[Mesa-dev] r600/sb loop issue

[Dave Airlie]

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 does seem to fix the problems in piglit, and looks close to what
I was attempting but written by someone who knows what they are doing :-)

What is the sb_sched.cpp change for at the end for?

Dave.