gallium: add PIPE_CAP_CULL_DISTANCE_NOCOMBINE

for drivers where separate cull distance variables are required, this
lets them avoid having to write yet another pass to undo gallium's mangling
of shader info

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14878>

docs/gallium/screen.rst

@@ -374,6 +374,9 @@ The integer capabilities:
   and accesses to unbound resources.
 * ``PIPE_CAP_CULL_DISTANCE``: Whether the driver supports the arb_cull_distance
   extension and thus implements proper support for culling planes.
+* ``PIPE_CAP_CULL_DISTANCE_NOCOMBINE``: Whether the driver wants to skip
+  running the `nir_lower_clip_cull_distance_arrays` pass in order to get
+  VARYING_SLOT_CULL_DIST0 slot variables.
 * ``PIPE_CAP_PRIMITIVE_RESTART_FOR_PATCHES``: Whether primitive restart is
   supported for patch primitives.
 * ``PIPE_CAP_TGSI_VOTE``: Whether the ``VOTE_*`` ops can be used in shaders.

src/gallium/auxiliary/util/u_screen.c

@@ -275,6 +275,7 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_FRAMEBUFFER_NO_ATTACHMENT:
    case PIPE_CAP_ROBUST_BUFFER_ACCESS_BEHAVIOR:
    case PIPE_CAP_CULL_DISTANCE:
+   case PIPE_CAP_CULL_DISTANCE_NOCOMBINE:
    case PIPE_CAP_TGSI_VOTE:
    case PIPE_CAP_MAX_WINDOW_RECTANGLES: /* Enables EXT_window_rectangles */
    case PIPE_CAP_POLYGON_OFFSET_UNITS_UNSCALED:

src/gallium/include/pipe/p_defines.h

@@ -875,6 +875,7 @@ enum pipe_cap
    PIPE_CAP_FRAMEBUFFER_NO_ATTACHMENT,
    PIPE_CAP_ROBUST_BUFFER_ACCESS_BEHAVIOR,
    PIPE_CAP_CULL_DISTANCE,
+   PIPE_CAP_CULL_DISTANCE_NOCOMBINE,
    PIPE_CAP_TGSI_VOTE,
    PIPE_CAP_MAX_WINDOW_RECTANGLES,
    PIPE_CAP_POLYGON_OFFSET_UNITS_UNSCALED,

src/mesa/state_tracker/st_glsl_to_nir.cpp

@@ -768,6 +768,7 @@ st_link_nir(struct gl_context *ctx,
       NIR_PASS_V(nir, nir_lower_system_values);
       NIR_PASS_V(nir, nir_lower_compute_system_values, NULL);
 
+      if (!st->screen->get_param(st->screen, PIPE_CAP_CULL_DISTANCE_NOCOMBINE))
          NIR_PASS_V(nir, nir_lower_clip_cull_distance_arrays);
 
       st_shader_gather_info(nir, shader->Program);