nir: Add a lowering pass for point smoothing

When point smoothing is enabled then this lowering pass will
modifies the alpha component of every write to fragment output.
Anti-aliased points get rounded with respect to their radius instead
of square.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15117>

src/compiler/nir/meson.build

@@ -187,6 +187,7 @@ files_libnir = files(
   'nir_lower_pntc_ytransform.c',
   'nir_lower_point_size.c',
   'nir_lower_point_size_mov.c',
+  'nir_lower_point_smooth.c',
   'nir_lower_poly_line_smooth.c',
   'nir_lower_printf.c',
   'nir_lower_regs_to_ssa.c',

src/compiler/nir/nir.h

@@ -5680,8 +5680,8 @@ nir_opt_preamble(nir_shader *shader,
 
 nir_function_impl *nir_shader_get_preamble(nir_shader *shader);
 
-bool
-nir_lower_poly_line_smooth(nir_shader *shader, unsigned num_smooth_aa_sample);
+bool nir_lower_point_smooth(nir_shader *shader);
+bool nir_lower_poly_line_smooth(nir_shader *shader, unsigned num_smooth_aa_sample);
 
 #include "nir_inline_helpers.h"
 

src/compiler/nir/nir_lower_point_smooth.c

@@ -0,0 +1,94 @@
+/*
+ * Copyright © 2022 Advanced Micro Devices, Inc.
+ *
+ * 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"
+#include "nir_builtin_builder.h"
+
+/**
+ * This NIR lowers pass for point smoothing by modifying the alpha value of
+ * fragment outputs using the distance from the center of the point.
+ * Anti-aliased points get rounded with respect to their radius.
+ */
+
+static bool
+lower_point_smooth(nir_builder *b, nir_instr *instr, UNUSED void *_state)
+{
+   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;
+
+   int location = nir_intrinsic_io_semantics(intr).location;
+   if ((location != FRAG_RESULT_COLOR && location < FRAG_RESULT_DATA0) ||
+       nir_intrinsic_src_type(intr) != nir_type_float32)
+      return false;
+
+   assert(intr->src[0].is_ssa);
+   assert(intr->num_components == 4);
+
+   b->cursor = nir_before_instr(&intr->instr);
+
+   nir_ssa_def *coord = nir_build_load_point_coord_maybe_flipped(b);
+
+   /* point_size = 1.0 / dFdx(gl_PointCoord.x); */
+   nir_ssa_def *point_size = nir_frcp(b, nir_fddx(b, nir_channel(b, coord, 0)));
+
+   /* radius = point_size * 0.5 */
+   nir_ssa_def *radius = nir_fmul_imm(b, point_size, 0.5);;
+
+   /**
+    * Compute the distance of point from centre
+    * distance = √ (x - 0.5)^2 + (y - 0.5)^2
+    */
+   nir_ssa_def *distance = nir_fast_distance(b, coord,
+                                             nir_imm_vec2(b, 0.5, 0.5));
+   distance = nir_fmul(b, distance, point_size);
+
+   /* alpha = min(max(radius - distance, 0.0), 1.0) */
+   nir_ssa_def *coverage = nir_fsat(b, nir_fsub(b, radius, distance));
+
+   /* Discard fragments that are not covered by the point */
+   nir_discard_if(b, nir_feq(b, nir_imm_float(b, 0.0f), coverage));
+
+   /* Write out the fragment color*vec4(1, 1, 1, coverage)*/
+   nir_ssa_def *one = nir_imm_float(b, 1.0f);
+   nir_ssa_def *new_val = nir_fmul(b, nir_vec4(b, one, one, one, coverage),
+                                   intr->src[0].ssa);
+   nir_instr_rewrite_src(instr, &intr->src[0], nir_src_for_ssa(new_val));
+
+   return true;
+}
+
+bool
+nir_lower_point_smooth(nir_shader *shader)
+{
+   assert(shader->info.stage == MESA_SHADER_FRAGMENT);
+   return nir_shader_instructions_pass(shader, lower_point_smooth,
+                                       nir_metadata_loop_analysis |
+                                       nir_metadata_block_index |
+                                       nir_metadata_dominance, NULL);
+}