[turbofan] First version of loop peeling.

src/compiler/osr.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_OSR_H_
 #define V8_COMPILER_OSR_H_
 
 #include "src/zone.h"
 
 // TurboFan structures OSR graphs in a way that separates almost all phases of
 // compilation from OSR implementation details. This is accomplished with
-// special
-// control nodes that are added at graph building time. In particular, the graph
-// is built in such a way that typing still computes the best types and
-// optimizations and lowering work unchanged. All that remains is to deconstruct
-// the OSR artifacts before scheduling. The helper class below performs the
-// necessary graph rewriting.
+// special control nodes that are added at graph building time. In particular,
+// the graph is built in such a way that typing still computes the best types
+// and optimizations and lowering work unchanged. All that remains is to
+// deconstruct the OSR artifacts before scheduling and code generation.
 
 // Graphs built for OSR from the AstGraphBuilder are structured as follows:
 //                             Start
 //          +-------------------^^-----+
 //          |                          |
 //   OsrNormalEntry               OsrLoopEntry <-------------+
 //          |                          |                     |
 //   control flow before loop          |           A     OsrValue
 //          |                          |           |         |
 //          | +------------------------+           | +-------+
 //          | | +-------------+                    | | +--------+
 //          | | |             |                    | | |        |
 //        ( Loop )<-----------|------------------ ( phi )       |
 //          |                 |                                 |
 //      loop body             | backedge(s)                     |
 //          |  |              |                                 |
 //          |  +--------------+                        B  <-----+
 //          |
 //         end
 
 // The control structure expresses the relationship that the loop has a separate
-// entrypoint which corresponds to entering the loop directly from start.
+// entrypoint which corresponds to entering the loop directly from the middle
+// of unoptimized code.
 // Similarly, the values that come in from unoptimized code are represented with
 // {OsrValue} nodes that merge into any phis associated with the OSR loop.
-// The nodes {A} and {B} represent values in the "normal" graph that correspond
-// to the values of those phis before the loop and on any backedges,
-// respectively.
+// In the above diagram, nodes {A} and {B} represent values in the "normal"
+// graph that correspond to the values of those phis before the loop and on any
+// backedges, respectively.
 
 // To deconstruct OSR, we simply replace the uses of the {OsrNormalEntry}
-// control
-// node with {Dead} and {OsrLoopEntry} with start and run the {ControlReducer}.
-// Control reduction propagates the dead control forward, essentially "killing"
-// all the code before the OSR loop. The entrypoint to the loop corresponding
-// to the "normal" entry path will also be removed, as well as the inputs to
-// the loop phis, resulting in the reduced graph:
+// control node with {Dead} and {OsrLoopEntry} with start and run the
+// {ControlReducer}. Control reduction propagates the dead control forward,
+// essentially "killing" all the code before the OSR loop. The entrypoint to the
+// loop corresponding to the "normal" entry path will also be removed, as well
+// as the inputs to the loop phis, resulting in the reduced graph:
 
 //                             Start
 //         Dead                  |^-------------------------+
 //          |                    |                          |
 //          |                    |                          |
 //          |                    |                          |
 //    disconnected, dead         |           A=dead      OsrValue
 //                               |                          |
 //            +------------------+                   +------+
 //            | +-------------+                      | +--------+
@@ skipping 56 matching lines @@
  private:
   size_t parameter_count_;
   size_t stack_slot_count_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_OSR_H_