i965: Consign COORD_REPLACE VS hacks to Pre-Gen6.

Pre-Gen6, the SF thread requires exact matching between VS output
slots (aka VUE slots) and FS input slots, even when the corresponding
VS output slot is unused due to being overwritten by point coordinate
replacement (glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, GL_TRUE)).
As a result, we have a special hack in the VS to ensure when any
texture coordinate is subject to point coordinate replacement, it is
always allocated space in the VUE, even if it isn't written to by the
VS.

This hack isn't needed from Gen6 onwards, since SF (Gen7: SBE)
swizzling has the ability to insert the point coordinate into
gl_TexCoord[] without needing a corresponding unused VUE slot.

Note that no modification of SF setup code is required for this
patch--get_attr_override() already does the right thing.  However, we
make a slight comment change to clarify why this works.

In addition to eliminating unnecessary VS recompiles and saving
precious URB space on Gen6+, this will save us the trouble of having
to adjust this hack when we implement geometry shaders.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

3 files changed, 34 insertions, 11 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_vs.c b/src/mesa/drivers/dri/i965/brw_vs.c
index 0810471..64659c0 100644
--- a/src/mesa/drivers/dri/i965/brw_vs.c
+++ b/src/mesa/drivers/dri/i965/brw_vs.c
@@ -258,15 +258,17 @@ do_vs_prog(struct brw_context *brw,
       c.prog_data.inputs_read |= VERT_BIT_EDGEFLAG;
    }
 
-   /* Put dummy slots into the VUE for the SF to put the replaced
-    * point sprite coords in.  We shouldn't need these dummy slots,
-    * which take up precious URB space, but it would mean that the SF
-    * doesn't get nice aligned pairs of input coords into output
-    * coords, which would be a pain to handle.
-    */
-   for (i = 0; i < 8; i++) {
-      if (c.key.point_coord_replace & (1 << i))
-	 c.prog_data.outputs_written |= BITFIELD64_BIT(VERT_RESULT_TEX0 + i);
+   if (intel->gen < 6) {
+      /* Put dummy slots into the VUE for the SF to put the replaced
+       * point sprite coords in.  We shouldn't need these dummy slots,
+       * which take up precious URB space, but it would mean that the SF
+       * doesn't get nice aligned pairs of input coords into output
+       * coords, which would be a pain to handle.
+       */
+      for (i = 0; i < 8; i++) {
+         if (c.key.point_coord_replace & (1 << i))
+            c.prog_data.outputs_written |= BITFIELD64_BIT(VERT_RESULT_TEX0 + i);
+      }
    }
 
    brw_compute_vue_map(brw, &c);
@@ -429,7 +431,7 @@ static void brw_upload_vs_prog(struct brw_context *brw)
    key.clamp_vertex_color = ctx->Light._ClampVertexColor;
 
    /* _NEW_POINT */
-   if (ctx->Point.PointSprite) {
+   if (intel->gen < 6 && ctx->Point.PointSprite) {
       for (i = 0; i < 8; i++) {
 	 if (ctx->Point.CoordReplace[i])
 	    key.point_coord_replace |= (1 << i);
diff --git a/src/mesa/drivers/dri/i965/brw_vs.h b/src/mesa/drivers/dri/i965/brw_vs.h
index 75c8a5f..caa8f7c 100644
--- a/src/mesa/drivers/dri/i965/brw_vs.h
+++ b/src/mesa/drivers/dri/i965/brw_vs.h
@@ -86,7 +86,17 @@ struct brw_vs_prog_key {
    GLuint userclip_planes_enabled_gen_4_5:MAX_CLIP_PLANES;
 
    GLuint copy_edgeflag:1;
+
+   /**
+    * For pre-Gen6 hardware, a bitfield indicating which texture coordinates
+    * are going to be replaced with point coordinates (as a consequence of a
+    * call to glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, GL_TRUE)).  Because
+    * our SF thread requires exact matching between VS outputs and FS inputs,
+    * these texture coordinates will need to be unconditionally included in
+    * the VUE, even if they aren't written by the vertex shader.
+    */
    GLuint point_coord_replace:8;
+
    GLuint clamp_vertex_color:1;
 
    struct brw_sampler_prog_key_data tex;
diff --git a/src/mesa/drivers/dri/i965/gen6_sf_state.c b/src/mesa/drivers/dri/i965/gen6_sf_state.c
index 7071acc..87f5bb1 100644
--- a/src/mesa/drivers/dri/i965/gen6_sf_state.c
+++ b/src/mesa/drivers/dri/i965/gen6_sf_state.c
@@ -78,7 +78,18 @@ get_attr_override(struct brw_vue_map *vue_map, int urb_entry_read_offset,
 
    if (slot == -1) {
       /* This attribute does not exist in the VUE--that means that the vertex
-       * shader did not write to it.  Behavior is undefined in this case, so
+       * shader did not write to it.  This means that either:
+       *
+       * (a) This attribute is a texture coordinate, and it is going to be
+       * replaced with point coordinates (as a consequence of a call to
+       * glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, GL_TRUE)), so the
+       * hardware will ignore whatever attribute override we supply.
+       *
+       * (b) This attribute is read by the fragment shader but not written by
+       * the vertex shader, so its value is undefined.  Therefore the
+       * attribute override we supply doesn't matter.
+       *
+       * In either case the attribute override we supply doesn't matter, so
        * just reference the first available attribute.
        */
       return 0;