here - Stefan-Marr.de

4. Experimentation Platform
for the currently executing method and block. The POP_FIELD bytecode implemented
by #doPopField in line 6, modifies the current object at the field index
indicated by the bytecode. It stores the current top element of the stack into
that field, and pops the top off the stack. The reverse operation is realized in
line 11 by the PUSH_FIELD bytecode, which reads the field of an object and
pushes the result onto the operand stack. The implementations of the SEND
and SUPER_SEND bytecode given in lines 16 and 24 first determine the object
that receives the message send, based on the number of arguments on the
stack, and will then determine the class where the lookup starts to eventually
delegate to the #performSend:to:lookupCls: method.
How the invocation is implemented depends on whether the message leads
to an application of a SOM method representing bytecodes, or a primitive
that is implemented in the implementation language. The application of a
method, as shown in line 6 of Lst. 4.6, will result in the creation of a new
frame (cf. line 29) that will be initialized with the receiver and the arguments.
The invocation of a primitive on the other hand, is performed in AweSOM by
taking arguments from the operand stack, executing the primitive with these
arguments and pushing the result value back onto the stack. In other SOM
implementations, and in typical Smalltalk VMs like the CogVM or RoarVM,
primitives will obtain a reference to the context object, i. e., frame instead
and will manipulate it directly, because it gives primitives more freedom and
power in terms of what they can implement. AweSOM optimizes for the common
case, reducing the burden on the primitive implementer.
The remaining bytecodes of SOM complement the ones already discussed.
Since their details are beyond the scope of this dissertation, this section only
gives a brief description of the operation each bytecode represents. Note however
that bytecodes represent access to object fields and local variables with
indexes. This is a typical optimization known as lexical addressing [Abelson
et al., 1996].
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