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<p>I managed to come up with a reproducer, and I've filed this:</p>
<p><a class="moz-txt-link-freetext" href="https://bugs.openjdk.org/browse/JDK-8336664">https://bugs.openjdk.org/browse/JDK-8336664</a></p>
<p>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".</p>
<p>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.</p>
<p>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.<br>
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<p>Thanks<br>
Maurizio<br>
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<p><br>
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<div class="moz-cite-prefix">On 17/07/2024 13:14, David Lloyd wrote:<br>
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<div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><span style="font-family:Arial,Helvetica,sans-serif">On Wed, Jul
17, 2024 at 4:41 AM Maurizio Cimadamore <<a href="mailto:maurizio.cimadamore@oracle.com" moz-do-not-send="true" class="moz-txt-link-freetext">maurizio.cimadamore@oracle.com</a>>
wrote:</span><br>
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<p>Hi David,<br>
I agree that having to second-guess signedness is not
fun.</p>
<p>However, I'd like to understand the problem more. I do
not see in SysV ABI any reference to the need
zero/sign-extend arguments. Do you have an example of an
ABI with stricter requirements? The SO post you show
says something about clang zero/sign extending all
arguments that are smaller than 32 bits, but it's not
clear to me whether that's a standard, or just something
that clang does.</p>
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<div class="gmail_default" style="font-family:arial,helvetica,sans-serif">As far as
system V goes, this behavior definitely isn't standard (in
fact my understanding is that it is undefined). However,
many compiler systems, libraries, and even the linux
kernel have seem to have relied on caller-side sign/zero
extension for values less than 32 bits in size. I think
the system V ABI on most (?) CPU types require callee-side
extension for 32-bit values and I think most (?)
compilers conform to that. But, having a way to indicate
signedness for all types would still be safest and most
future-proof I think, even if the information isn't always
used on all platforms.</div>
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<p>There are several ways to address this issue that were
discussed in the past:</p>
<p>* add carriers for unsigned types (e.g.
Unsigned<byte>) - this will likely require
Valhalla<br>
* add a sign property to value layouts. This is
relatively harmless. And will also allow
Linker::canonicalLayouts to expand the set of canonical
layouts it reports (by including the unsigned ones)<br>
* deal with this like clang does - e.g. as a Linker
option that can be added to function parameter/return
types</p>
<p>Of these, I think my preferred option would be to add
the property to value layouts. This will turn out useful
if, in the future, we will allow the memory part of the
FFM API to e.g. take a JAVA_INT and turn it into a
`long` (because we could inspect the sign, and decide
whether to zero or sign-extend).</p>
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<div class="gmail_default" style="font-family:arial,helvetica,sans-serif">I think any
of these options sounds good (though I don't like the idea
of waiting for Valhalla).</div>
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<span class="gmail_signature_prefix">-- </span><br>
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<div dir="ltr">- DML • he/him<br>
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