[Mesa-dev] per-vertex array max_index

[José Fonseca]

On 04/15/2011 08:51 PM, Brian Paul wrote:
> On 04/15/2011 01:41 PM, José Fonseca wrote:
>    
>> On 04/15/2011 07:20 PM, Christoph Bumiller wrote:
>>      
>>> On 15.04.2011 18:04, Brian Paul wrote:
>>>
>>>        
>>>> Hi Marek,
>>>>
>>>> Back on Jan 10 you removed the per-vertex array max_index field
>>>> (commit cdca3c58aa2d9549f5188910e2a77b438516714f).  I believe this was
>>>> a mistake.
>>>>
>>>> I noticed today that the piglit draw-instanced-divisor test is
>>>> failing.  A bisection points to Jose's commit
>>>> 17bbc1f0425b3768e26473eccea5f2570dcb26d3.  But that's a red herring.
>>>> If I disable the SSE code paths, the regression goes back to the Jan
>>>> 10 change.
>>>>
>>>> With the GL_ARB_instanced_arrays extension vertex array indexing
>>>> changes quite a bit.  Specifically, some arrays (those with divisors
>>>> != 0) are now indexed by instance ID, not the primitive's vertex
>>>> index.  The draw_info::max_index field doesn't let us take this into
>>>> account.  I believe that we really need a per-array max_index value.
>>>>
>>>> As an example, suppose we're drawing a star field with 1 million
>>>> instanced stars, each star drawn as a 4-vertex quad.  We might use a
>>>> vertex array indexed by the instance ID to color the stars.
>>>>
>>>> The draw call would look like:
>>>>
>>>> glDrawArraysInstanced(GL_QUADS, 0, 4, 1000*1000);
>>>>
>>>> In this case we'd have two vertex arrays.  The first array is the quad
>>>> vertex positions with four elements.  The second array is the star
>>>> colors with 1 million elements.  As it is now, we're setting
>>>> draw_info::max_index = 4 and we errantly clamp the index into the
>>>> second array to 4 instead of 1 million.
>>>>
>>>> As a temporary work around we can disable clamping of array indexes
>>>> for instance arrays.  But I think we need to revert the Jan 10 commit
>>>> and then rework some of Jose's related changes.
>>>>
>>>> -Brian
>>>>
>>>>
>>>>          
>>> You know which vertex elements are per-instance, you know the divisor,
>>> and you know the max instance index - that should be all the information
>>> you need.
>>>
>>> You just have to clamp it to startInstance + instanceCount for them instead.
>>>
>>>
>>>        
>> draw_info::max_index is an optimization hint. In theory it could be always 0xfffffff and the pipe driver should still cope with it. Provided it is an upper bound of the true max index in the index buffer it should cause no visible difference.
>>
>>
>> My u_draw.c code already ignores instanced elements when computing
>> max_index.
>>
>> And I believe the translate module doesn't clamp to max_index when
>> fetching instanced elements, but I'll have to double check.
>>
>> I didn't look too closely at current st_draw.c code though yet. But it
>> appears the bug is in st_draw.c, as there is no need to artificially
>> limit the max_index passed by the app.
>>      
> I thought we were using max_index to prevent out-of-bounds buffer
> reads.  Are we not doing that anywhere?
>
>    

Inside the draw module there is a max_index with that meaning that: 
draw_context->pt.max_index.

This is all very subtle, but min_index/max_index is really a property of 
the index_buffer alone, independent of vertex buffers size and/or any 
index_bias.

src/gallium/docs/source/context.rst could be a bit better... This is 
what it has currently:

    All vertex indices must fall inside the range given by ``min_index`` and
    ``max_index``.  In case non-indexed draw, ``min_index`` should be set to
    ``start`` and ``max_index`` should be set to ``start``+``count``-1.

    ``index_bias`` is a value added to every vertex index before
    fetching vertex
    attributes.  It does not affect ``min_index`` and ``max_index``.

    If there is an index buffer bound, and ``indexed`` field is true,
    all vertex
    indices will be looked up in the index buffer.  ``min_index``,
    ``max_index``,
    and ``index_bias`` apply after index lookup.


This would be a better description:

    In indexed draw, all vertex indices in the vertex buffer should fall
    inside the range given by ``min_index`` and
    ``max_index``, but the driver should not crash or hang if not.  It
    is merely an optimization hint so the driver knows which range of
    vertices will be referenced without having to scan the index buffer
    with the CPU.  A state tracker could always set ``min_index`` and
    ``max_index`` to 0 and 0xffffffff respectively, and the only
    prejudice would be performance, not stability, which means that the
    driver should internally guarantee there will be no out of bounds
    accesses.

    In case non-indexed draw, ``min_index`` should be set to
    ``start`` and ``max_index`` should be set to ``start``+``count``-1.

    ``index_bias`` is a value added to every vertex index from the index
    buffer before fetching vertex
    attributes.  It does not affect ``min_index`` and ``max_index``.

    If there is an index buffer bound, and ``indexed`` field is true,
    all vertex
    indices will be looked up in the index buffer.

Any suggestions for better english welcome.

Jose
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