nir: Add alpha_to_coverage lowering pass

Importing this pass from fs_visitor::emit_alpha_to_coverage_workaround()
in intel/compiler.

v2 (Caio Marcelo de Oliveira Filho):
- Track store output and sample mask instruction
- Nest math insturction for more readability
- Bail out early if no gl_SampleMask

v3: (Caio Marcelo de Oliveira Filho):
- Do math instructions after instruction block
- Restructure code
- Move pass under src/intel/compiler

v4: (Caio Marcelo de Oliveira Filho):
- Organize dither mask calculation

Signed-off-by: Sagar Ghuge <[email protected]>
Reviewed-by: Caio Marcelo de Oliveira Filho <[email protected]>

3 files changed, 171 insertions, 0 deletions

diff --git a/src/intel/compiler/brw_nir.h b/src/intel/compiler/brw_nir.h
index cccd9927273..6f685478fd8 100644
--- a/src/intel/compiler/brw_nir.h
+++ b/src/intel/compiler/brw_nir.h
@@ -102,6 +102,7 @@ brw_nir_link_shaders(const struct brw_compiler *compiler,
 
 bool brw_nir_lower_cs_intrinsics(nir_shader *nir,
                                  unsigned dispatch_width);
+void brw_nir_lower_alpha_to_coverage(nir_shader *shader);
 void brw_nir_lower_legacy_clipping(nir_shader *nir,
                                    int nr_userclip_plane_consts,
                                    struct brw_stage_prog_data *prog_data);
diff --git a/src/intel/compiler/brw_nir_lower_alpha_to_coverage.c b/src/intel/compiler/brw_nir_lower_alpha_to_coverage.c
new file mode 100644
index 00000000000..660d218e764
--- /dev/null
+++ b/src/intel/compiler/brw_nir_lower_alpha_to_coverage.c
@@ -0,0 +1,169 @@
+/*
+ * Copyright © 2019 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 "compiler/nir/nir_builder.h"
+#include "brw_nir.h"
+
+/**
+ * We need to compute alpha to coverage dithering manually in shader
+ * and replace sample mask store with the bitwise-AND of sample mask and
+ * alpha to coverage dithering.
+ *
+ * The following formula is used to compute final sample mask:
+ *  m = int(16.0 * clamp(src0_alpha, 0.0, 1.0))
+ *  dither_mask = 0x1111 * ((0xfea80 >> (m & ~3)) & 0xf) |
+ *     0x0808 * (m & 2) | 0x0100 * (m & 1)
+ *  sample_mask = sample_mask & dither_mask
+ *
+ * It gives a number of ones proportional to the alpha for 2, 4, 8 or 16
+ * least significant bits of the result:
+ *  0.0000 0000000000000000
+ *  0.0625 0000000100000000
+ *  0.1250 0001000000010000
+ *  0.1875 0001000100010000
+ *  0.2500 1000100010001000
+ *  0.3125 1000100110001000
+ *  0.3750 1001100010011000
+ *  0.4375 1001100110011000
+ *  0.5000 1010101010101010
+ *  0.5625 1010101110101010
+ *  0.6250 1011101010111010
+ *  0.6875 1011101110111010
+ *  0.7500 1110111011101110
+ *  0.8125 1110111111101110
+ *  0.8750 1111111011111110
+ *  0.9375 1111111111111110
+ *  1.0000 1111111111111111
+ */
+static nir_ssa_def *
+build_dither_mask(nir_builder b, nir_intrinsic_instr *store_instr)
+{
+   nir_ssa_def *alpha =
+      nir_channel(&b, nir_ssa_for_src(&b, store_instr->src[0], 4), 3);
+
+   nir_ssa_def *m =
+      nir_f2i32(&b, nir_fmul_imm(&b, nir_fsat(&b, alpha), 16.0));
+
+   nir_ssa_def *part_a =
+      nir_iand(&b,
+               nir_imm_int(&b, 0xf),
+               nir_ushr(&b,
+                        nir_imm_int(&b, 0xfea80),
+                        nir_iand(&b, m, nir_imm_int(&b, ~3))));
+
+   nir_ssa_def *part_b = nir_iand(&b, m, nir_imm_int(&b, 2));
+
+   nir_ssa_def *part_c = nir_iand(&b, m, nir_imm_int(&b, 1));
+
+   return nir_ior(&b,
+                  nir_imul_imm(&b, part_a, 0x1111),
+                  nir_ior(&b,
+                          nir_imul_imm(&b, part_b, 0x0808),
+                          nir_imul_imm(&b, part_c, 0x0100)));
+}
+
+void
+brw_nir_lower_alpha_to_coverage(nir_shader *shader)
+{
+   assert(shader->info.stage == MESA_SHADER_FRAGMENT);
+
+   /* Bail out early if we don't have gl_SampleMask */
+   bool is_sample_mask = false;
+   nir_foreach_variable(var, &shader->outputs) {
+      if (var->data.location == FRAG_RESULT_SAMPLE_MASK) {
+         is_sample_mask = true;
+         break;
+      }
+   }
+
+   if (!is_sample_mask)
+      return;
+
+   nir_foreach_function(function, shader) {
+      nir_function_impl *impl = function->impl;
+      nir_builder b;
+      nir_builder_init(&b, impl);
+
+      nir_foreach_block(block, impl) {
+         nir_intrinsic_instr *sample_mask_instr = NULL;
+         nir_intrinsic_instr *store_instr = NULL;
+
+         nir_foreach_instr_safe(instr, block) {
+            if (instr->type == nir_instr_type_intrinsic) {
+               nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+               nir_variable *out = NULL;
+
+               switch (intr->intrinsic) {
+               case nir_intrinsic_store_output:
+                  nir_foreach_variable(var, &shader->outputs) {
+                     int drvloc = var->data.driver_location;
+                     if (nir_intrinsic_base(intr) == drvloc) {
+                        out = var;
+                        break;
+                     }
+                  }
+
+                  if (out->data.mode != nir_var_shader_out)
+                     continue;
+
+                  /* save gl_SampleMask instruction pointer */
+                  if (out->data.location == FRAG_RESULT_SAMPLE_MASK) {
+                     assert(!sample_mask_instr);
+                     sample_mask_instr = intr;
+                  }
+
+                  /* save out_color[0] instruction pointer */
+                  if ((out->data.location == FRAG_RESULT_COLOR ||
+                      out->data.location == FRAG_RESULT_DATA0)) {
+                     nir_src *offset_src = nir_get_io_offset_src(intr);
+                     if (nir_src_is_const(*offset_src) && nir_src_as_uint(*offset_src) == 0) {
+                        assert(!store_instr);
+                        store_instr = intr;
+                     }
+                  }
+                  break;
+               default:
+                  continue;
+               }
+            }
+         }
+
+         if (sample_mask_instr && store_instr) {
+            b.cursor = nir_before_instr(&store_instr->instr);
+            nir_ssa_def *dither_mask = build_dither_mask(b, store_instr);
+
+            /* Combine dither_mask and reorder gl_SampleMask store instruction
+             * after render target 0 store instruction.
+             */
+            nir_instr_remove(&sample_mask_instr->instr);
+            dither_mask = nir_iand(&b, sample_mask_instr->src[0].ssa, dither_mask);
+            nir_instr_insert_after(&store_instr->instr, &sample_mask_instr->instr);
+            nir_instr_rewrite_src(&sample_mask_instr->instr,
+                                  &sample_mask_instr->src[0],
+                                  nir_src_for_ssa(dither_mask));
+         }
+      }
+      nir_metadata_preserve(impl, nir_metadata_block_index |
+                            nir_metadata_dominance);
+   }
+}
diff --git a/src/intel/compiler/meson.build b/src/intel/compiler/meson.build
index 2d938bc48b2..3b144561372 100644
--- a/src/intel/compiler/meson.build
+++ b/src/intel/compiler/meson.build
@@ -78,6 +78,7 @@ libintel_compiler_files = files(
   'brw_nir_attribute_workarounds.c',
   'brw_nir_lower_conversions.c',
   'brw_nir_lower_cs_intrinsics.c',
+  'brw_nir_lower_alpha_to_coverage.c',
   'brw_nir_lower_image_load_store.c',
   'brw_nir_lower_mem_access_bit_sizes.c',
   'brw_nir_opt_peephole_ffma.c',