[Interpreter] Add support for calling runtime functions which return a pair.

src/interpreter/bytecodes.h

 // Copyright 2015 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_INTERPRETER_BYTECODES_H_
 #define V8_INTERPRETER_BYTECODES_H_
 
 #include <iosfwd>
 
 // Clients of this interface shouldn't depend on lots of interpreter internals.
@@ skipping 123 matching lines @@
   V(DeletePropertySloppy, OperandType::kReg8)                                  \
   V(DeleteLookupSlot, OperandType::kNone)                                      \
                                                                                \
   /* Call operations */                                                        \
   V(Call, OperandType::kReg8, OperandType::kReg8, OperandType::kCount8,        \
     OperandType::kIdx8)                                                        \
   V(CallWide, OperandType::kReg8, OperandType::kReg8, OperandType::kCount16,   \
     OperandType::kIdx16)                                                       \
   V(CallRuntime, OperandType::kIdx16, OperandType::kMaybeReg8,                 \
     OperandType::kCount8)                                                      \
+  V(CallRuntimePair, OperandType::kIdx16, OperandType::kMaybeReg8,             \
+    OperandType::kCount8, OperandType::kReg8)                                  \

[oth, 2016/01/06 16:26:07]

The problem with this bytecode is that the set of operands ends with an implicit
range.

For register-mapping there needs to be a consistent way to identify ranges of
registers, preferably without hardcoding the information for every bytecode. The
current wide register CL does this by looking for a count operand after any
8-bit register operand.

This bytecode could avoid the problem by returning one element in the
accumulator and one element as a register. Or changing the first maybe register
to be a register that takes the result. Neither is pretty, but perhaps better
than increasing the maximum number of operands or having to manually hardcode
patterns of registers.

[rmcilroy, 2016/01/06 16:48:34]

We should talk about this when your back in the office tomorrow, however we
can't do one in the accumulator, one in a register for deoptimization reasons -
the values need to be co-located on the stack. 
8-bit register operand.

I think looking for a count after any kReg8 bytecode is pretty brittle. Could we
not have a switch statement on the bytecode which special cases calls (with the
count and first_arg) and this bytecode (with the first_return having a fixed
"count" of 2)? That would seem safer and would mean this bytecode isn't a
problem. WDYT?

[rmcilroy, 2016/01/07 17:21:43]

As discussed offline, I've changed this to use a new bytecode operand called
kRegPair8, which should make it clear that this specifies a pair of registers.

   V(CallJSRuntime, OperandType::kIdx16, OperandType::kReg8,                    \
     OperandType::kCount8)                                                      \
                                                                                \
   /* New operator */                                                           \
   V(New, OperandType::kReg8, OperandType::kMaybeReg8, OperandType::kCount8)    \
                                                                                \
   /* Test Operators */                                                         \
   V(TestEqual, OperandType::kReg8)                                             \
   V(TestNotEqual, OperandType::kReg8)                                          \
   V(TestEqualStrict, OperandType::kReg8)                                       \
@@ skipping 232 matching lines @@
 
 std::ostream& operator<<(std::ostream& os, const Bytecode& bytecode);
 std::ostream& operator<<(std::ostream& os, const OperandType& operand_type);
 std::ostream& operator<<(std::ostream& os, const OperandSize& operand_type);
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_INTERPRETER_BYTECODES_H_