nir: Add a pass for lowering shaders to single-sampled

On Intel, we have to do this because we can't ask for the per-sample
barycentrics without setting the per-sample dispatch bit or the GPU will
hang.  However, nothing we're doing in this pass is Intel-specific and
it may be a useful optimization for someone else so we may as well make
it a generic NIR pass.  This version actually does a bit more than the
current brw_nir_demote_sample_qualifiers() pass as it also handles
pre-nir_lower_io interp_dref_at* as well as a couple system values which
we can easily constant-fold.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14020>

src/compiler/nir/meson.build

@@ -194,6 +194,7 @@ files_libnir = files(
   'nir_lower_samplers.c',
   'nir_lower_scratch.c',
   'nir_lower_shader_calls.c',
+  'nir_lower_single_sampled.c',
   'nir_lower_ssbo.c',
   'nir_lower_subgroups.c',
   'nir_lower_system_values.c',

src/compiler/nir/nir.h

@@ -4860,6 +4860,8 @@ 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);
+
 typedef struct nir_lower_subgroups_options {
    uint8_t subgroup_size;
    uint8_t ballot_bit_size;

src/compiler/nir/nir_lower_single_sampled.c

@@ -0,0 +1,128 @@
+/*
+ * Copyright © 2021 Intel Corporation
+ *
+ * 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.h"
+#include "nir_builder.h"
+
+static bool
+lower_single_sampled_instr(nir_builder *b,
+                           nir_instr *instr,
+                           UNUSED void *cb_data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+
+   nir_ssa_def *lowered;
+   switch (intrin->intrinsic) {
+   case nir_intrinsic_load_sample_id:
+      b->cursor = nir_before_instr(instr);
+      lowered = nir_imm_int(b, 0);
+      break;
+
+   case nir_intrinsic_load_sample_pos:
+      b->cursor = nir_before_instr(instr);
+      lowered = nir_imm_vec2(b, 0.5, 0.5);
+      break;
+
+   case nir_intrinsic_load_sample_mask_in:
+      /* Don't lower to helper invocations if helper invocations are going
+       * to be lowered right back to sample mask.
+       */
+      if (b->shader->options->lower_helper_invocation)
+         return false;
+
+      b->cursor = nir_before_instr(instr);
+      lowered = nir_b2i32(b, nir_inot(b, nir_load_helper_invocation(b, 1)));
+      break;
+
+   case nir_intrinsic_interp_deref_at_centroid:
+   case nir_intrinsic_interp_deref_at_sample:
+      b->cursor = nir_before_instr(instr);
+      assert(intrin->src[0].is_ssa);
+      lowered = nir_load_deref(b, nir_src_as_deref(intrin->src[0]));
+      break;
+
+   case nir_intrinsic_load_barycentric_centroid:
+   case nir_intrinsic_load_barycentric_sample:
+   case nir_intrinsic_load_barycentric_at_sample:
+      b->cursor = nir_before_instr(instr);
+      lowered = nir_load_barycentric(b, nir_intrinsic_load_barycentric_pixel,
+                                        nir_intrinsic_interp_mode(intrin));
+
+      if (nir_intrinsic_interp_mode(intrin) == INTERP_MODE_NOPERSPECTIVE) {
+         BITSET_SET(b->shader->info.system_values_read,
+                    SYSTEM_VALUE_BARYCENTRIC_LINEAR_PIXEL);
+      } else {
+         BITSET_SET(b->shader->info.system_values_read,
+                    SYSTEM_VALUE_BARYCENTRIC_PERSP_PIXEL);
+      }
+      break;
+
+   default:
+      return false;
+   }
+
+   nir_ssa_def_rewrite_uses(&intrin->dest.ssa, lowered);
+   nir_instr_remove(instr);
+   return true;
+}
+
+/* Assume the fragment shader is single-sampled and lower accordingly
+ *
+ * This drops sample/centroid qualifiers from all input variables, forces
+ * barycentrics to pixel, and constant-folds various built-ins.
+ */
+bool
+nir_lower_single_sampled(nir_shader *shader)
+{
+   assert(shader->info.stage == MESA_SHADER_FRAGMENT);
+
+   bool progress = false;
+   nir_foreach_shader_in_variable(var, shader) {
+      if (var->data.sample) {
+         var->data.sample = false;
+         progress = true;
+      }
+      if (var->data.centroid) {
+         var->data.centroid = false;
+         progress = true;
+      }
+   }
+
+   /* We're going to get rid of any uses of these */
+   BITSET_CLEAR(shader->info.system_values_read,
+                SYSTEM_VALUE_BARYCENTRIC_PERSP_SAMPLE);
+   BITSET_CLEAR(shader->info.system_values_read,
+                SYSTEM_VALUE_BARYCENTRIC_PERSP_CENTROID);
+   BITSET_CLEAR(shader->info.system_values_read,
+                SYSTEM_VALUE_BARYCENTRIC_LINEAR_SAMPLE);
+   BITSET_CLEAR(shader->info.system_values_read,
+                SYSTEM_VALUE_BARYCENTRIC_LINEAR_CENTROID);
+
+   return nir_shader_instructions_pass(shader, lower_single_sampled_instr,
+                                       nir_metadata_block_index |
+                                       nir_metadata_dominance,
+                                       NULL) || progress;
+}