A first implementation of a loop optimization framework.

Rationale:
We are planning to add more and more loop related optimizations
and this framework provides the basis to do so. For starters,
the framework optimizes dead induction, induction that can be
replaced with a simpler closed-form, and eliminates dead loops
completely (either pre-existing or as a result of induction
removal).

Speedup on e.g. Benchpress Loop is 73x (17.5us. -> 0.24us.)
[with the potential for more exploiting outer loop too]

Test: 618-checker-induction et al.

Change-Id: If80a809acf943539bf6726b0030dcabd50c9babc

1 files changed, 88 insertions, 0 deletions

diff --git a/compiler/optimizing/loop_optimization.h b/compiler/optimizing/loop_optimization.h
new file mode 100644
index 0000000000..e7980ce89e
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+++ b/compiler/optimizing/loop_optimization.h
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+/*
+ * Copyright (C) 2016 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_LOOP_OPTIMIZATION_H_
+#define ART_COMPILER_OPTIMIZING_LOOP_OPTIMIZATION_H_
+
+#include <string>
+
+#include "induction_var_range.h"
+#include "nodes.h"
+#include "optimization.h"
+
+namespace art {
+
+/**
+ * Loop optimizations. Builds a loop hierarchy and applies optimizations to
+ * the detected nested loops, such as removal of dead induction and empty loops.
+ */
+class HLoopOptimization : public HOptimization {
+ public:
+  HLoopOptimization(HGraph* graph, HInductionVarAnalysis* induction_analysis);
+
+  void Run() OVERRIDE;
+
+  static constexpr const char* kLoopOptimizationPassName = "loop_optimization";
+
+ private:
+  /**
+   * A single loop inside the loop hierarchy representation.
+   */
+  struct LoopNode : public ArenaObject<kArenaAllocInductionVarAnalysis> {
+    explicit LoopNode(HLoopInformation* lp_info)
+        : loop_info(lp_info),
+          outer(nullptr),
+          inner(nullptr),
+          previous(nullptr),
+          next(nullptr) {}
+    const HLoopInformation* const loop_info;
+    LoopNode* outer;
+    LoopNode* inner;
+    LoopNode* previous;
+    LoopNode* next;
+  };
+
+  void AddLoop(HLoopInformation* loop_info);
+  void RemoveLoop(LoopNode* node);
+
+  void TraverseLoopsInnerToOuter(LoopNode* node);
+
+  void SimplifyInduction(LoopNode* node);
+  void RemoveIfEmptyLoop(LoopNode* node);
+
+  void ReplaceAllUses(HInstruction* instruction,
+                      HInstruction* replacement,
+                      HInstruction* exclusion);
+
+  // Range analysis based on induction variables.
+  InductionVarRange induction_range_;
+
+  // Phase-local heap memory allocator for the loop optimizer. Storage obtained
+  // through this allocator is released when the loop optimizer is done.
+  ArenaAllocator loop_allocator_;
+
+  // Entries into the loop hierarchy representation.
+  LoopNode* top_loop_;
+  LoopNode* last_loop_;
+
+  friend class LoopOptimizationTest;
+
+  DISALLOW_COPY_AND_ASSIGN(HLoopOptimization);
+};
+
+}  // namespace art
+
+#endif  // ART_COMPILER_OPTIMIZING_LOOP_OPTIMIZATION_H_