1 // Copyright 2016 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 #ifndef V8_COMPILER_STORE_STORE_ELIMINATION_H_
6 #define V8_COMPILER_STORE_STORE_ELIMINATION_H_
7 
8 #include "src/common/globals.h"
9 
10 namespace v8 {
11 namespace internal {
12 
13 class TickCounter;
14 class Zone;
15 
16 namespace compiler {
17 
18 class JSGraph;
19 
20 // Store-store elimination.
21 //
22 // The aim of this optimization is to detect the following pattern in the
23 // effect graph:
24 //
25 // - StoreField[+24, kRepTagged](263, ...)
26 //
27 //   ... lots of nodes from which the field at offset 24 of the object
28 //       returned by node #263 cannot be observed ...
29 //
30 // - StoreField[+24, kRepTagged](263, ...)
31 //
32 // In such situations, the earlier StoreField cannot be observed, and can be
33 // eliminated. This optimization should work for any offset and input node, of
34 // course.
35 //
36 // The optimization also works across splits. It currently does not work for
37 // loops, because we tend to put a stack check in loops, and like deopts,
38 // stack checks can observe anything.
39 
40 // Assumption: every byte of a JS object is only ever accessed through one
41 // offset. For instance, byte 15 of a given object may be accessed using a
42 // two-byte read at offset 14, or a four-byte read at offset 12, but never
43 // both in the same program.
44 //
45 // This implementation needs all dead nodes removed from the graph, and the
46 // graph should be trimmed.
47 class StoreStoreElimination final : public AllStatic {
48  public:
49   static void Run(JSGraph* js_graph, TickCounter* tick_counter,
50                   Zone* temp_zone);
51 };
52 
53 }  // namespace compiler
54 }  // namespace internal
55 }  // namespace v8
56 
57 #endif  // V8_COMPILER_STORE_STORE_ELIMINATION_H_
58