[Bug 243958] net-p2p/libtorrent-rasterbar : Fails to build on FreeBSD 12.1-RELEASE-p1 powerpc 32 bit : libtorrent-rasterbar.so.1.2.3: undefined reference to `__atomic_compare_exchange_8'

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=243958

--- Comment #11 from Mark Millard <marklmi26-fbsd at yahoo.com> ---
(In reply to Piotr Kubaj from comment #9)

For head, if gcc* is to match the system clang code
that is currently produced by default, gcc* needs to
be told to use -maix-struct-return instead of its
default -msvr4-struct-return .

It will be this way until system-clang itself is
updated to use the historical -msvr4-struct-return
style that was in use prior to the conversion to
using clang as the system compiler for 32-bit
powerpc.

This, unfortunately, is a mismatch with prior-to-head
versions of 32-bit FreeBSD: for those -maix-struct-return
in a gcc* would be wrong and would introduce an ABI
difference.

I'll note that, when code on head hits the ABI
difference, it treats non-pointer-values as pointer
values and tries to dereference or update memory
through the garbage addresses.

(FYI: I'm a tester of the llvm proposal, so my context
is non-default for the system-clang behavior for
targeting 32-bit powerpc.)

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