RFR: 8308031: Linkers should reject unpromoted variadic parameters [v2]

[Jorn Vernee]

On Wed, 31 May 2023 23:41:25 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   review comments
>
> src/java.base/share/classes/java/lang/foreign/Linker.java line 379:
> 
>> 377:  * type {@code float} is converted to {@code double}, and each integral type smaller than {@code int} is converted to
>> 378:  * {@code int}. As such, the native linker will reject attempts to link function descriptors with certain variadic argument
>> 379:  * layouts. Namely, {@linkplain ValueLayout value layouts} that have a carrier type of {@code boolean}, {@code byte},
> 
> Is there any reason as to why you decided to say which layouts are **not** allowed as variadic layouts? I'm wondering whether, if we add more carriers in the future, this list will probably need to be updated? Or do the restriction only involve these "small" types, and stuff like `long double` is always allowed? (in which case the set of unsupported layouts would remain stable over time)

No real reason for the current polarity. The specification explicitly calls out the float -> double conversion, and then for the integral types it refers to ['integer promotions'](https://en.cppreference.com/w/c/language/conversion#Integer_promotions), which is a somewhat complex rule set for assigning ranks to integer types.

So, I think if we add more carriers in the future, we would have to worry about integral types that have a rank less than (unsigned) int when considering this list. This is currently only the types I've listed (though, `char` is a bit of a special case), and this seems relatively stable to me. e.g. `long double` would not need to be added here.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14225#discussion_r1212432229