@SlowPath renaming discussion

[Christian Humer]

Hi Folks,

I've heard from a few people (including myself) that the @SlowPath
annotation naming might be misleading. That is why I want to raise a
discussion on this list, which will hopefully lead to a good permanent
decision for a new name.

I would suggest these names:
@Boundary
@TruffleBoundary
@PartialEvaluationBoundary (or @PEBoundary)

Please add more suggestions and vote for whatever you think is best.

---

Here are some additional thoughts regarding the old name:

The @SlowPath annotation marks the border for the Truffle partial
evaluator. So methods marked with @SlowPath are not partially evaluated and
treated as if they would be normal Java methods.

Here is an example:

@SlowPath
String stringConcat(String s1, String s2) {
   return new StringBuilder(s1).append(s2).toString();
}

If there would be no @SlowPath annotation on this method then the partial
evaluator would just inline through the constructor and the append method.
This will produce too much code and is not desirable. Another issue is that
the implementation of append is not under the control of the guest language
implementation and might contain e.g. recursions which will make the
partial evaluator fail.

Virtual objects like VirtualFrame instances must not be passed to @SlowPath
annotated methods because they leave the scope of the partial evaluator.
Such cases are considered Truffle compilation errors.

To summarize:
 * @SlowPath should be applied on every call to a method that is not under
the control of the guest language implementation.
 * @SlowPath should be applied on guest language methods that are very big
and might produce too much code.
 * Virtual objects must not be passed to @SlowPath annotated methods.

These are arguments against using "SlowPath" as name:
 * Its name does not explain the semantics in any way
 * @SlowPath might also be used for "fast path" code.
 * The use of @SlowPath does not imply slow execution.



- Christian Humer