[PATCH] Mitigate PR85222 a bit

[Richard Biener]

On Fri, 6 Apr 2018, Jonathan Wakely wrote:

> On 6 April 2018 at 10:39, Richard Biener wrote:
> >
> > The following allows people configuring the gcc-4 compatible ABI
> > as the default ABI to retain compatibility with old programs
> > catching ios_base::failure by matching the abi version thrown
> > to the configured default ABI.
> >
> > This doesn't really fix the PR but it makes behavior between
> > the dual-ABI with default gcc-4 compatible consistent with that
> > of the non-dual-ABI which is what I had expected.
> >
> > Whether an ABI break is really desired for the case of a c++11
> > default ABI is still questionable and any programs that differ
> > from the default ABI choice will now be broken (compared to
> > those differing from c++11).
> >
> > Bootstrapped on x86_64-unknown-linux-gnu, ok for trunk?  Ok for
> > the GCC 7 branch?
> 
> Thinking about this further, I seem to recall we decided *not* to do
> this. Currently the --with-default-libstdcxx-abi flag only affects the
> default value of a macro in the headers, it doesn't affect the library
> ABI. The header macro can be overriden per translation unit, but the
> library ABI is always the same (it always throws the new ios::failure
> type). With this change the option would affect the library ABI and so
> --with-default-libstdcxx-abi would cause a new effect on libstdc++.so

But --disable-libstdcxx-dual-abi does so as well.  Currently the
behavior with that compared to 
--with-default-libstdcxx-abi=gcc4-compatible is inconsistent with
respect to whether the following testcase works or not (note it
doesn't explicitely specify the ABI version to use)

#include <iostream>
#include <fstream>
using namespace std;
int main ()
{
  std::ifstream pstats;
  pstats.exceptions(ifstream::failbit | ifstream::badbit | 
ifstream::eofbit);
  try {
      printf("\n Opening file : /proc/0/stat ");
      pstats.open("/proc/0/stat");
  }
  catch(ifstream::failure e)
    {
      printf("\n!!Caught ifstream exception!!\n");
      if(pstats.is_open()) {
          pstats.close();
      }
    }
  return 0;
}

I call that a bug.  Yes, we talked about ways to eventually fix that.
But is anybody working on that?

I think the current default behavior with 
--with-default-libstdcxx-abi=gcc4-compatible is undesirable.

Thanks,
Richard.

> > I'm not sure if we want to revert r245167 after this?  I'm
> > re-running the testsuite with a gcc4-compatible ABI right now.
> > At least with a "real" fix we should be able to run the affected
> > tests twice, once with the new and once with the old ABI.
> >
> > Thanks,
> > Richard.
> >
> > 2018-04-06  Richard Biener  <rguenther@suse.de>
> >
> >         PR libstdc++/85222
> >         * src/c++11/ios.cc: Remove hard define of _GLIBCXX_USE_CXX11_ABI to 1.
> >         Instead use the configured default ABI to decide the ABI version
> >         of ios_base::failure thrown by __throw_ios_failure.
> >
> > Index: libstdc++-v3/src/c++11/ios.cc
> > ===================================================================
> > --- libstdc++-v3/src/c++11/ios.cc       (revision 258812)
> > +++ libstdc++-v3/src/c++11/ios.cc       (working copy)
> > @@ -26,9 +26,8 @@
> >  // ISO C++ 14882: 27.4  Iostreams base classes
> >  //
> >
> > -// Determines the version of ios_base::failure thrown by __throw_ios_failure.
> > -// If !_GLIBCXX_USE_DUAL_ABI this will get undefined automatically.
> > -#define _GLIBCXX_USE_CXX11_ABI 1
> > +// The ABI version of ios_base::failure thrown by __throw_ios_failure
> > +// is determined by the default ABI version choosed at configure time
> >
> >  #include <ios>
> >  #include <limits>
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)