Partial LSE analysis & store removal

This is the first piece of partial LSE for art. This CL adds analysis
tools needed to implement partial LSE. More immediately, it improves
LSE so that it will remove stores that are provably non-observable
based on the location they occur. For example:

```
Foo o = new Foo();
if (xyz) {
  check(foo);
  foo.x++;
} else {
  foo.x = 12;
}
return foo.x;
```

The store of 12 can be removed because the only escape in this method
is unreachable and was not executed by the point we reach the store.

The main purpose of this CL is to add the analysis tools needed to
implement partial Load-Store elimination. Namely it includes tracking
of which blocks are escaping and the groups of blocks that we cannot
remove allocations from.

The actual impact of this change is incredibly minor, being triggered
only once in a AOSP code. go/lem shows only minor effects to
compile-time and no effect on the compiled code. See
go/lem-allight-partial-lse-2 for numbers. Compile time shows an
average of 1.4% regression (max regression is 7% with 0.2 noise).

This CL adds a new 'reachability' concept to the HGraph. If this has
been calculated it allows one to quickly query whether there is any
execution path containing two blocks in a given order. This is used to
define a notion of sections of graph from which the escape of some
allocation is inevitable.

Test: art_compiler_tests
Test: treehugger
Bug: 67037140
Change-Id: I0edc8d6b73f7dd329cb1ea7923080a0abe913ea6

1 files changed, 38 insertions, 0 deletions

diff --git a/compiler/optimizing/execution_subgraph_test.h b/compiler/optimizing/execution_subgraph_test.h
new file mode 100644
index 0000000000..13cb2bc7c5
--- /dev/null
+++ b/compiler/optimizing/execution_subgraph_test.h
@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2020 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_EXECUTION_SUBGRAPH_TEST_H_
+#define ART_COMPILER_OPTIMIZING_EXECUTION_SUBGRAPH_TEST_H_
+
+#include "android-base/macros.h"
+
+namespace art {
+
+class HGraph;
+class ExecutionSubgraph;
+
+class ExecutionSubgraphTestHelper {
+ public:
+  static bool CalculateValidity(HGraph* graph, const ExecutionSubgraph* subgraph);
+
+ private:
+  ExecutionSubgraphTestHelper() = delete;
+
+  DISALLOW_COPY_AND_ASSIGN(ExecutionSubgraphTestHelper);
+};
+}  // namespace art
+
+#endif  // ART_COMPILER_OPTIMIZING_EXECUTION_SUBGRAPH_TEST_H_