[Mesa-dev] [PATCH 2/2] i965 Gen6: Implement gl_ClipVertex.

[Paul Berry]

This patch implements proper support for gl_ClipVertex by causing the
new VS backend to populate the clip distance VUE slots using
VERT_RESULT_CLIP_VERTEX when appropriate, and by using the
untransformed clip planes in ctx->Transform.EyeUserPlane rather than
the transformed clip planes in ctx->Transform._ClipUserPlane.

When fixed functionality is in use the driver needs to do the old
behavior (clip based on VERT_RESULT_HPOS and
ctx->Transform._ClipUserPlane).  This happens automatically because we
use the old VS backend for fixed functionality.

Fixes the following Piglit tests on i965 Gen6:
- vs-clip-vertex-const-accept
- vs-clip-vertex-const-reject
- vs-clip-vertex-different-from-position
- vs-clip-vertex-equal-to-position
- vs-clip-vertex-homogeneity
- vs-clip-based-on-position
- vs-clip-based-on-position-homogeneity
- clip-plane-transformation clipvert_pos
- clip-plane-transformation pos_clipvert
- clip-plane-transformation pos
---
 src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp |   31 ++++++++++++++++++++++-
 src/mesa/drivers/dri/i965/brw_vs.c             |    8 +++++-
 2 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
index e5eda22..f335a86 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
@@ -553,7 +553,16 @@ vec4_visitor::setup_uniform_clipplane_values()
          this->userplane[compacted_clipplane_index] = dst_reg(UNIFORM, this->uniforms);
          this->userplane[compacted_clipplane_index].type = BRW_REGISTER_TYPE_F;
          for (int j = 0; j < 4; ++j) {
-            c->prog_data.param[this->uniforms * 4 + j] = &ctx->Transform._ClipUserPlane[i][j];
+            /* For fixed functionality shaders, we need to clip based on
+             * ctx->Transform._ClipUserPlane (which has been transformed by
+             * Mesa core into clip coordinates).  For user-supplied vertex
+             * shaders, we need to use the untransformed clip planes in
+             * ctx->Transform.EyeUserPlane.  Since vec4_visitor is currently
+             * only used for user-supplied vertex shaders, we can hardcode
+             * this to EyeUserPlane for now.
+             */
+            c->prog_data.param[this->uniforms * 4 + j]
+               = &ctx->Transform.EyeUserPlane[i][j];
          }
          ++compacted_clipplane_index;
          ++this->uniforms;
@@ -1840,9 +1849,27 @@ vec4_visitor::emit_clip_distances(struct brw_reg reg, int offset)
       return;
    }
 
+   /* From the GLSL 1.30 spec, section 7.1 (Vertex Shader Special Variables):
+    *
+    *     "If a linked set of shaders forming the vertex stage contains no
+    *     static write to gl_ClipVertex or gl_ClipDistance, but the
+    *     application has requested clipping against user clip planes through
+    *     the API, then the coordinate written to gl_Position is used for
+    *     comparison against the user clip planes."
+    *
+    * This function is only called if the shader didn't write to
+    * gl_ClipDistance.  Accordingly, we use gl_ClipVertex to perform clipping
+    * if the user wrote to it; otherwise we use gl_Position.
+    */
+   gl_vert_result clip_vertex = VERT_RESULT_CLIP_VERTEX;
+   if (!(c->prog_data.outputs_written
+         & BITFIELD64_BIT(VERT_RESULT_CLIP_VERTEX))) {
+      clip_vertex = VERT_RESULT_HPOS;
+   }
+
    for (int i = 0; i + offset < c->key.nr_userclip && i < 4; ++i) {
       emit(DP4(dst_reg(brw_writemask(reg, 1 << i)),
-               src_reg(output_reg[VERT_RESULT_HPOS]),
+               src_reg(output_reg[clip_vertex]),
                src_reg(this->userplane[i + offset])));
    }
 }
diff --git a/src/mesa/drivers/dri/i965/brw_vs.c b/src/mesa/drivers/dri/i965/brw_vs.c
index 93c6838..4fd260f 100644
--- a/src/mesa/drivers/dri/i965/brw_vs.c
+++ b/src/mesa/drivers/dri/i965/brw_vs.c
@@ -137,11 +137,17 @@ brw_compute_vue_map(struct brw_vue_map *vue_map,
    /* The hardware doesn't care about the rest of the vertex outputs, so just
     * assign them contiguously.  Don't reassign outputs that already have a
     * slot.
+    *
+    * Also, don't assign a slot for VERT_RESULT_CLIP_VERTEX, since it is
+    * unsupported in pre-GEN6, and in GEN6+ the vertex shader converts it into
+    * clip distances.
     */
    for (int i = 0; i < VERT_RESULT_MAX; ++i) {
       if ((outputs_written & BITFIELD64_BIT(i)) &&
           vue_map->vert_result_to_slot[i] == -1) {
-         assign_vue_slot(vue_map, i);
+         if (i != VERT_RESULT_CLIP_VERTEX) {
+            assign_vue_slot(vue_map, i);
+         }
       }
    }
 }
-- 
1.7.6.2