nir: add an optimization for removing dead control flow

v2: use nir_cf_node_remove_after() instead of our own broken thing.
v3: use the new control flow modification helpers.

Reviewed-by: Kenneth Graunke <[email protected]>

1 files changed, 155 insertions, 0 deletions

diff --git a/src/glsl/nir/nir_opt_dead_cf.c b/src/glsl/nir/nir_opt_dead_cf.c
new file mode 100644
index 00000000000..fca842caedd
--- /dev/null
+++ b/src/glsl/nir/nir_opt_dead_cf.c
@@ -0,0 +1,155 @@
+/*
+ * Copyright © 2014 Connor Abbott
+ *
+ * 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.
+ *
+ * Authors:
+ *    Connor Abbott ([email protected])
+ *
+ */
+
+#include "nir.h"
+#include "nir_control_flow.h"
+
+/*
+ * This file implements an optimization that deletes statically unreachable
+ * code. In NIR, one way this can happen if if an if statement has a constant
+ * condition:
+ *
+ * if (true) {
+ *    ...
+ * }
+ *
+ * We delete the if statement and paste the contents of the always-executed
+ * branch into the surrounding control flow, possibly removing more code if
+ * the branch had a jump at the end.
+ */
+
+static void
+remove_after_cf_node(nir_cf_node *node)
+{
+   nir_cf_node *end = node;
+   while (!nir_cf_node_is_last(end))
+      end = nir_cf_node_next(end);
+
+   nir_cf_list list;
+   nir_cf_extract(&list, nir_after_cf_node(node), nir_after_cf_node(end));
+   nir_cf_delete(&list);
+}
+
+static void
+opt_constant_if(nir_if *if_stmt, bool condition)
+{
+   void *mem_ctx = ralloc_parent(if_stmt);
+
+   /* First, we need to remove any phi nodes after the if by rewriting uses to
+    * point to the correct source.
+    */
+   nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&if_stmt->cf_node));
+   nir_block *last_block =
+      nir_cf_node_as_block(condition ? nir_if_last_then_node(if_stmt)
+                                     : nir_if_last_else_node(if_stmt));
+
+   nir_foreach_instr_safe(after, instr) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+
+      nir_phi_instr *phi = nir_instr_as_phi(instr);
+      nir_ssa_def *def = NULL;
+      nir_foreach_phi_src(phi, phi_src) {
+         if (phi_src->pred != last_block)
+            continue;
+
+         assert(phi_src->src.is_ssa);
+         def = phi_src->src.ssa;
+      }
+
+      assert(def);
+      assert(phi->dest.is_ssa);
+      nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def), mem_ctx);
+      nir_instr_remove(instr);
+   }
+
+   /* The control flow list we're about to paste in may include a jump at the
+    * end, and in that case we have to delete the rest of the control flow
+    * list after the if since it's unreachable and the validator will balk if
+    * we don't.
+    */
+
+   if (!exec_list_is_empty(&last_block->instr_list)) {
+      nir_instr *last_instr = nir_block_last_instr(last_block);
+      if (last_instr->type == nir_instr_type_jump)
+         remove_after_cf_node(&if_stmt->cf_node);
+   }
+
+   /* Finally, actually paste in the then or else branch and delete the if. */
+   struct exec_list *cf_list = condition ? &if_stmt->then_list
+                                         : &if_stmt->else_list;
+
+   nir_cf_list list;
+   nir_cf_extract(&list, nir_before_cf_list(cf_list),
+                  nir_after_cf_list(cf_list));
+   nir_cf_reinsert(&list, nir_after_cf_node(&if_stmt->cf_node));
+   nir_cf_node_remove(&if_stmt->cf_node);
+}
+
+static bool
+dead_cf_cb(nir_block *block, void *state)
+{
+   bool *progress = state;
+
+   nir_if *following_if = nir_block_get_following_if(block);
+   if (!following_if)
+      return true;
+
+  nir_const_value *const_value =
+     nir_src_as_const_value(following_if->condition);
+
+  if (!const_value)
+     return true;
+
+   opt_constant_if(following_if, const_value->u[0] != 0);
+   *progress = true;
+   return true;
+}
+
+static bool
+opt_dead_cf_impl(nir_function_impl *impl)
+{
+   bool progress = false;
+   nir_foreach_block(impl, dead_cf_cb, &progress);
+
+   if (progress)
+      nir_metadata_preserve(impl, nir_metadata_none);
+
+   return progress;
+}
+
+bool
+nir_opt_dead_cf(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_overload(shader, overload)
+      if (overload->impl)
+         progress |= opt_dead_cf_impl(overload->impl);
+
+   return progress;
+}