here - Stefan-Marr.de

7.1. AST Transformation
all have the same receiver object, which might be the result of an expression.
However, the object may change its owner. This indirection, to determine the
domain for every execution request directly, avoids a complete restructuring
of the code as part of the transformation. With that, it also avoids the need of
complex transformations and the use of additional temporary variables. The
latter is a problem when a naive 1 transformation is used. The transformed
method could require more temporary variables than what is supported by
the bytecode set and VM. Furthermore, besides keeping the transformation
simple, the approach has another benefit. It preserves the structure of the
original code, which is beneficial while debugging the transformed code.
In Squeak/Pharo, globals such as classes and class variables are realized by
mutable associations of a symbol with a value. These accesses are rewritten by
sending either #readGlobal: or #write:toGlobal: to the current execution
domain. Note that the current execution domain does not need to be the
receiver’s owner domain. Certain shared memory concurrency models might
want to enable access from processes from multiple domains on the same
object.
Strategy for Controlled Enforcement The AST-OMOP realizes the notion of
controlled enforcement by applying the approach Renggli and Nierstrasz
utilized to differentiate between normal and transactional execution.
As illustrated in Lst. 7.2, every method implementation needs to be available
in two versions: the normal unchanged version, which corresponds to unenforced
execution, and the transformed method. Enforced, i. e., transformed
methods are made available with a name prefixed with __enforced__. The
prefix is used to have the enforced methods side by side with the unenforced
ones in the same Smalltalk method dictionary.
Switching to enforced execution is done by sending the #enter: message to
a block. The parameter to #enter: is the domain which #evaluateEnforced:
is going to be sent to with the block as argument. This double dispatch is
used for idiomatic/esthetic reasons. However, it also simplifies the correct
implementation of enforced execution. The example block given in line 25
needs to be transformed in the same way normal methods are transformed
into their enforced equivalents. The current restriction of this implementation
is that only blocks that directly receive the #enter: message statically in the
1 Other approaches would require analysis of the transformed code to minimize the number
of temporary variables required.
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