Sponsor Request: 8241100: Make Boolean, Character, Byte, and Short implement Constable
jorn.vernee at oracle.com
Fri Mar 20 17:49:09 UTC 2020
W.r.t. the source type being needed, I see the following 4 major cases:
1. src=Prim & dst=Prim -> cast convert. For boolean the
least-significant-bit is used to convert it to/from an integer type.
2. src=Prim & dst=Ref -> box the source value and potentially cast
3. src=Ref & dst=Prim -> apply an unboxing conversion if possible and
then a casting conversion (with same trick for boolean as (1))
4. src=Ref & dst=Ref -> reference cast
Without the source type we can't disambiguate between cases (2) and (4),
or (1) and (3) because the bootstrap takes Object as an input. For (2)
and (4) the bootstrap invocation mechanism takes care of the boxing for
us, and the cast is performed by (4). For (1) and (3) the conversion
code for the latter handles conversion of wrapper types to Number and
then to the target primitive per (1). In the end things seems to work
out to the same result (though maybe I'm missing some subtle difference
in a failure case).
What I'm mostly worried about is that the source type already affects
_how_ the conversion is done, and the fact that this difference is not
observable seems somewhat incidental.... Coupled with the fact that
asType and explicitCastArguments also have access to both the source and
destination type, I think maybe the new bootstrap method should as well.
After all, what if the set of cases is extended (valhalla?) and/or not
being able to disambiguate starts to matter?
I've written a more in-depth specification for the bootstrap, and
re-implemented it using explicitCastArguments, since that helps to catch
discrepancies between the input value and the source type. Here is the
next iteration: http://cr.openjdk.java.net/~jvernee/8241100/webrev.01
I've kept the source type for now, if it should be removed the
specification can be trimmed down (since there would be less cases to
As for the name; I think "asType" might be confusing since the applied
conversion is not quite the same as MethodHandle::asType. Since the
bootstrap is implemented in terms of explicitCastArguments I went with
"explicitCast", how is that?
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