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

[Maurizio Cimadamore]

On Wed, 31 May 2023 22:44:52 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> In C, arguments smaller than `int` are promoted to (`unsigned`) `int`, and `float` is promoted to `double`, when being passed as variadic argument (see e.g. https://en.cppreference.com/w/c/language/conversion#Default_argument_promotions). This patch restricts the layouts that can be used as variadic layouts to what is allowed by the C specification.
>> 
>> The fallback linker is also updated to use to correct function to link variadic calls (not doing this turned out not to be a problem so far, but it is problematic for instance on Mac/AArch64 when using the fallback linker). Adding the restriction on layouts for all linkers is also partly motivated by the fallback linker rejecting such unsupported variadic layouts already.
>> 
>> I've added a small paragraph to the Linker javadoc as well that explains the restriction. Comments on that are welcome, but please explain.
>> 
>> The tests are updated to no longer try to link variadic functions with the illegal layouts, and I've added some more negative tests to TestIllegalLink.
>> 
>> Testing:
>> - local testing on Windows/x64
>> - tier1-3 + jdk-tier5 (ongoing)
>> - manual test run on mac/aarch64 with the fallback linker to verify the correctness of the fallback linker changes.
>
> 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 368:

> 366:  * <h3 id="variadic-funcs">Variadic functions</h3>
> 367:  *
> 368:  * Variadic functions (e.g. a C function declared with a trailing ellipses {@code ...} at the end of the formal parameter

Optional suggestion for improvement (maybe now, or maybe later). When reading this great para, I understand that there are two things that fall under the "variadic function" umbrella. Some are declared with `...` and some with `()`. This is a very good definition and I wonder if we should expand a bit more on it - e.g. in a way, we never explain what a variadic function is - we merely define it by saying how it is declared in C. I wonder if we might very very briefly explain that in C, some functions (variadic functions) can take a variable number of parameters. Then we go on to say that these functions can be declared in two ways (e.g. ellipsis and prototype-less). Perhaps if we used a bullet-list to define the two ways in which variadic function can be declared, the definition could stand out even more?

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PR Review Comment: https://git.openjdk.org/jdk/pull/14225#discussion_r1212415909