1 files changed, 358 insertions, 0 deletions

diff --git a/src/compiler/nir/nir_opt_dead_cf.c b/src/compiler/nir/nir_opt_dead_cf.c
new file mode 100644
index 0000000000..4cc6798702
--- /dev/null
+++ b/src/compiler/nir/nir_opt_dead_cf.c
@@ -0,0 +1,358 @@
+/*
+ * 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 (cwabbott0@gmail.com)
+ *
+ */
+
+#include "nir.h"
+#include "nir_control_flow.h"
+
+/*
+ * This file implements an optimization that deletes statically
+ * unreachable/dead 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.
+ *
+ * Another way is that control flow can end in a jump so that code after it
+ * never gets executed. In particular, this can happen after optimizing
+ * something like:
+ *
+ * if (true) {
+ *    ...
+ *    break;
+ * }
+ * ...
+ *
+ * We also consider the case where both branches of an if end in a jump, e.g.:
+ *
+ * if (...) {
+ *    break;
+ * } else {
+ *    continue;
+ * }
+ * ...
+ *
+ * Finally, we also handle removing useless loops, i.e. loops with no side
+ * effects and without any definitions that are used elsewhere. This case is a
+ * little different from the first two in that the code is actually run (it
+ * just never does anything), but there are similar issues with needing to
+ * be careful with restarting after deleting the cf_node (see dead_cf_list())
+ * so this is a convenient place to remove them.
+ */
+
+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)
+{
+   /* 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));
+      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
+block_has_no_side_effects(nir_block *block, void *state)
+{
+   (void) state;
+
+   nir_foreach_instr(block, instr) {
+      if (instr->type == nir_instr_type_call)
+         return false;
+
+      /* Return instructions can cause us to skip over other side-effecting
+       * instructions after the loop, so consider them to have side effects
+       * here.
+       */
+
+      if (instr->type == nir_instr_type_jump &&
+          nir_instr_as_jump(instr)->type == nir_jump_return)
+         return false;
+
+      if (instr->type != nir_instr_type_intrinsic)
+         continue;
+
+      nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+      if (!nir_intrinsic_infos[intrin->intrinsic].flags &
+          NIR_INTRINSIC_CAN_ELIMINATE)
+         return false;
+   }
+
+   return true;
+}
+
+static bool
+def_not_live_out(nir_ssa_def *def, void *state)
+{
+   nir_block *after = state;
+
+   return !BITSET_TEST(after->live_in, def->live_index);
+}
+
+/*
+ * Test if a loop is dead. A loop is dead if:
+ *
+ * 1) It has no side effects (i.e. intrinsics which could possibly affect the
+ * state of the program aside from producing an SSA value, indicated by a lack
+ * of NIR_INTRINSIC_CAN_ELIMINATE).
+ *
+ * 2) It has no phi nodes after it, since those indicate values inside the
+ * loop being used after the loop.
+ *
+ * 3) If there are no phi nodes after the loop, then the only way a value
+ * defined inside the loop can be used outside the loop is if its definition
+ * dominates the block after the loop. If none of the definitions that
+ * dominate the loop exit are used outside the loop, then the loop is dead
+ * and it can be deleted.
+ */
+
+static bool
+loop_is_dead(nir_loop *loop)
+{
+   nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
+   nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
+
+   if (!exec_list_is_empty(&after->instr_list) &&
+       nir_block_first_instr(after)->type == nir_instr_type_phi)
+      return false;
+
+   if (!nir_foreach_block_in_cf_node(&loop->cf_node, block_has_no_side_effects,
+                                     NULL))
+      return false;
+
+   nir_function_impl *impl = nir_cf_node_get_function(&loop->cf_node);
+   nir_metadata_require(impl, nir_metadata_live_ssa_defs |
+                              nir_metadata_dominance);
+
+   for (nir_block *cur = after->imm_dom; cur != before; cur = cur->imm_dom) {
+      nir_foreach_instr(cur, instr) {
+         if (!nir_foreach_ssa_def(instr, def_not_live_out, after))
+            return false;
+      }
+   }
+
+   return true;
+}
+
+static bool
+dead_cf_block(nir_block *block)
+{
+   nir_if *following_if = nir_block_get_following_if(block);
+   if (following_if) {
+     nir_const_value *const_value =
+        nir_src_as_const_value(following_if->condition);
+
+     if (!const_value)
+        return false;
+
+      opt_constant_if(following_if, const_value->u[0] != 0);
+      return true;
+   }
+
+   nir_loop *following_loop = nir_block_get_following_loop(block);
+   if (!following_loop)
+      return false;
+
+   if (!loop_is_dead(following_loop))
+      return false;
+
+   nir_cf_node_remove(&following_loop->cf_node);
+   return true;
+}
+
+static bool
+ends_in_jump(nir_block *block)
+{
+   if (exec_list_is_empty(&block->instr_list))
+      return false;
+
+   nir_instr *instr = nir_block_last_instr(block);
+   return instr->type == nir_instr_type_jump;
+}
+
+static bool
+dead_cf_list(struct exec_list *list, bool *list_ends_in_jump)
+{
+   bool progress = false;
+   *list_ends_in_jump = false;
+
+   nir_cf_node *prev = NULL;
+
+   foreach_list_typed(nir_cf_node, cur, node, list) {
+      switch (cur->type) {
+      case nir_cf_node_block: {
+         nir_block *block = nir_cf_node_as_block(cur);
+         if (dead_cf_block(block)) {
+            /* We just deleted the if or loop after this block, so we may have
+             * deleted the block before or after it -- which one is an
+             * implementation detail. Therefore, to recover the place we were
+             * at, we have to use the previous cf_node.
+             */
+
+            if (prev) {
+               cur = nir_cf_node_next(prev);
+            } else {
+               cur = exec_node_data(nir_cf_node, exec_list_get_head(list),
+                                    node);
+            }
+
+            block = nir_cf_node_as_block(cur);
+
+            progress = true;
+         }
+
+         if (ends_in_jump(block)) {
+            *list_ends_in_jump = true;
+
+            if (!exec_node_is_tail_sentinel(cur->node.next)) {
+               remove_after_cf_node(cur);
+               return true;
+            }
+         }
+
+         break;
+      }
+
+      case nir_cf_node_if: {
+         nir_if *if_stmt = nir_cf_node_as_if(cur);
+         bool then_ends_in_jump, else_ends_in_jump;
+         progress |= dead_cf_list(&if_stmt->then_list, &then_ends_in_jump);
+         progress |= dead_cf_list(&if_stmt->else_list, &else_ends_in_jump);
+
+         if (then_ends_in_jump && else_ends_in_jump) {
+            *list_ends_in_jump = true;
+            nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur));
+            if (!exec_list_is_empty(&next->instr_list) ||
+                !exec_node_is_tail_sentinel(next->cf_node.node.next)) {
+               remove_after_cf_node(cur);
+               return true;
+            }
+         }
+
+         break;
+      }
+
+      case nir_cf_node_loop: {
+         nir_loop *loop = nir_cf_node_as_loop(cur);
+         bool dummy;
+         progress |= dead_cf_list(&loop->body, &dummy);
+
+         break;
+      }
+
+      default:
+         unreachable("unknown cf node type");
+      }
+
+      prev = cur;
+   }
+
+   return progress;
+}
+
+static bool
+opt_dead_cf_impl(nir_function_impl *impl)
+{
+   bool dummy;
+   bool progress = dead_cf_list(&impl->body, &dummy);
+
+   if (progress)
+      nir_metadata_preserve(impl, nir_metadata_none);
+
+   return progress;
+}
+
+bool
+nir_opt_dead_cf(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_function(shader, function)
+      if (function->impl)
+         progress |= opt_dead_cf_impl(function->impl);
+
+   return progress;
+}