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      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/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 
     78 namespace v8 {
     79 namespace internal {
     80 
     81 class CompilationInfo;
     82 
     83 namespace compiler {
     84 
     85 class JSGraph;
     86 class CommonOperatorBuilder;
     87 class Frame;
     88 class Linkage;
     89 
     90 // Encapsulates logic relating to OSR compilations as well has handles some
     91 // details of the frame layout.
     92 class 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