[Mesa-dev] [PATCH 2/2] i965: Don't replicate data for zero-stride arrays when copying to VBOs.

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> When copy_array_to_vbo_array encountered an array with src_stride == 0
> and dst_stride != 0, we would replicate out the single element to the
> whole size (max - min + 1).  This is unnecessary: we can simply upload
> one copy and set the buffer's stride to 0.
>
> Decreases vertex upload overhead in an upcoming Steam for Linux title.
> Prior to this patch, copy_array_to_vbo_array appeared very high in the
> profile (Eric quoted 20%).  After the patch, it disappeared completely.

> diff --git a/src/mesa/drivers/dri/i965/brw_draw_upload.c b/src/mesa/drivers/dri/i965/brw_draw_upload.c
> index 722166c..ad7fe7c 100644
> --- a/src/mesa/drivers/dri/i965/brw_draw_upload.c
> +++ b/src/mesa/drivers/dri/i965/brw_draw_upload.c
> @@ -311,13 +311,13 @@ copy_array_to_vbo_array(struct brw_context *brw,
>  			struct brw_vertex_buffer *buffer,
>  			GLuint dst_stride)
>  {
> -   if (min == -1) {
> -      /* If we don't have computed min/max bounds, then this must be a use of
> -       * the current attribute, which has a 0 stride.  Otherwise, we wouldn't
> -       * know what data to upload.
> -       */
> -      assert(element->glarray->StrideB == 0);
> +   const int src_stride = element->glarray->StrideB;

I hate const on local variable declarations with a passion.  But this is
still such a good fix that:

Reviewed-by: Eric Anholt <eric at anholt.net>
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