nir: Move ST's force-persample-shading NIR pass to shared code.

This is about to grow a little.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36496>

src/compiler/nir/meson.build

@@ -215,6 +215,7 @@ else
   'nir_lower_returns.c',
   'nir_lower_robust_access.c',
   'nir_lower_samplers.c',
+  'nir_lower_sample_shading.c',
   'nir_lower_scratch.c',
   'nir_lower_scratch_to_var.c',
   'nir_lower_shader_calls.c',

src/compiler/nir/nir.h

@@ -5417,6 +5417,7 @@ bool nir_lower_uniforms_to_ubo(nir_shader *shader, bool dword_packed, bool load_
 bool nir_lower_is_helper_invocation(nir_shader *shader);
 
 bool nir_lower_single_sampled(nir_shader *shader);
+bool nir_lower_sample_shading(nir_shader *shader);
 
 bool nir_lower_atomics(nir_shader *shader, nir_instr_filter_cb filter);
 

src/compiler/nir/nir_lower_sample_shading.c

@@ -0,0 +1,49 @@
+/*
+ * Copyright © 2025 Igalia SL
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include "compiler/nir/nir.h"
+#include "compiler/nir/nir_builder.h"
+
+static bool
+force_persample_shading(struct nir_builder *b, nir_intrinsic_instr *intr,
+                        void *data)
+{
+   if (intr->intrinsic == nir_intrinsic_load_barycentric_pixel ||
+       intr->intrinsic == nir_intrinsic_load_barycentric_centroid) {
+      intr->intrinsic = nir_intrinsic_load_barycentric_sample;
+      return true;
+   }
+
+   return false;
+}
+
+/** Lowering to set up interpolation for sample shading. */
+bool
+nir_lower_sample_shading(nir_shader *nir)
+{
+   assert(nir->info.stage == MESA_SHADER_FRAGMENT);
+   assert(nir->info.fs.uses_sample_shading);
+
+   return nir_shader_intrinsics_pass(nir, force_persample_shading,
+                                     nir_metadata_all, NULL);
+}

src/mesa/state_tracker/st_program.c

@@ -693,19 +693,6 @@ lower_ucp(struct st_context *st,
    }
 }
 
-static bool
-force_persample_shading(struct nir_builder *b, nir_intrinsic_instr *intr,
-                        void *data)
-{
-   if (intr->intrinsic == nir_intrinsic_load_barycentric_pixel ||
-       intr->intrinsic == nir_intrinsic_load_barycentric_centroid) {
-      intr->intrinsic = nir_intrinsic_load_barycentric_sample;
-      return true;
-   }
-
-   return false;
-}
-
 static int
 xfb_compare_dst_offset(const void *a, const void *b)
 {
@@ -1075,8 +1062,6 @@ st_create_fp_variant(struct st_context *st,
 
    if (key->persample_shading) {
       nir_shader *shader = state.ir.nir;
-      nir_shader_intrinsics_pass(shader, force_persample_shading,
-                                 nir_metadata_all, NULL);
 
       /* In addition to requiring per-sample interpolation, sample shading
        * changes the behaviour of gl_SampleMaskIn, so we need per-sample shading
@@ -1084,6 +1069,7 @@ st_create_fp_variant(struct st_context *st,
        * uses_sample_shading won't be set by glsl_to_nir. We need to do so here.
        */
       shader->info.fs.uses_sample_shading = true;
+      nir_lower_sample_shading(shader);
 
       finalize = true;
    }