[Bug c++/106568] New: -freorder-blocks-algorithm appears to causes a crash in stable code, no way to disable it

["noloader at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106568

            Bug ID: 106568
           Summary: -freorder-blocks-algorithm appears to causes a crash
                    in stable code, no way to disable it
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: noloader at gmail dot com
  Target Milestone: ---

Hi Everyone,

This is going to be a shitty bug report because we don't have a reproducer. We
believe we have it narrowed down to a particular optimization, however.

Debian Unstable, Fedora 37 and Gentoo 17.1 are reporting crashes in Crypto++
test program.[1,2] The distros use GCC 12. We found a particular function
crashes without explanation (and a garbage backtrace) at -O2 and -O3. The
function is Ok at -O0, -O1 and -Os.

(The code has been fairly stable for years. It is -Wall, -Wextra, Asan, UBsan
and Valgrind clean. We would be surprised to learn we have undetected UB. But
we don't rule it out).

According to GCC Optimization docs, the difference between -Os (no crash) and
-O2 (crash) are:[3]

    -falign-functions
    -falign-jumps
    -falign-labels
    -falign-loops
    -fprefetch-loop-arrays
    -freorder-blocks-algorithm=stc

We used CFLAGS and CXXFLAGS with -Os plus listed opts less
-freorder-blocks-algorithm=stc. The crash went away.

We are fairly certain the problem is with the -freorder-blocks-algorithm
optimization. The problem we are now having is, we don't know how to disable
it. The following fails to compile:

    -fno-reorder-blocks-algorithm
    -freorder-blocks-algorithm=none
    -freorder-blocks-algorithm=

So, we believe we have a bad option in -freorder-blocks-algorithm, but we can't
disable it for typical opt settings used by distros. The typical opt setting is
-O2 or -O3.

I sincerely apologize for not having a reproducer. I'm not sure where to begin
when it comes to -freorder-blocks-algorithm.

Please advise.

[1] https://github.com/weidai11/cryptopp/issues/1134
[2] https://github.com/weidai11/cryptopp/issues/1141
[3] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html