1// Copyright 2014 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_OSR_H_
6#define V8_COMPILER_OSR_H_
7
8#include "src/zone.h"
9
10// TurboFan structures OSR graphs in a way that separates almost all phases of
11// compilation from OSR implementation details. This is accomplished with
12// special control nodes that are added at graph building time. In particular,
13// the graph is built in such a way that typing still computes the best types
14// and optimizations and lowering work unchanged. All that remains is to
15// deconstruct the OSR artifacts before scheduling and code generation.
16
17// Graphs built for OSR from the AstGraphBuilder are structured as follows:
18//                             Start
19//          +-------------------^^-----+
20//          |                          |
21//   OsrNormalEntry               OsrLoopEntry <-------------+
22//          |                          |                     |
23//   control flow before loop          |           A     OsrValue
24//          |                          |           |         |
25//          | +------------------------+           | +-------+
26//          | | +-------------+                    | | +--------+
27//          | | |             |                    | | |        |
28//        ( Loop )<-----------|------------------ ( phi )       |
29//          |                 |                                 |
30//      loop body             | backedge(s)                     |
31//          |  |              |                                 |
32//          |  +--------------+                        B  <-----+
33//          |
34//         end
35
36// The control structure expresses the relationship that the loop has a separate
37// entrypoint which corresponds to entering the loop directly from the middle
38// of unoptimized code.
39// Similarly, the values that come in from unoptimized code are represented with
40// {OsrValue} nodes that merge into any phis associated with the OSR loop.
41// In the above diagram, nodes {A} and {B} represent values in the "normal"
42// graph that correspond to the values of those phis before the loop and on any
43// backedges, respectively.
44
45// To deconstruct OSR, we simply replace the uses of the {OsrNormalEntry}
46// control node with {Dead} and {OsrLoopEntry} with start and run the
47// {ControlReducer}. Control reduction propagates the dead control forward,
48// essentially "killing" all the code before the OSR loop. The entrypoint to the
49// loop corresponding to the "normal" entry path will also be removed, as well
50// as the inputs to the loop phis, resulting in the reduced graph:
51
52//                             Start
53//         Dead                  |^-------------------------+
54//          |                    |                          |
55//          |                    |                          |
56//          |                    |                          |
57//    disconnected, dead         |           A=dead      OsrValue
58//                               |                          |
59//            +------------------+                   +------+
60//            | +-------------+                      | +--------+
61//            | |             |                      | |        |
62//        ( Loop )<-----------|------------------ ( phi )       |
63//          |                 |                                 |
64//      loop body             | backedge(s)                     |
65//          |  |              |                                 |
66//          |  +--------------+                        B  <-----+
67//          |
68//         end
69
70// Other than the presences of the OsrValue nodes, this is a normal, schedulable
71// graph. OsrValue nodes are handled specially in the instruction selector to
72// simply load from the unoptimized frame.
73
74// For nested OSR loops, loop peeling must first be applied as many times as
75// necessary in order to bring the OSR loop up to the top level (i.e. to be
76// an outer loop).
77
78namespace v8 {
79namespace internal {
80
81class CompilationInfo;
82
83namespace compiler {
84
85class JSGraph;
86class CommonOperatorBuilder;
87class Frame;
88class Linkage;
89
90// Encapsulates logic relating to OSR compilations as well has handles some
91// details of the frame layout.
92class OsrHelper {
93 public:
94  explicit OsrHelper(CompilationInfo* info);
95  // Only for testing.
96  OsrHelper(size_t parameter_count, size_t stack_slot_count)
97      : parameter_count_(parameter_count),
98        stack_slot_count_(stack_slot_count) {}
99
100  // Deconstructs the artificial {OsrNormalEntry} and rewrites the graph so
101  // that only the path corresponding to {OsrLoopEntry} remains.
102  void Deconstruct(JSGraph* jsgraph, CommonOperatorBuilder* common,
103                   Zone* tmp_zone);
104
105  // Prepares the frame w.r.t. OSR.
106  void SetupFrame(Frame* frame);
107
108  // Returns the number of unoptimized frame slots for this OSR.
109  size_t UnoptimizedFrameSlots() { return stack_slot_count_; }
110
111  // Returns the environment index of the first stack slot.
112  static int FirstStackSlotIndex(int parameter_count) {
113    // n.b. unlike Crankshaft, TurboFan environments do not contain the context.
114    return 1 + parameter_count;  // receiver + params
115  }
116
117 private:
118  size_t parameter_count_;
119  size_t stack_slot_count_;
120};
121
122}  // namespace compiler
123}  // namespace internal
124}  // namespace v8
125
126#endif  // V8_COMPILER_OSR_H_
127