nir: add constant clip/cull distance optimization

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32518>

src/compiler/nir/meson.build

@@ -221,6 +221,7 @@ files_libnir = files(
   'nir_normalize_cubemap_coords.c',
   'nir_opt_access.c',
   'nir_opt_barriers.c',
+  'nir_opt_clip_cull_const.c',
   'nir_opt_combine_stores.c',
   'nir_opt_comparison_pre.c',
   'nir_opt_conditional_discard.c',

src/compiler/nir/nir.h

@@ -5718,6 +5718,8 @@ void nir_assign_io_var_locations(nir_shader *shader,
                                  unsigned *size,
                                  gl_shader_stage stage);
 
+bool nir_opt_clip_cull_const(nir_shader *shader);
+
 typedef enum {
    /* If set, this causes all 64-bit IO operations to be lowered on-the-fly
     * to 32-bit operations.  This is only valid for nir_var_shader_in/out

src/compiler/nir/nir_opt_clip_cull_const.c

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2024 Valve Corporation
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "nir.h"
+#include "nir_builder.h"
+
+/**
+ * If a clip/cull distance is constant >= 0,
+ * we know that it will never cause clipping/culling.
+ * Remove the sysval_output in that case.
+ *
+ * Assumes that nir_lower_io_to_temporaries was run,
+ * and works best with scalar store_outputs.
+ */
+
+static bool
+opt_clip_cull(nir_builder *b, nir_intrinsic_instr *intr, void *unused)
+{
+   if (intr->intrinsic != nir_intrinsic_store_output)
+      return false;
+
+   const nir_io_semantics io_sem = nir_intrinsic_io_semantics(intr);
+   const unsigned location = io_sem.location;
+
+   if (io_sem.no_sysval_output)
+      return false;
+
+   if (location != VARYING_SLOT_CLIP_DIST0 && location != VARYING_SLOT_CLIP_DIST1)
+      return false;
+
+   nir_def *val = intr->src[0].ssa;
+   for (unsigned i = 0; i < val->num_components; i++) {
+      nir_scalar s = nir_scalar_resolved(val, i);
+      if (!nir_scalar_is_const(s))
+         return false;
+      float distance = nir_scalar_as_float(s);
+
+      /* NaN gets clipped, and INF after interpolation is NaN. */
+      if (isnan(distance) || distance < 0.0 || distance == INFINITY)
+         return false;
+   }
+
+   nir_remove_sysval_output(intr);
+   return true;
+}
+
+bool
+nir_opt_clip_cull_const(nir_shader *shader)
+{
+   return nir_shader_intrinsics_pass(shader, opt_clip_cull, nir_metadata_control_flow, NULL);
+}