[Mesa-dev] [RFC] [PATCH] mesa/st: set max_int instance_divisor for constant elements

[Luca Barbieri]

Currently constant elements are specified only with stride = 0.

However, drivers often need to know whether an element is constant
or not when setting up the vertex element CSO instead of when setting
the buffers.

Hence, in addition to stride = 0, also set instance_divisor = ~0 in
the vertex element, in all places where Mesa sets up both at the same time.

Drivers that ignore instancing will behave as before since stride = 0 is
set too.

Drivers that support instancing should continue to behave correctly, and
if not can just ignore this divisor.

Drivers that take advantage of this hint will however perform best.
---
 src/mesa/state_tracker/st_draw.c          |    4 ++--
 src/mesa/state_tracker/st_draw_feedback.c |    2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/mesa/state_tracker/st_draw.c b/src/mesa/state_tracker/st_draw.c
index 5b05489..ee69572 100644
--- a/src/mesa/state_tracker/st_draw.c
+++ b/src/mesa/state_tracker/st_draw.c
@@ -388,7 +388,7 @@ setup_interleaved_attribs(GLcontext *ctx,
 
       velements[attr].src_offset =
          (unsigned) (arrays[mesaAttr]->Ptr - offset0);
-      velements[attr].instance_divisor = 0;
+      velements[attr].instance_divisor = stride ? 0 : ~0;
       velements[attr].vertex_buffer_index = 0;
       velements[attr].src_format =
          st_pipe_vertex_format(arrays[mesaAttr]->Type,
@@ -483,7 +483,7 @@ setup_non_interleaved_attribs(GLcontext *ctx,
       /* common-case setup */
       vbuffer[attr].stride = stride; /* in bytes */
       vbuffer[attr].max_index = max_index;
-      velements[attr].instance_divisor = 0;
+      velements[attr].instance_divisor = stride ? 0 : ~0;
       velements[attr].vertex_buffer_index = attr;
       velements[attr].src_format
          = st_pipe_vertex_format(arrays[mesaAttr]->Type,
diff --git a/src/mesa/state_tracker/st_draw_feedback.c b/src/mesa/state_tracker/st_draw_feedback.c
index 5cf2666..41c9ecc 100644
--- a/src/mesa/state_tracker/st_draw_feedback.c
+++ b/src/mesa/state_tracker/st_draw_feedback.c
@@ -180,7 +180,7 @@ st_feedback_draw_vbo(GLcontext *ctx,
       /* common-case setup */
       vbuffers[attr].stride = arrays[mesaAttr]->StrideB; /* in bytes */
       vbuffers[attr].max_index = max_index;
-      velements[attr].instance_divisor = 0;
+      velements[attr].instance_divisor = vbuffers[attr].stride ? 0 : ~0;
       velements[attr].vertex_buffer_index = attr;
       velements[attr].src_format = 
          st_pipe_vertex_format(arrays[mesaAttr]->Type,
-- 
1.7.0.4