clang++-21: massive failures in Perl 5 test suite

[jkeenan]

Smoke-test results, mainly from @cjg-cguevara, indicate that as various Linux distributions roll out upgrades of LLVM to include clang version 21, we are running into massive test failures when we compile perl with clang++-21. Consider these smoke test reports run at c0b8ab1:

linux 6.12.60_1 [Void Linux rolling]
clang++ Clang 21.1.4
https://perl.develop-help.com/db/5522079

linux 6.17.9-300.fc43.x86_64 [Fedora Linux 43 (Server Edition)]
clang++ version Clang 21.1.6 (Fedora 21.1.6-1.fc43)
https://perl.develop-help.com/db/5522092

linux 6.17.9-arch1-1 [Arch Linux]
clang++ version Clang 21.1.6 (*1)
https://perl.develop-help.com/db/5522090

linux 6.18.0-3-lts [Alpine Linux v3.23 3.23.0]
clang++ version Alpine Clang 21.1.2
https://perl.develop-help.com/db/5522075

It appears that we don't yet have any smoke-test reports from other OSes, e.g., FreeBSD, run with clang++-21. On my own machines, clang++-19 is as new as I can get. I suspect, however, that this problem will not be confined just to Linux.

(We're also getting lots of build-time warnings on these, but at least the build is completing.)

Can we get this investigated?

Thank you very much.

[thesamesam]

llvm/llvm-project#152241

[Leont]

The -fno-strict-aliasing, -pipe and -fstack-protector flags are only enabled if the code example compiles without warnings. And that code example now generates a warning when compiled with clang++:

clang++: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]

It seems we need a better check for these flags.

There's a minor secondary issue that those warnings aren't outputted in the right place, that should be fixed by

diff --git Configure Configure
index ef2257f438..443ec99b58 100755
--- Configure
+++ Configure
@@ -5420,8 +5420,8 @@ echo "int main(void) { return 0; }" > gcctest.c;
 if $cc $_sysroot -O2 $flag -o gcctest gcctest.c 2>gcctest.out && $run ./gcctest; then
     echo "Yes, it does." >&4;
     if $test -s gcctest.out ; then
-        echo "But your platform does not like it:";
-        cat gcctest.out;
+        echo "But your platform does not like it:" >&4;
+        cat gcctest.out >&4;
     else
    case "$ccflags" in
    *$check*)

Also note that I could only reproduce the failures with a threaded perl.

[tonycoz]

The deprecation means that using clang++ to build as C++ without -xc++ is going to stop working at some point (this has been warning since at least clang-16).

Unfortunately we can't just add -xc++ to -Dcc, since the generate_uudmap link rule uses $(CC), and the -xc++ then treats the object file as C++ with useless results:

generate_uudmap.o:1:1: error: expected unqualified-id
    1 | <U+007F>ELF<U+0002><U+0001><U+0001><U+0000><U+0000><U+0000>...
      | ^

Changing this rule to use $(LD) instead broke the build on AIX and HP-UX (see 0c77b72).

Maybe my simple change from 07e3b5c should be reapplied and exclude HP-UX and AIX.

Or we could drop building as C++.