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

src/interpreter/bytecodes.h

Index: src/interpreter/bytecodes.h
diff --git a/src/interpreter/bytecodes.h b/src/interpreter/bytecodes.h
index 5a3e46805f3b0df4ebbbbdcabd6548ca6a720341..71d327b0a7b70a758d594c0470e933df28fe40a6 100644
--- a/src/interpreter/bytecodes.h
+++ b/src/interpreter/bytecodes.h
@@ -141,6 +141,8 @@ namespace interpreter {
     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)                                                      \
                                                                                \