i965 gen6: Fix transform feedback of triangle strips.

When rendering triangle strips, vertices come down the pipeline in the
order specified, even though this causes alternate triangles to have
reversed winding order.  For example, if the vertices are ABCDE, then
the GS is invoked on triangles ABC, BCD, and CDE, even though this
means that triangle BCD is in the reverse of the normal winding order.
The hardware automatically flags the triangles with reversed winding
order as _3DPRIM_TRISTRIP_REVERSE, so that face culling and two-sided
coloring can be adjusted to account for the reversed order.

In order to ensure that winding order is correct when streaming
vertices out to a transform feedback buffer, we need to alter the
ordering of BCD to BDC when the first provoking vertex convention is
in use, and to CBD when the last provoking vertex convention is in
use.

To do this, we precompute an array of indices indicating where each
vertex will be placed in the transform feedback buffer; normally this
is SVBI[0] + (0, 1, 2), indicating that vertex order should be
preserved.  When the primitive type is _3DPRIM_TRISTRIP_REVERSE, we
change this order to either SVBI[0] + (0, 2, 1) or SVBI[0] + (1, 0,
2), depending on the provoking vertex convention.

Fixes piglit tests "EXT_transform_feedback/tessellation
triangle_strip" on Gen6.

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

2 files changed, 72 insertions, 18 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_gs.h b/src/mesa/drivers/dri/i965/brw_gs.h
index 7bf2248a5ed..2ab8b724eac 100644
--- a/src/mesa/drivers/dri/i965/brw_gs.h
+++ b/src/mesa/drivers/dri/i965/brw_gs.h
@@ -83,6 +83,12 @@ struct brw_gs_compile {
       struct brw_reg vertex[MAX_GS_VERTS];
       struct brw_reg header;
       struct brw_reg temp;
+
+      /**
+       * Register holding destination indices for streamed buffer writes.
+       * Only used for SOL programs.
+       */
+      struct brw_reg destination_indices;
    } reg;
 
    /* Number of registers used to store vertex data */
diff --git a/src/mesa/drivers/dri/i965/brw_gs_emit.c b/src/mesa/drivers/dri/i965/brw_gs_emit.c
index 1f96a164637..607ee7516eb 100644
--- a/src/mesa/drivers/dri/i965/brw_gs_emit.c
+++ b/src/mesa/drivers/dri/i965/brw_gs_emit.c
@@ -45,13 +45,16 @@
 /**
  * Allocate registers for GS.
  *
- * If svbi_payload_enable is true, then the thread will be spawned with the
- * "SVBI Payload Enable" bit set, so GRF 1 needs to be set aside to hold the
- * streamed vertex buffer indices.
+ * If sol_program is true, then:
+ *
+ * - The thread will be spawned with the "SVBI Payload Enable" bit set, so GRF
+ *   1 needs to be set aside to hold the streamed vertex buffer indices.
+ *
+ * - The thread will need to use the destination_indices register.
  */
 static void brw_gs_alloc_regs( struct brw_gs_compile *c,
 			       GLuint nr_verts,
-                               bool svbi_payload_enable )
+                               bool sol_program )
 {
    GLuint i = 0,j;
 
@@ -60,7 +63,7 @@ static void brw_gs_alloc_regs( struct brw_gs_compile *c,
    c->reg.R0 = retype(brw_vec8_grf(i, 0), BRW_REGISTER_TYPE_UD); i++;
 
    /* Streamed vertex buffer indices */
-   if (svbi_payload_enable)
+   if (sol_program)
       c->reg.SVBI = retype(brw_vec8_grf(i++, 0), BRW_REGISTER_TYPE_UD);
 
    /* Payload vertices plus space for more generated vertices:
@@ -73,6 +76,11 @@ static void brw_gs_alloc_regs( struct brw_gs_compile *c,
    c->reg.header = retype(brw_vec8_grf(i++, 0), BRW_REGISTER_TYPE_UD);
    c->reg.temp = retype(brw_vec8_grf(i++, 0), BRW_REGISTER_TYPE_UD);
 
+   if (sol_program) {
+      c->reg.destination_indices =
+         retype(brw_vec4_grf(i++, 0), BRW_REGISTER_TYPE_UD);
+   }
+
    c->prog_data.urb_read_length = c->nr_regs; 
    c->prog_data.total_grf = i;
 }
@@ -351,16 +359,17 @@ gen6_sol_program(struct brw_gs_compile *c, struct brw_gs_prog_key *key,
 
    if (key->num_transform_feedback_bindings > 0) {
       unsigned vertex, binding;
+      struct brw_reg destination_indices_uw =
+         vec8(retype(c->reg.destination_indices, BRW_REGISTER_TYPE_UW));
+
       /* Note: since we use the binding table to keep track of buffer offsets
        * and stride, the GS doesn't need to keep track of a separate pointer
        * into each buffer; it uses a single pointer which increments by 1 for
        * each vertex.  So we use SVBI0 for this pointer, regardless of whether
        * transform feedback is in interleaved or separate attribs mode.
+       *
+       * Make sure that the buffers have enough room for all the vertices.
        */
-      brw_MOV(p, get_element_ud(c->reg.header, 5),
-              get_element_ud(c->reg.SVBI, 0));
-
-      /* Make sure that the buffers have enough room for all the vertices. */
       brw_ADD(p, get_element_ud(c->reg.temp, 0),
 	         get_element_ud(c->reg.SVBI, 0), brw_imm_ud(num_verts));
       brw_CMP(p, vec1(brw_null_reg()), BRW_CONDITIONAL_LE,
@@ -368,10 +377,58 @@ gen6_sol_program(struct brw_gs_compile *c, struct brw_gs_prog_key *key,
 	         get_element_ud(c->reg.SVBI, 4));
       brw_IF(p, BRW_EXECUTE_1);
 
+      /* Compute the destination indices to write to.  Usually we use SVBI[0]
+       * + (0, 1, 2).  However, for odd-numbered triangles in tristrips, the
+       * vertices come down the pipeline in reversed winding order, so we need
+       * to flip the order when writing to the transform feedback buffer.  To
+       * ensure that flatshading accuracy is preserved, we need to write them
+       * in order SVBI[0] + (0, 2, 1) if we're using the first provoking
+       * vertex convention, and in order SVBI[0] + (1, 0, 2) if we're using
+       * the last provoking vertex convention.
+       *
+       * Note: since brw_imm_v can only be used in instructions in
+       * packed-word execution mode, and SVBI is a double-word, we need to
+       * first move the appropriate immediate constant ((0, 1, 2), (0, 2, 1),
+       * or (1, 0, 2)) to the destination_indices register, and then add SVBI
+       * using a separate instruction.  Also, since the immediate constant is
+       * expressed as packed words, and we need to load double-words into
+       * destination_indices, we need to intersperse zeros to fill the upper
+       * halves of each double-word.
+       */
+      brw_MOV(p, destination_indices_uw,
+              brw_imm_v(0x00020100)); /* (0, 1, 2) */
+      if (num_verts == 3) {
+         /* Get primitive type into temp register. */
+         brw_AND(p, get_element_ud(c->reg.temp, 0),
+                 get_element_ud(c->reg.R0, 2), brw_imm_ud(0x1f));
+
+         /* Test if primitive type is TRISTRIP_REVERSE.  We need to do this as
+          * an 8-wide comparison so that the conditional MOV that follows
+          * moves all 8 words correctly.
+          */
+         brw_CMP(p, vec8(brw_null_reg()), BRW_CONDITIONAL_EQ,
+                 get_element_ud(c->reg.temp, 0),
+                 brw_imm_ud(_3DPRIM_TRISTRIP_REVERSE));
+
+         /* If so, then overwrite destination_indices_uw with the appropriate
+          * reordering.
+          */
+         brw_MOV(p, destination_indices_uw,
+                 brw_imm_v(key->pv_first ? 0x00010200    /* (0, 2, 1) */
+                                         : 0x00020001)); /* (1, 0, 2) */
+         brw_set_predicate_control(p, BRW_PREDICATE_NONE);
+      }
+      brw_ADD(p, c->reg.destination_indices,
+              c->reg.destination_indices, get_element_ud(c->reg.SVBI, 0));
+
       /* For each vertex, generate code to output each varying using the
        * appropriate binding table entry.
        */
       for (vertex = 0; vertex < num_verts; ++vertex) {
+         /* Set up the correct destination index for this vertex */
+         brw_MOV(p, get_element_ud(c->reg.header, 5),
+                 get_element_ud(c->reg.destination_indices, vertex));
+
          for (binding = 0; binding < key->num_transform_feedback_bindings;
               ++binding) {
             unsigned char vert_result =
@@ -398,15 +455,6 @@ gen6_sol_program(struct brw_gs_compile *c, struct brw_gs_prog_key *key,
                           SURF_INDEX_SOL_BINDING(binding), /* binding_table_index */
                           final_write); /* send_commit_msg */
          }
-
-         /* If there are more vertices to output, increment the pointer so
-          * that we will start outputting to the next location in the
-          * transform feedback buffers.
-          */
-         if (vertex != num_verts - 1) {
-            brw_ADD(p, get_element_ud(c->reg.header, 5),
-                    get_element_ud(c->reg.header, 5), brw_imm_ud(1));
-         }
       }
       brw_ENDIF(p);