i965: new VS: Clarify comments about max_usable_mrf and add an assertion.

max_usable_mrf has been carefully set such that (max_usable_mrf -
base_mrf) is a multiple of 2, so that an even number of VUE slots are
emitted with each URB write (which Gen6 requires).  This patch adds an
assertion to confirm that this is the case, and moves the comment to
this effect to be near the assertion.

Reviewed-by: Eric Anholt <eric@anholt.net>

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
index 478c1c8..9738ee3 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
@@ -1898,6 +1898,12 @@ vec4_visitor::emit_urb_writes()
     */
    int max_usable_mrf = 13;
 
+   /* The following assertion verifies that max_usable_mrf causes an
+    * even-numbered amount of URB write data, which will meet gen6's
+    * requirements for length alignment.
+    */
+   assert ((max_usable_mrf - base_mrf) % 2 == 0);
+
    /* FINISHME: edgeflag */
 
    brw_compute_vue_map(&c->vue_map, intel, c->key.nr_userclip,
@@ -1917,10 +1923,8 @@ vec4_visitor::emit_urb_writes()
    for (slot = 0; slot < c->vue_map.num_slots; ++slot) {
       emit_urb_slot(mrf++, c->vue_map.slot_to_vert_result[slot]);
 
-      /* If this was MRF 15, we can't fit anything more into this URB
-       * WRITE.  Note that base_mrf of 1 means that MRF 15 is an
-       * even-numbered amount of URB write data, which will meet
-       * gen6's requirements for length alignment.
+      /* If this was max_usable_mrf, we can't fit anything more into this URB
+       * WRITE.
        */
       if (mrf > max_usable_mrf) {
         slot++;