[Bug c++/67918] -fdevirtualize causes binary to crash with Segmentation Fault (CryptoPP involved)

["11throwaway11 at outlook dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67918

--- Comment #6 from Nameless <11throwaway11 at outlook dot com> ---
Another update:

In Crypto++ there was a class with a virtual function, in a header file:
https://github.com/weidai11/cryptopp/blob/b7de164d6251dc066123b59bc15d30c74e920756/modes.h#L71

There were two derived classes that overridden this function, but called the
version of the base class:
1)
https://github.com/weidai11/cryptopp/blob/b7de164d6251dc066123b59bc15d30c74e920756/modes.h#L173
2)
https://github.com/weidai11/cryptopp/blob/b7de164d6251dc066123b59bc15d30c74e920756/modes.h#L232

Moving definitions of these functions from a header file to a .cpp-file and
recompiling the library made the testcase work:
https://github.com/weidai11/cryptopp/commit/8134f2cd502e457201d65f1dc557268ba13e3663

Worth noting that only -fdevirtualize is actually not enough, it's a
combination of -O1 and -fdevirtualize. -O0 -fdevirtualize doesn't crash.

I couldn't narrow it down to a specific -O1 optimization, because applying all
those flags mentioned under -O1:
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

... did not make the output binary crash. I guess it's a case of "Not all
optimizations are controlled directly by a flag. Only optimizations that have a
flag are listed in this section. Most optimizations are only enabled if an -O
level is set on the command line".