JIT compiler optimizes the function pointers
>> +     in offload_func_table to NULL, which can be prevented by adding a
>> +     dummy procedure. With CUDA 11.1, it seems to work fine without
>> +     workaround while CUDA 10.2 as some ancient version have need the
>> +     workaround. Assuming CUDA 11.0 fixes it, emitting it could be
>> +     restricted to 'if (sm_ver2[0] < 8 && version2[0] < 7)' as sm_80 and
>> +     PTX ISA 7.0 are new in CUDA 11.0; for 11.1 it would be sm_86 and
>> +     PTX ISA 7.1.  */
>> +      fprintf (out, "\n\t\".func __dummy$func ( );\"\n");
>> +      fprintf (out, "\t\".func __dummy$func ( )\"\n");
>> +      fprintf (out, "\t\"{\"\n");
>> +      fprintf (out, "\t\"}\"\n");
>> +
>>        size_t fidx = 0;
>>        for (id = func_ids; id; id = id->next)
>>      {
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