here - Stefan-Marr.de

3. Which Concepts for Concurrent and Parallel Progr. does a VM need to Support?
specification is deemed sufficiently precise to answer the questions for which
and how concepts are exposed accurately.
Other aspects have a stronger impact on the confidence in the results. In
general, the survey is based on the specifications, language manuals, and
inspection of actual implementations. While it initially included the Parrot
VM 16 as an interesting subject, it was omitted, since the mismatch between
documentation and implementation was major. It was not clear whether the
documentation was outdated or visionary, or whether the understanding of
the implementation was not sufficient, making a proper assessment impossible.
This example raises the question of how valid the results are. Most implementations
use customary names and make an effort to be comprehensible,
which results in a high confidence in the results. However, it is not possible
to eliminate the possibility that the analysis is to a lesser extent inaccurate or
incomplete.
Exposure Assessment The confidence in the assessment that a given concept
is supported directly by a VM is high, because the VM implementations
provide direct evidence that a concept is supported as primitive or as part
of the the instruction set. However, a concept may have been missed. Thus,
some concepts may in fact be supported by the VM directly. Completeness is
thus an issue for the question of whether concepts are directly supported by
a VM. This assessment could also not by verify trivially. However, this form
of completeness has only little impact on the overall results, since it includes
a larger number of different VMs.
Concept Completeness The confidence in the completeness of the overall
identified set of concepts that is exposed by a VM and its libraries is high,
because the typically sufficiently extensive and well structured language documentations
enable a trivial verification for completeness.
Problematic are concepts that are solely available in the implementations
and remain undocumented. One example for such a case is the relatively
well know Unsafe class in Oracle’s JVM implementation. 17 Other similarly
private APIs might have been missed. Arguably, such concepts are not meant
to be used by programmers targeting the VM, and therefore do not need to be
considered as concepts that are offered by the VM. Instead, these concepts are
16 http://www.parrot.org/
17 http://hg.openjdk.java.net/jdk7/jdk7/jdk/log/tip/src/share/classes/sun/misc/U
nsafe.java
54