[Mesa-dev] [PATCH 5/5] st/mesa: clean up min/max_index handling in st_draw_vbo

[Nicolai Hähnle]

On 24.04.2017 15:31, Marek Olšák wrote:
> On Mon, Apr 24, 2017 at 12:01 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
>> On 23.04.2017 01:10, Marek Olšák wrote:
>>>
>>> From: Marek Olšák <marek.olsak at amd.com>
>>>
>>> There is no reason to check for ~0.
>>> Also remove the incorrect comment.
>>> ---
>>>  src/mesa/state_tracker/st_draw.c | 9 ++-------
>>>  1 file changed, 2 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/src/mesa/state_tracker/st_draw.c
>>> b/src/mesa/state_tracker/st_draw.c
>>> index d710284..e510d43 100644
>>> --- a/src/mesa/state_tracker/st_draw.c
>>> +++ b/src/mesa/state_tracker/st_draw.c
>>> @@ -200,28 +200,23 @@ st_draw_vbo(struct gl_context *ctx,
>>>     if (ib) {
>>>        /* Get index bounds for user buffers. */
>>>        if (!index_bounds_valid)
>>>           if (!all_varyings_in_vbos(arrays))
>>>              vbo_get_minmax_indices(ctx, prims, ib, &min_index,
>>> &max_index,
>>>                                     nr_prims);
>>>
>>>        setup_index_buffer(st, ib);
>>>
>>>        info.indexed = TRUE;
>>> -      if (min_index != ~0U && max_index != ~0U) {
>>> -         info.min_index = min_index;
>>> -         info.max_index = max_index;
>>> -      }
>>> +      info.min_index = min_index;
>>> +      info.max_index = max_index;
>>
>>
>> For this and the previous patch, I think it would be cleaner to keep the
>> semantics of pipe_draw_info::min_index/max_index to always make sense. This
>> would mean dropping the previous patch, and changing this patch to only set
>> the min_index/max_index if index_bounds_valid (rather than the ~0U check).
>
> The old code didn't check index_bounds_valid, and u_vbuf already
> treats max_index == ~0 as invalid range. Instead of starting to use
> index_bounds_valid here, I'd prefer it if we removed
> index_bounds_valid and only rely on max_index == ~0 from vbo.

Hmm. Legitimate and valid max_index == ~0 is definitely possible on 
64-bit systems. It's ridiculously unlikely in practice though, so I 
guess it's worth considering this change in the name of eliminating a 
branch here.

That said, it looks like some drivers are actually using max_index, so 
at least it needs to be checked that this change is okay for them.

And definitely, if this change is made, document the changed semantics 
of pipe_draw_info.

Cheers,
Nicolai
-- 
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Aber vergiss niemals, wie sie sein sollte.