Assign yield ids in ast-numbering rather than in bytecode-generator.

src/ast/ast-numbering.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_AST_AST_NUMBERING_H_
 #define V8_AST_AST_NUMBERING_H_
 
 namespace v8 {
 namespace internal {
 
 // Forward declarations.
 class FunctionLiteral;
 class Isolate;
 class Zone;
 
 namespace AstNumbering {
-// Assign type feedback IDs and bailout IDs to an AST node tree.  For a
-// generator function, also annotate the function itself and any loops therein
-// with the number of contained yields.
+// Assign type feedback IDs, bailout IDs, and generator yield IDs to an AST node
+// tree.
 bool Renumber(Isolate* isolate, Zone* zone, FunctionLiteral* function);
 }
 
+// Some details on yield IDs
+// -------------------------
+//
+// In order to assist Ignition in generating bytecode for a generator function,
+// we assign a unique number (the yield ID) to each Yield node in its AST. We
+// also annotate loops with the number of yields they contain (loop.yield_count)
+// and the smallest ID of those (loop.first_yield_id), and we annotate the
+// function itself with the number of yields it contains (function.yield_count).
+//
+// The way in which we choose the IDs is simply by enumerating the Yield nodes.
+// Ignition relies on the following properties:
+// - For each loop l and each yield y of l:
+//     l.first_yield_id  <=  y.yield_id  <  l.first_yield_id + l.yield_count
+// - For the generator function f itself and each yield y of f:
+//                    0  <=  y.yield_id  <  f.yield_count
+
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_AST_NUMBERING_H_