[Mesa-dev] [PATCH 0/5] Volatile and invariant LDS memory ops

[Connor Abbott]

On Fri, Nov 10, 2017 at 1:19 PM, Marek Olšák <maraeo at gmail.com> wrote:
> On Fri, Nov 10, 2017 at 6:55 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
>> On 10.11.2017 18:43, Marek Olšák wrote:
>>>
>>> On Fri, Nov 10, 2017 at 2:09 AM, Connor Abbott <cwabbott0 at gmail.com>
>>> wrote:
>>>>
>>>> On Thu, Nov 9, 2017 at 7:17 PM, Marek Olšák <maraeo at gmail.com> wrote:
>>>>>
>>>>> On Fri, Nov 10, 2017 at 12:40 AM, Matt Arsenault <arsenm2 at gmail.com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>>> On Nov 10, 2017, at 07:41, Marek Olšák <maraeo at gmail.com> wrote:
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> This fixes the TCS gl_ClipDistance piglit failure that was uncovered
>>>>>>> by a recent LLVM change. The solution is to set volatile on loads
>>>>>>> and stores to enforce proper ordering.
>>>>>>>
>>>>>>> Please review.
>>>>>>>
>>>>>>
>>>>>>
>>>>>> Every LDS access certainly should not be volatile. This kills all
>>>>>> optimizations, like formation of ds_read2_b32. What ordering issue are you
>>>>>> having?
>>>>>
>>>>>
>>>>> It might be caused by inttoptr(NULL) that we do to declare LDS. There
>>>>> is simply no ordering enforced, which is weird.
>>>>
>>>>
>>>> As soon as you do inttoptr(NULL), you've generated a poison value (in
>>>> LLVM legalese), so LLVM will assume that you never dereference it and
>>>> optimize accordingly. I think a GEP instruction without the inbounds
>>>> parameter set will get rid of the poison value, although I'm not sure
>>>> about the case where the offset is known to be zero. At least, that's
>>>> my reading of the langref text for the GEP instruction
>>>> (https://llvm.org/docs/LangRef.html#id215). If zero is a valid address
>>>> in LDS, then it sounds like LLVM needs to be fixed to disable this
>>>> optimization for certain address spaces. On the other hand, if you're
>>>> doing inttoptr(NULL) + offset, where "offset" is the result of a
>>>> ptrtoint somewhere, you should be doing inttoptr(offset) instead, and
>>>> then LLVM should never misbehave.
>>>
>>>
>>> I don't think that using inttoptr before every load and store would be
>>> better than volatile. The must be a better solution.
>>
>>
>> Can't we just allocate the required LDS memory explicitly like we did for
>> the LDS-based derivative computations?
>>
>> It may require shuffling around a bit how/when we calculate the required
>> sizes, but it doesn't seem impossible.
>
> We want to share the same declaration in TCS main and epilog parts.
>
> Does LLVM know that LDS declarations are pre-initialized?
> Do sized LDS declarations affect SIMD-occupancy-based optimizations?
> Because Mesa always declares 64kB of LDS and the real value is
> determined at runtime.

I don't know about the latter, but for the former, if you declare the
LDS variable as having external linkage, LLVM should assume that it
might be initialized beforehand -- exactly like a global non-static
variable in C.

>
> Marek