nir: Add a function to count the number of vertices a GS emits.

Some hardware (such as Broadwell) can run geometry shaders more
efficiently when the number of vertices emitted is statically known.

This pass provides a way to obtain the constant vertex count, or
-1 indicating that the vertex count is unknown/non-constant.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jordan Justen <[email protected]>

1 files changed, 93 insertions, 0 deletions

diff --git a/src/glsl/nir/nir_gs_count_vertices.c b/src/glsl/nir/nir_gs_count_vertices.c
new file mode 100644
index 00000000000..e0bdf170d22
--- /dev/null
+++ b/src/glsl/nir/nir_gs_count_vertices.c
@@ -0,0 +1,93 @@
+/*
+ * Copyright © 2015 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 nir_intrinsic_instr *
+as_intrinsic(nir_instr *instr, nir_intrinsic_op op)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return NULL;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+   if (intrin->intrinsic != op)
+      return NULL;
+
+   return intrin;
+}
+
+static nir_intrinsic_instr *
+as_set_vertex_count(nir_instr *instr)
+{
+   return as_intrinsic(instr, nir_intrinsic_set_vertex_count);
+}
+
+/**
+ * If a geometry shader emits a constant number of vertices, return the
+ * number of vertices.  Otherwise, return -1 (unknown).
+ *
+ * This only works if you've used nir_lower_gs_intrinsics() to do vertex
+ * counting at the NIR level.
+ */
+int
+nir_gs_count_vertices(nir_shader *shader)
+{
+   int count = -1;
+
+   nir_foreach_overload(shader, overload) {
+      if (!overload->impl)
+         continue;
+
+      /* set_vertex_count intrinsics only appear in predecessors of the
+       * end block.  So we don't need to walk all of them.
+       */
+      struct set_entry *entry;
+      set_foreach(overload->impl->end_block->predecessors, entry) {
+         nir_block *block = (nir_block *) entry->key;
+
+         nir_foreach_instr_reverse(block, instr) {
+            nir_intrinsic_instr *intrin = as_set_vertex_count(instr);
+            if (!intrin)
+               continue;
+
+            nir_const_value *val = nir_src_as_const_value(intrin->src[0]);
+            /* We've found a non-constant value.  Bail. */
+            if (!val)
+               return -1;
+
+            if (count == -1)
+               count = val->i[0];
+
+            /* We've found contradictory set_vertex_count intrinsics.
+             * This can happen if there are early-returns in main() and
+             * different paths emit different numbers of vertices.
+             */
+            if (count != val->i[0])
+               return -1;
+         }
+      }
+   }
+
+   return count;
+}