[Mesa-dev] [PATCH v2 00/31] Nir support for Nouveau

[Ilia Mirkin]

On Thu, Jan 4, 2018 at 3:07 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On January 4, 2018 14:02:25 Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>
>> On Thu, Jan 4, 2018 at 2:57 PM, Karol Herbst <kherbst at redhat.com> wrote:
>>>
>>> On Thu, Jan 4, 2018 at 8:56 PM, Jason Ekstrand <jason at jlekstrand.net>
>>> wrote:
>>>>
>>>> What is the nature of the immediate problem?  We may have a similar
>>>> issue.
>>>
>>>
>>>  we don't do rescheduling, so all the immediates are at the top of the
>>> shader.
>>
>>
>> The implication being that any values that are initialized to an
>> immediate (which is not infrequent) are done so at the top of the
>> shader, extending live ranges dramatically.
>>
>> It's not difficult to move them somewhere closer to first use, just
>> needs to be done.
>
>
> I think our back-end compiler's copy propagation and constant communication
> passes handle that for us fairly well.  The primary place it's likely to
> still be an issue for is loop counters and the like.

Well, even without anything fancy, let's say you have

x = uniform + 1

NVIDIA instruction encoding doesn't allow both a uniform and an
immediate at the same time, so one has to stay behind in a register.

If the move to the register happens at the top of the program, then
its live range is substantially extended.

  -ilia