[Mesa-dev] [PATCH v4 6/6] i965: gl_BaseVertex must be zero for non-indexed draw calls

[Ian Romanick]

On 04/10/2018 09:26 AM, Jason Ekstrand wrote:
> On Tue, Apr 10, 2018 at 1:28 AM, Antia Puentes <apuentes at igalia.com
> <mailto:apuentes at igalia.com>> wrote:
> 
>     On 07/04/18 08:21, Jason Ekstrand wrote:
> 
>>     Oh, boy, this is tricky... First of all, it's a bit of a bummer
>>     that we can't just load the indirect buffer again for this.  Not
>>     too much to do about it, I guess.
>>
>>     Second, there be very scary dragons here.  It turns out that, at
>>     least on Haswell (and possibly other platforms), reading from
>>     state registers while rendering is in-flight can lead to GPU
>>     hangs.  Yes, I said "reading".  We found this out the hard way
>>     while working on Vulkan indirect clear colors.  The better thing
>>     to do here would be to use GPRs when available (I think they're
>>     safe but I'm not sure) or to do a MI_COPY_MEM_MEM which, of
>>     course, is only available on gen8+.  On Ivy Bridge (and haswell if
>>     we're going to do a store_register_mem from a state register), we
>>     need to do a mi_flush *before* the store as well.
>>      
> 
>     I see that this is complicated, I have thought in a different way to
>     implement this.
>     Instead of moving gl_BaseVertex to a VE2 and reading its value from
>     state registers:
> 
>     - VE1 remains as: <firstvertex, BaseInstance, VertexID, InstanceID>
>     -> Patches 1-5 are still valid (I think) and we can still calculate
>     the VertexID as FirstVertex + VertexIDZeroBased.
> 
>     - VE2 contains: <Draw ID, IsIndexedDraw, 0, 0>,
>     ->when asked for glBaseVertex (nir_instrinsic_load_base_vertex), we
>     would return the value stored in FirstVertex is the draw call is
>     indexed, zero if it is not.
> 
>     How does it sound?.
> 
> That sounds fine to me.  It's one extra instruciton in the shader (it

Unless there are objections, I'm going to push patches 1 through 5 (with
the extra comment Jason suggested in patch 4) in the morning.

> could be an AND if IsIndexedDraw is 0/~0) and lets us avoid trying to do
> command-stream copying of data.  If I understand correcctly, Vulkan will
> never hit this path because it will always use FIRST_VERETX and not
> BASE_VERTEX.  Is that correct?