panfrost: Effectively lower gl_FragColor late

nir_lower_fragcolor takes the number of colour buffers as input, but it's an
early pass, so we don't want to use the key for it. Instead, we can overestimate
and then optimize out late with an easy pass.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20906>

src/gallium/drivers/panfrost/meson.build

@@ -34,6 +34,7 @@ files_panfrost = files(
   'pan_shader.c',
   'pan_mempool.c',
   'pan_mempool.h',
+  'pan_nir_remove_fragcolor_stores.c',
 )
 
 panfrost_includes = [

src/gallium/drivers/panfrost/pan_context.h

@@ -369,6 +369,8 @@ bool panfrost_disk_cache_retrieve(
 
 void panfrost_disk_cache_init(struct panfrost_screen *screen);
 
+bool panfrost_nir_remove_fragcolor_stores(nir_shader *s, unsigned nr_cbufs);
+
 /** (Vertex buffer index, divisor) tuple that will become an Attribute Buffer
  * Descriptor at draw-time on Midgard
  */

src/gallium/drivers/panfrost/pan_nir_remove_fragcolor_stores.c

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2023 Collabora, Ltd.
+ *
+ * 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 "nir_builder.h"
+#include "pan_context.h"
+
+static bool
+pass(nir_builder *b, nir_instr *instr, void *data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+   if (intr->intrinsic != nir_intrinsic_store_output)
+      return false;
+
+   unsigned *nr_cbufs = data;
+   unsigned location = nir_intrinsic_io_semantics(intr).location;
+
+   if (location >= FRAG_RESULT_DATA0 &&
+       (location - FRAG_RESULT_DATA0) >= (*nr_cbufs)) {
+      nir_instr_remove(instr);
+      return true;
+   } else {
+      return false;
+   }
+}
+
+bool
+panfrost_nir_remove_fragcolor_stores(nir_shader *s, unsigned nr_cbufs)
+{
+   return nir_shader_instructions_pass(
+      s, pass, nir_metadata_block_index | nir_metadata_dominance, &nr_cbufs);
+}

src/gallium/drivers/panfrost/pan_shader.c

@@ -97,7 +97,7 @@ panfrost_shader_compile(struct panfrost_screen *screen, const nir_shader *ir,
       inputs.fixed_varying_mask = key->fs.fixed_varying_mask;
 
       if (s->info.outputs_written & BITFIELD_BIT(FRAG_RESULT_COLOR)) {
-         NIR_PASS_V(s, nir_lower_fragcolor, key->fs.nr_cbufs_for_fragcolor);
+         NIR_PASS_V(s, nir_lower_fragcolor, 8);
       }
    } else if (s->info.stage == MESA_SHADER_VERTEX) {
       inputs.fixed_varying_mask = fixed_varying_mask;
@@ -110,6 +110,11 @@ panfrost_shader_compile(struct panfrost_screen *screen, const nir_shader *ir,
    pan_shader_preprocess(s, inputs.gpu_id);
 
    if (s->info.stage == MESA_SHADER_FRAGMENT) {
+      if (key->fs.nr_cbufs_for_fragcolor) {
+         NIR_PASS_V(s, panfrost_nir_remove_fragcolor_stores,
+                    key->fs.nr_cbufs_for_fragcolor);
+      }
+
       if (key->fs.sprite_coord_enable) {
          NIR_PASS_V(s, nir_lower_texcoord_replace_late,
                     key->fs.sprite_coord_enable,