nir: Move vc4's alpha test lowering to core NIR.

I've been doing this inside of vc4, but vc5 wants it as well and it may be
useful for other drivers (Intel has a related path for pre-gen6 with MRT,
and freedreno had a TGSI path for it at one point).

This required defining a common enum for the standard comparison
functions, but other lowering passes are likely to also want that enum.

v2: Add to meson.build as well.

Acked-by: Rob Clark <[email protected]>

1 files changed, 111 insertions, 0 deletions

diff --git a/src/compiler/nir/nir_lower_alpha_test.c b/src/compiler/nir/nir_lower_alpha_test.c
new file mode 100644
index 00000000000..bd433b8ec66
--- /dev/null
+++ b/src/compiler/nir/nir_lower_alpha_test.c
@@ -0,0 +1,111 @@
+/*
+ * Copyright © 2017 Broadcom
+ *
+ * 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.
+ */
+
+/**
+ * @file
+ *
+ * Implements GL alpha testing by comparing the output color's alpha to the
+ * alpha_ref intrinsic and emitting a discard based on it.
+ *
+ * The alpha_to_one value overrides the source alpha to 1.0 to implement
+ * GL_SAMPLE_ALPHA_TO_ONE, which applies before the alpha test (and would be
+ * rather silly to use with alpha test, but the spec permits).
+ */
+
+#include "nir/nir.h"
+#include "nir/nir_builder.h"
+
+void
+nir_lower_alpha_test(nir_shader *shader, enum compare_func func,
+                     bool alpha_to_one)
+{
+   assert(shader->stage == MESA_SHADER_FRAGMENT);
+
+   nir_foreach_function(function, shader) {
+      nir_function_impl *impl = function->impl;
+      nir_builder b;
+      nir_builder_init(&b, impl);
+      b.cursor = nir_before_cf_list(&impl->body);
+
+      nir_foreach_block(block, impl) {
+         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_var:
+                  out = intr->variables[0]->var;
+                  break;
+               case nir_intrinsic_store_output:
+                  /* already had i/o lowered.. lookup the matching output var: */
+                  nir_foreach_variable(var, &shader->outputs) {
+                     int drvloc = var->data.driver_location;
+                     if (nir_intrinsic_base(intr) == drvloc) {
+                        out = var;
+                        break;
+                     }
+                  }
+                  assume(out);
+                  break;
+               default:
+                  continue;
+               }
+
+               if (out->data.mode != nir_var_shader_out)
+                  continue;
+
+               if (out->data.location != FRAG_RESULT_COLOR &&
+                   out->data.location != FRAG_RESULT_DATA0)
+                  continue;
+
+               b.cursor = nir_before_instr(&intr->instr);
+
+               nir_ssa_def *alpha;
+               if (alpha_to_one) {
+                  alpha = nir_imm_float(&b, 1.0);
+               } else {
+                  alpha = nir_channel(&b, nir_ssa_for_src(&b, intr->src[0], 4),
+                                      3);
+               }
+
+               nir_ssa_def *condition =
+                  nir_compare_func(&b, func,
+                                   alpha, nir_load_alpha_ref_float(&b));
+
+               nir_intrinsic_instr *discard =
+                  nir_intrinsic_instr_create(b.shader,
+                                             nir_intrinsic_discard_if);
+               discard->num_components = 1;
+               discard->src[0] = nir_src_for_ssa(nir_inot(&b, condition));
+               nir_builder_instr_insert(&b, &discard->instr);
+               shader->info.fs.uses_discard = true;
+            }
+         }
+      }
+
+      nir_metadata_preserve(impl, nir_metadata_block_index |
+                            nir_metadata_dominance);
+   }
+}