[Mesa-dev] [PATCH] i965: Don't use texture swizzling to force alpha to 1.0 if unnecessary.

[Ian Romanick]

On 03/15/2013 03:14 PM, Kenneth Graunke wrote:
> Commit 33599433c7 began setting the texture swizzle mode to XYZ1 for
> RED, RG, and RGB textures in order to force alpha to 1.0 in case we
> actually stored the texture as RGBA.
>
> This had a unforseen performance implication: the shader precompile
> assumes that the texture swizzle mode will be XYZW for non-shadow
> sampler types.  By setting it to XYZ1, this means every shader used with
> a RED, RG, or RGB texture has to be recompiled.  This is a very common
> case.
>
> Unfortunately, there's no way to improve the precompile, since RGBA
> textures still need XYZW, and there's no way to know by looking at
> the shader source what texture formats might be used.

I suspect that in a lot of cases the other components of the texture 
aren't used by the shader.  Could we use that information somehow?  We 
ought to be able to track which components of each sampler variable are 
used.

> However, we only need to smash alpha to 1.0 if the texture's memory
> format actually has alpha bits.  If not, the sampler already returns 1.0
> for us without any special swizzling.  XRGB8888, for example, is a very
> common case where this occurs.
>
> This partially fixes a performance regression since commit 33599433c7.
> More work is required to fully fix it in all cases.  This at least helps
> Warsow.
>
> NOTE: This is a candidate for the 9.1 branch.
>
> Cc: Carl Worth <cworth at cworth.org>
> Cc: Eric Anholt <eric at anholt.net>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>   src/mesa/drivers/dri/i965/brw_wm_surface_state.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_wm_surface_state.c b/src/mesa/drivers/dri/i965/brw_wm_surface_state.c
> index 0cb4b2d..771655d 100644
> --- a/src/mesa/drivers/dri/i965/brw_wm_surface_state.c
> +++ b/src/mesa/drivers/dri/i965/brw_wm_surface_state.c
> @@ -773,7 +773,8 @@ brw_get_texture_swizzle(const struct gl_context *ctx,
>      case GL_RED:
>      case GL_RG:
>      case GL_RGB:
> -      swizzles[3] = SWIZZLE_ONE;
> +      if (_mesa_get_format_bits(img->TexFormat, GL_ALPHA_BITS) > 0)
> +         swizzles[3] = SWIZZLE_ONE;
>         break;
>      }
>
>