[Bug c++/66223] New: Diagnostic of pure virtual function call broken, including __cxa_pure_virtual

[Bug c++/66223] New: Diagnostic of pure virtual function call broken, including __cxa_pure_virtual

[d.frey at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66223

            Bug ID: 66223
           Summary: Diagnostic of pure virtual function call broken,
                    including __cxa_pure_virtual
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: d.frey at gmx dot de
  Target Milestone: ---

Consider this small and certainly broken program:

    struct B {
        B* self;
        B() : self( this ) { self->f(); }
        virtual void f() = 0;
    };

    struct D : B
    {
        void f() {}
    };

    int main()
    {
        D d;
    }

The ctor of B calls (indirectly) the pure virtual function f(), but the vtbl is
still from B, not D (yet). Hence the program crashes. With GCC 4.9, I got:

    > ./a.out
    pure virtual method called
    terminate called without an active exception
    Aborted (core dumped)
    >

Which is a good hint and I got a core dump. Fine so far. With GCC 5.1, I get
this:

    > ./a.out
    Segmentation fault (core dumped)
    >

Which is certainly less helpful.

What is actually a lot worse is that even __cxa_pure_virtual is severly broken.
I used to have my own __cxa_pure_virtual method to provide more output
including a backtrace, something like this was my output for GCC 4.9:

    > ./a.out
    ### EMERGENCY ###
    pure virtual function called
    ### BACKTRACE ###
   
build/release/test/emergency/pure_virtual_XFAIL(coin::core::output::backtrace()+0x23)
[0x4034a3]
    build/release/test/emergency/pure_virtual_XFAIL(__cxa_pure_virtual+0x47)
[0x4031f7]
    build/release/test/emergency/pure_virtual_XFAIL() [0x402a09]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7ff9dd52dec5]
    build/release/test/emergency/pure_virtual_XFAIL() [0x402b14]
    ### ABORTING ###
    Aborted (core dumped)
    >

After the backtrace was printed to stdout, a core dump was written. With GCC
5.1, all I get is:

    > ./a.out
    >

where at least the result code is not 0 (it's 128 if it helps). But no message
and no core dump.

Further experiments have shown that GCC 5.1 actually calls the terminate
handler (which I also registered via std::set_terminate). This handler prints a
backtrace and some other information when called on other errors, but for a
pure virtual call it seems to be unable to even call a simple write() to
stdout.

Please let me know if you need further help to debug and fix this problem. I
realize it's "just" a diagnostic in case of calling an unimplemented pure
virtual method which should not be done in the first place, but I think the
current situation is really hurting people when there is absolutely no message
and no core dump and the process just returns with a non-zero exit code.

[Bug c++/66223] Diagnostic of pure virtual function call broken, including __cxa_pure_virtual

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66223

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |WAITING

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I still get the same output for 5.1:

pure virtual method called
terminate called without an active exception
Aborted (core dumped)

How did you configure GCC? (Please provide the output of 'gcc -v`' as requested
in the bug reporting instructions)

[Bug c++/66223] Diagnostic of pure virtual function call broken, including __cxa_pure_virtual

[d.frey at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66223

--- Comment #2 from Daniel Frey <d.frey at gmx dot de> ---
I'm using Ubuntu 14.04 in a VM, packages are installed from
http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu trusty main. Package
version is 5.1.0-0ubuntu11~14.04.1

Output from g++-5 -v:

Using built-in specs.
COLLECT_GCC=g++-5
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5.1.0-0ubuntu11~14.04.1' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=c++98 --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.1.0 (Ubuntu 5.1.0-0ubuntu11~14.04.1) 

I'm compiling the example code with:

g++-5 -std=c++14 -O3 -Wall -Wextra t.cpp

I tried it without -O3 and it then prints the old output:

pure virtual method called
terminate called without an active exception
Aborted (core dumped)

So that probably means it has something to do with the optimizer?

[Bug ipa/66223] [5/6 Regression] Diagnostic of pure virtual function call broken, including __cxa_pure_virtual

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66223

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
          Component|c++                         |ipa
            Summary|Diagnostic of pure virtual  |[5/6 Regression] Diagnostic
                   |function call broken,       |of pure virtual function
                   |including                   |call broken, including
                   |__cxa_pure_virtual          |__cxa_pure_virtual

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
With -O2 you just get a segfault.

With -O1 or -O2 -fno-devirtualize you get the old output.

[Bug ipa/66223] [5/6 Regression] Diagnostic of pure virtual function call broken, including __cxa_pure_virtual

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66223

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |5.3