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, 7 insertions, 0 deletions

diff --git a/src/gallium/drivers/svga/svga_tgsi.c b/src/gallium/drivers/svga/svga_tgsi.c
index 202eee276b7..4c16f4313a0 100644
--- a/src/gallium/drivers/svga/svga_tgsi.c
+++ b/src/gallium/drivers/svga/svga_tgsi.c
@@ -240,6 +240,13 @@ svga_tgsi_vgpu9_translate(struct svga_context *svga,
 
    variant->pstipple_sampler_unit = emit.pstipple_sampler_unit;
 
+   /* If there was exactly one write to a fragment shader output register
+    * and it came from a constant buffer, we know all fragments will have
+    * the same color (except for blending).
+    */
+   variant->constant_color_output =
+      emit.constant_color_output && emit.num_output_writes == 1;
+
 #if 0
    if (!svga_shader_verify(variant->tokens, variant->nr_tokens) ||
        SVGA_DEBUG & DEBUG_TGSI) {