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4.4. RoarVM
that is performance critical and benefits from avoiding the interpretation overhead.
The RoarVM and CogVM are derived from the same original code base and
share the implementation of primitives. While the implementations diverged
over time, the RoarVM supports the same primitives and collections of primitives
in the form of plugins to be compatible with the CogVM. Thus, on the
language level the provided mechanism are identical.
Smalltalk has access to primitives via an encoding in the method header of
a method object. If the primitive part of the method header is set to a value
different from zero, the VM is asked to execute the primitive referenced by
this method instead of executing the bytecodes encoded in the method. For
the execution of a primitive the current context object is used, instead of creating
a new one, as is done for standard message sends. This gives the primitive
access to the current operand stack, the VM, and perhaps the underlying system.
A number of primitive identifiers is however reserved for so-called quick
methods. Quick methods do not encode bytecodes, but instead use the primitive
identifier to encode presumably common short methods. This includes
return of self, true, false, and nil. Furthermore, quick methods encode
accessors, i. e., methods that only return the object stored in a field.
Tab. 4.1 shows the encoding of the method header in the RoarVM. It uses
the same encoding as the CogVM for compatibility. To avoid issues with the
garbage collector, and because the method header is a normal field in the
method object, it is encoded using a SmallInt, which is indicated by the
least significant bit set to one. The header encodes the number of arguments
(#args) the method expects, the number of slots the context object should provide
for temporary variables (#temporaries), whether a small or large context
object should be allocated for the operand stack (FS: frame size), and the number
of literals encoded in the method. For historical reasons, the primitive is
encoded with 10 non-consecutive bits. Bit 29 was presumably added when
the need arose. The 30th bit (F) is a flag bit that is reserved for custom use at
the language side, and bit 31 remains unused.
Table 4.1.: RoarVM Method Header
F P. #args #temporaries FS #literals Primitive 1
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
103