[Mesa-dev] r600/sb loop issue

[Dave Airlie]

On 9 December 2014 at 10:25, Dave Airlie <airlied at gmail.com> wrote:
> On 8 December 2014 at 20:41, Vadim Girlin <vadimgirlin at gmail.com> wrote:
>> On 12/06/2014 07:13 AM, Vadim Girlin wrote:
>>>
>>> On 12/04/2014 01:43 AM, Dave Airlie wrote:
>>>>
>>>> Hi Vadim,
>>>>
>>>> I've been looking with Glenn's help into a bug in sb for a couple of
>>>> weeks now triggered by a change in how GLSL generates switch
>>>> statements.
>>>>
>>>> I understand you probably aren't too interested in r600g but I believe
>>>> I'm hitting a design level problem and I would like some advice.
>>>>
>>>> So it appears that GLSL can create loops that don't repeat for switch
>>>> statements, and it appears SB wasn't ready to handle such a thing.
>>>
>>>
>>> Hi, Dave,
>>>
>>> I suspect we should rather get rid of such loops somehow, i.e. convert
>>> to something else, the loop that never repeats is not really a loop
>>> anyway. AFAICS "continue" is not supported in switch statements
>>> according to GLSL specs, so the loops generated for switch will never be
>>> repeated. Am I missing something? Even if repeating is possible somehow,
>>> at least we can get rid of the loops that are not repeated.
>>>
>>> I think loops are less efficient than other control flow instructions on
>>> r600g hw (at least because they increase stack usage), and possibly on
>>> other hw too.
>>>
>>> In fact it seems sb basically gets rid of it already in IR, it just
>>> doesn't know how to translate resulting control flow to ISA, because so
>>> far it only supports specific control flow structure for if-then-else
>>> that was previously preserved during optimizations. I think it may be
>>> not very hard to implement support for that in finalizer, I'll look into
>>> it.
>>
>>
>> In fact handling that control flow in finalizer is not as easy as I hoped,
>> probably impossible, at least if we want to make it efficient. I forgot
>> about the limitations of R600 ISA.
>>
>> OTOH it seems I've managed to fix the issues with loops, the patch is
>> attached (it's meant to be used instead of 7b0067d2). There are no piglit
>> regressions on evergreen, but I didn't test any real apps.
>>
>
> This fixes one thing, but the switches are still broken here on cayman at least
>
Actually ignore that, another regression snuck into r600g that I had to fix.

Dave.