perl 5.40.2 fails to run simple script when compiled with Clang-21.1.0-rc2

[kraj]

I use yocto to build a system using clang as default system compiler. When using clang-21 RC2, resulting perl is not working correctly. A simple reproducer is

perl -e 'use Getopt::Std;'

instead of executing successfully results in

panic: magic_killbackrefs (flags=0) at /usr/lib/perl5/5.40.2/warnings.pm line 620.
Compilation failed in require at /usr/lib/perl5/5.40.2/Getopt/Std.pm line 4.
BEGIN failed--compilation aborted at /usr/lib/perl5/5.40.2/Getopt/Std.pm line 4.
Compilation failed in require at -e line 1.
BEGIN failed--compilation aborted at -e line 1.

This happens on x86_64 and also on arm64 targets.

If perl is built with -Doptimize='-O2 -fno-strict-aliasing' added to configure then it works correctly. It also works file when using just -O1

I do not have a further narrowed down reproducer than this yet.

[thesamesam]

Note that Perl builds with -fno-strict-aliasing in a bunch of environments (looks like it does it for non-ancient GCC, not sure if that branch will fire for Clang or not). It's also easy to accidentally override those *FLAGS that configure sets with Perl's setup.

I wouldn't be surprised if there's aliasing issues there, anyway. There was some interest in fixing those but it's a complex codebase.