Optimizing: Rewrite search for common dominators.

Provide a utility class that can be used to quickly search
for common dominators of two or more blocks. Change the
algorithm to avoid memory allocations.

Change-Id: Id72c975fc42377cb7622902f87c4262ea7b3cc38

1 files changed, 93 insertions, 0 deletions

diff --git a/compiler/optimizing/common_dominator.h b/compiler/optimizing/common_dominator.h
new file mode 100644
index 0000000000..b459d24d7c
--- /dev/null
+++ b/compiler/optimizing/common_dominator.h
@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef ART_COMPILER_OPTIMIZING_COMMON_DOMINATOR_H_
+#define ART_COMPILER_OPTIMIZING_COMMON_DOMINATOR_H_
+
+#include "nodes.h"
+
+namespace art {
+
+// Helper class for finding common dominators of two or more blocks in a graph.
+// The domination information of a graph must not be modified while there is
+// a CommonDominator object as it's internal state could become invalid.
+class CommonDominator {
+ public:
+  // Convenience function to find the common dominator of 2 blocks.
+  static HBasicBlock* ForPair(HBasicBlock* block1, HBasicBlock* block2) {
+    CommonDominator finder(block1);
+    finder.Update(block2);
+    return finder.Get();
+  }
+
+  // Create a finder starting with a given block.
+  explicit CommonDominator(HBasicBlock* block)
+      : dominator_(block), chain_length_(ChainLength(block)) {
+    DCHECK(block != nullptr);
+  }
+
+  // Update the common dominator with another block.
+  void Update(HBasicBlock* block) {
+    DCHECK(block != nullptr);
+    HBasicBlock* block2 = dominator_;
+    DCHECK(block2 != nullptr);
+    if (block == block2) {
+      return;
+    }
+    size_t chain_length = ChainLength(block);
+    size_t chain_length2 = chain_length_;
+    // Equalize the chain lengths
+    for ( ; chain_length > chain_length2; --chain_length) {
+      block = block->GetDominator();
+      DCHECK(block != nullptr);
+    }
+    for ( ; chain_length2 > chain_length; --chain_length2) {
+      block2 = block2->GetDominator();
+      DCHECK(block2 != nullptr);
+    }
+    // Now run up the chain until we hit the common dominator.
+    while (block != block2) {
+      --chain_length;
+      block = block->GetDominator();
+      DCHECK(block != nullptr);
+      block2 = block2->GetDominator();
+      DCHECK(block2 != nullptr);
+    }
+    dominator_ = block;
+    chain_length_ = chain_length;
+  }
+
+  HBasicBlock* Get() const {
+    return dominator_;
+  }
+
+ private:
+  static size_t ChainLength(HBasicBlock* block) {
+    size_t result = 0;
+    while (block != nullptr) {
+      ++result;
+      block = block->GetDominator();
+    }
+    return result;
+  }
+
+  HBasicBlock* dominator_;
+  size_t chain_length_;
+};
+
+}  // namespace art
+
+#endif  // ART_COMPILER_OPTIMIZING_COMMON_DOMINATOR_H_