draw/gs: add clipvertex support for compatibility

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12374>

src/gallium/auxiliary/draw/draw_context.c

@@ -1003,7 +1003,7 @@ uint
 draw_current_shader_clipvertex_output(const struct draw_context *draw)
 {
    if (draw->gs.geometry_shader)
-      return draw->gs.position_output;
+      return draw->gs.clipvertex_output;
    if (draw->tes.tess_eval_shader)
       return draw->tes.position_output;
    return draw->vs.clipvertex_output;

src/gallium/auxiliary/draw/draw_gs.c

@@ -834,12 +834,18 @@ draw_create_geometry_shader(struct draw_context *draw,
    gs->primitive_boundary = gs->max_output_vertices + 1;
 
    gs->position_output = -1;
+   bool found_clipvertex = false;
    for (i = 0; i < gs->info.num_outputs; i++) {
       if (gs->info.output_semantic_name[i] == TGSI_SEMANTIC_POSITION &&
           gs->info.output_semantic_index[i] == 0)
          gs->position_output = i;
       if (gs->info.output_semantic_name[i] == TGSI_SEMANTIC_VIEWPORT_INDEX)
          gs->viewport_index_output = i;
+      if (gs->info.output_semantic_name[i] == TGSI_SEMANTIC_CLIPVERTEX &&
+          gs->info.output_semantic_index[i] == 0) {
+         found_clipvertex = true;
+         gs->clipvertex_output = i;
+      }
       if (gs->info.output_semantic_name[i] == TGSI_SEMANTIC_CLIPDIST) {
          debug_assert(gs->info.output_semantic_index[i] <
                       PIPE_MAX_CLIP_OR_CULL_DISTANCE_ELEMENT_COUNT);
@@ -847,6 +853,9 @@ draw_create_geometry_shader(struct draw_context *draw,
       }
    }
 
+   if (!found_clipvertex)
+      gs->clipvertex_output = gs->position_output;
+
    gs->machine = draw->gs.tgsi.machine;
 
    gs->num_vertex_streams = 1;
@@ -900,6 +909,7 @@ void draw_bind_geometry_shader(struct draw_context *draw,
       draw->gs.geometry_shader = dgs;
       draw->gs.num_gs_outputs = dgs->info.num_outputs;
       draw->gs.position_output = dgs->position_output;
+      draw->gs.clipvertex_output = dgs->clipvertex_output;
       draw_geometry_shader_prepare(dgs, draw);
    }
    else {

src/gallium/auxiliary/draw/draw_gs.h

@@ -75,6 +75,7 @@ struct draw_geometry_shader {
    struct tgsi_shader_info info;
    unsigned position_output;
    unsigned viewport_index_output;
+   unsigned clipvertex_output;
    unsigned ccdistance_output[PIPE_MAX_CLIP_OR_CULL_DISTANCE_ELEMENT_COUNT];
 
    unsigned max_output_vertices;

src/gallium/auxiliary/draw/draw_private.h

@@ -330,6 +330,7 @@ struct draw_context
       struct draw_geometry_shader *geometry_shader;
       uint num_gs_outputs;  /**< convenience, from geometry_shader */
       uint position_output;
+      uint clipvertex_output;
 
       /** Fields for TGSI interpreter / execution */
       struct {