Concerns about signedness and ABI

[Andrew Haley]

On 7/17/24 19:10, Maurizio Cimadamore wrote:
> I managed to come up with a reproducer, and I've filed this:
> 
> https://bugs.openjdk.org/browse/JDK-8336664
> 
> TBH, it is still not super clear to me how much this is a quirk in clang
> and how much this is really what should be considered a "de facto standard".

It's a known bug in clang:

https://groups.google.com/g/x86-64-abi/c/h7FFh30oS3s/m/NDV4lCRQAQAJ

This is x86-only, AFAIK. Other ABIs didn't fall into this trap. For
example, AArch64 states quite explicitly that only the lower n bits in
a register contain the argument.

> We are aware e.g. of 32 bit promotion of narrow arguments in variadic,
> or prototype-less functions (as noted in the issue above), but other
> than that, the ABI (and the C spec) is silent on this topic.
> 
> It would be useful to know if sign extension is required in ABIs _other_
> than SysV. Using compiler explorer, it seems like both gcc/clang on ARM
> both apply the expected masking at the callee (unlike for x64/clang).
> Same for Windows.

I don't think there's any way to fix this without requiring the user of
the downcall ABI to specify signedness.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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