i965/vs: Move compute_clip_distance() out of emit_urb_writes().

Legacy user clipping (using gl_Position or gl_ClipVertex) is handled by
turning those into the modern gl_ClipDistance equivalents.

This is unnecessary in Core Profile: if user clipping is enabled, but
the shader doesn't write the corresponding gl_ClipDistance entry,
results are undefined.  Hence, it is also unnecessary for geometry
shaders.

This patch moves the call up to run_vs().  This is equivalent for VS,
but removes the need to pass clip distances into emit_urb_writes().

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -3816,7 +3816,9 @@ fs_visitor::run_vs(gl_clip_plane *clip_planes)
    if (failed)
       return false;
 
-   emit_urb_writes(clip_planes);
+   compute_clip_distance(clip_planes);
+
+   emit_urb_writes();
 
    if (shader_time_index >= 0)
       emit_shader_time_end();

src/mesa/drivers/dri/i965/brw_fs.h

@@ -271,7 +271,7 @@ public:
                                  fs_reg src0_alpha, unsigned components,
                                  unsigned exec_size, bool use_2nd_half = false);
    void emit_fb_writes();
-   void emit_urb_writes(gl_clip_plane *clip_planes);
+   void emit_urb_writes();
    void emit_cs_terminate();
 
    void emit_barrier();

src/mesa/drivers/dri/i965/brw_fs_visitor.cpp

@@ -1731,6 +1731,12 @@ fs_visitor::setup_uniform_clipplane_values(gl_clip_plane *clip_planes)
    }
 }
 
+/**
+ * Lower legacy fixed-function and gl_ClipVertex clipping to clip distances.
+ *
+ * This does nothing if the shader uses gl_ClipDistance or user clipping is
+ * disabled altogether.
+ */
 void fs_visitor::compute_clip_distance(gl_clip_plane *clip_planes)
 {
    struct brw_vue_prog_data *vue_prog_data =
@@ -1738,6 +1744,10 @@ void fs_visitor::compute_clip_distance(gl_clip_plane *clip_planes)
    const struct brw_vue_prog_key *key =
       (const struct brw_vue_prog_key *) this->key;
 
+   /* Bail unless some sort of legacy clipping is enabled */
+   if (!key->userclip_active || prog->UsesClipDistanceOut)
+      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
@@ -1781,7 +1791,7 @@ void fs_visitor::compute_clip_distance(gl_clip_plane *clip_planes)
 }
 
 void
-fs_visitor::emit_urb_writes(gl_clip_plane *clip_planes)
+fs_visitor::emit_urb_writes()
 {
    int slot, urb_offset, length;
    struct brw_vs_prog_data *vs_prog_data =
@@ -1794,10 +1804,6 @@ fs_visitor::emit_urb_writes(gl_clip_plane *clip_planes)
    bool flush;
    fs_reg sources[8];
 
-   /* Lower legacy ff and ClipVertex clipping to clip distances */
-   if (key->base.userclip_active && !prog->UsesClipDistanceOut)
-      compute_clip_distance(clip_planes);
-
    /* If we don't have any valid slots to write, just do a minimal urb write
     * send to terminate the shader.  This includes 1 slot of undefined data,
     * because it's invalid to write 0 data: