[Bug c/94734] New: Program crashes when compiled with GCC 10

["john+gcc at hogberg dot online" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94734

            Bug ID: 94734
           Summary: Program crashes when compiled with GCC 10
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john+gcc at hogberg dot online
  Target Milestone: ---

Created attachment 48361
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48361&action=edit
Test case program

The attached program crashes when it has been compiled with GCC 10 and -O2.
With -O3 or -Os it returns an incorrect value instead.

There are no warnings with `-Wall -Wextra` or `-fanalyzer`, and strangely
enough `-fsanitize=undefined` hides the crash without reporting any errors.

-fno-aggressive-loop-optimizations also makes the crash go away which made me
suspect that the code is wrong somehow, but `frama-c` insists that it's correct
and should always return 0. If there's a bug in there, I'm not seeing it. :(

Steps to reproduce:

----
$ gcc -O2 test.c -o bug
$ ./bug
Segmentation fault (core dumped)
----

GCC version:

----
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-pc-linux-gnu/10.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ./configure
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.0.1 20200423 (experimental) (GCC) 
----

It appears to work fine with GCC 7, I tested it with:

----
$ gcc-7 -v
Using built-in specs.
COLLECT_GCC=gcc-7
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
7.5.0-3ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)
----