[Patch] [middle-end & nvptx] gcc/tracer.c: Don't split BB with SIMT LANE [PR95654]

[Tom de Vries]

On 9/16/20 1:24 PM, Alexander Monakov wrote:
> Can you supply a tar with tree dumps for me to look at please?
> 

Hi Alexander,

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95654#c6 .

> Also, if you can check if the problem can be triggered without a
> collapsed loop (e.g. try removing collapse(2), remove mentions of
> d2) and if so supply dumps from that instead, I'd appreciate that too.
> 

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95654#c8

Thanks,
- Tom

> Alexander
> 
> On Wed, 16 Sep 2020, Tom de Vries wrote:
> 
>> [ cc-ing author omp support for nvptx. ]
>>
>> On 9/16/20 12:39 PM, Tobias Burnus wrote:
>>> Hi Tom, hi Richard, hello all,
>>>
>>> @Richard: does it look okay from the ME side?
>>> @Tom: Can you check which IFN_GOMP_SIMT should be
>>> excluded with -ftracer?
>>>
>>> Pre-remark, I do not know much about SIMT – except that they
>>> only appear with nvptx and somehow relate to lanes on the
>>> GPU.
>>>
>>> In any case, as the testcase libgomp.fortran/pr66199-5.f90 shows,
>>> if a basic block with  GOMP_SIMT_VOTE_ANY in it is duplicated,
>>> which happens via -ftracer for this testcase, the result is wrong.
>>>
>>> The testcase ignores all the loop work but via "lastprivate" takes
>>> the upper loop bound (as assigned to the loop indices); instead of
>>> the expected 32*32 = 1024, either some number (like 4 or very large
>>> or negative) is returned.
>>>
>>> While GOMP_SIMT_VOTE_ANY fixes the issue for this testcase, I
>>> have the feeling that at least GOMP_SIMT_LAST_LANE should be
>>> not copied - but I might be wrong.
>>>
>>> Tom: Do you think GOMP_SIMT_LAST_LANE should be removed from
>>> that list – or any of the following added as well?
>>> GOMP_USE_SIMT, GOMP_SIMT_ENTER, GOMP_SIMT_ENTER_ALLOC, GOMP_SIMT_EXIT,
>>> GOMP_SIMT_VF, GOMP_SIMT_ORDERED_PRED, GOMP_SIMT_XCHG_BFLY,
>>> GOMP_SIMT_XCHG_IDX
>>>
>>> OK for mainline?
>>>
>>> Tobias
>>>
>>> -----------------
>>> Mentor Graphics (Deutschland) GmbH, Arnulfstraße 201, 80634 München /
>>> Germany
>>> Registergericht München HRB 106955, Geschäftsführer: Thomas Heurung,
>>> Alexander Walter