[Mesa-dev] [PATCH 1/9] i965/vec4: Don't emit MOVs for unused URB slots.

[Matt Turner]

On Tue, Oct 20, 2015 at 1:01 AM, Iago Toral <itoral at igalia.com> wrote:
> On Mon, 2015-10-19 at 21:08 -0700, Matt Turner wrote:
>> Otherwise we'd emit a MOV from the null register (which isn't allowed).
>>
>> Helps 24 programs in shader-db (the geometry shaders in GSCloth):
>>
>> instructions in affected programs:     302 -> 262 (-13.25%)
>
> Makes sense to me,
>
> Reviewed-by: Iago Toral Quiroga <itoral at igalia.com>
>
> Out of curiosity, does moving from the null register have any actual
> consequences?

The documentation says that it's not allowed, and I *think* I've seen
a case where it caused a GPU hang but I cannot remember for sure.