[generators] Implement %GeneratorGetSourcePosition.

src/interpreter/interpreter.cc

Index: src/interpreter/interpreter.cc
diff --git a/src/interpreter/interpreter.cc b/src/interpreter/interpreter.cc
index 8aea6c8209815b77e5d2d02d07d2a8600cfe5f96..284dbe34682a0c3e0f7bc8813a312ea873fb3dee 100644
--- a/src/interpreter/interpreter.cc
+++ b/src/interpreter/interpreter.cc
@@ -1791,7 +1791,8 @@ void Interpreter::DoNop(InterpreterAssembler* assembler) { __ Dispatch(); }
 // SuspendGenerator <generator>
 //
 // Exports the register file and stores it into the generator.  Also stores the
-// current context and the state given in the accumulator into the generator.
+// current context, the state given in the accumulator, and the current bytecode
+// offset (for debugging purposes) into the generator.
 void Interpreter::DoSuspendGenerator(InterpreterAssembler* assembler) {
   Node* generator_reg = __ BytecodeOperandReg(0);
   Node* generator = __ LoadRegister(generator_reg);
@@ -1816,6 +1817,13 @@ void Interpreter::DoSuspendGenerator(InterpreterAssembler* assembler) {
   __ StoreObjectField(generator, JSGeneratorObject::kContextOffset, context);
   __ StoreObjectField(generator, JSGeneratorObject::kContinuationOffset, state);
 
+  // The bytecode offsets here are relative to a different base than the ones
+  // used in the source position table, hence the subtraction.
+  Node* offset = __ SmiTag(__ IntPtrSub(
+      __ BytecodeOffset(),
+      __ IntPtrConstant(BytecodeArray::kHeaderSize - kHeapObjectTag)));
+  __ StoreObjectField(generator, JSGeneratorObject::kInputOffset, offset);

[rmcilroy, 2016/06/20 11:11:26]

Could we just store the non-subtracted value in the generator object here and do
the subtraction in JSGeneratorObject::source_position? This would be consistent
with the approach we use to push the bytecode offset on the stack used for stack
traces (where the subtraction happens in InterpretedFrame::GetBytecodeOffset())

+
   __ Dispatch();
 
   __ Bind(&if_stepping);