svga: detect constant color writes in fragment shaders

Examine the fragment shader to try to detect TGSI shaders which use
"MOV OUT[0], CONST[i]" to write a constant value for the fragment color.
In this case, all fragments will have the same color (unless blending is
enabled).

This is a common case for OpenGL code such as: glColor(), glBegin(),
glVertex(), ..., glEnd() when lighting/fog/etc are disabled.  In this
case, the Mesa/gallium state tracker actually generates a simple
"MOV OUT[0], CONST[i]" fragment shader.

This will be used by the next commit to avoid provoking vertex conversion
(creating/rewriting an index buffer) when drawing flat-shaded primitives.

Reviewed-by: Charmaine Lee <[email protected]>
Reviewed-by: José Fonseca <[email protected]>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/gallium/drivers/svga/svga_shader.h b/src/gallium/drivers/svga/svga_shader.h
index efcac408626..f49fdb46d0e 100644
--- a/src/gallium/drivers/svga/svga_shader.h
+++ b/src/gallium/drivers/svga/svga_shader.h
@@ -155,6 +155,9 @@ struct svga_shader_variant
                                 *  applied to any of the varyings.
                                 */
 
+   /** Is the color output just a constant value? (fragment shader only) */
+   boolean constant_color_output;
+
    /** For FS-based polygon stipple */
    unsigned pstipple_sampler_unit;