Re: [v3 PATCH] PR libstdc++/77288 and the newest proposed resolution for LWG 2756

[Christophe Lyon <christophe.lyon@linaro.org>]

On 22 September 2016 at 15:25, Jonathan Wakely <jwakely@redhat.com> wrote:
> On 22/09/16 12:15 +0100, Jonathan Wakely wrote:
>>
>> On 22/09/16 11:16 +0100, Jonathan Wakely wrote:
>>>
>>> (Somebody should fix PR58938 so exception_ptr is portable).
>>
>>
>> Christophe, would you be able to test this patch?
>>
>> It uses a single global mutex for exception_ptr objects, which doesn't
>> scale well but that probably isn't a problem for processors without
>> lock-free atomics for single words.
>>
>> This also solves the problem of mismatched -march options, where the
>> header is compiled for a CPU that supports the atomics but
>> libstdc++.so was built for an older CPU that doesn't support them, and
>> linking fails (as in https://gcc.gnu.org/PR58938#c13).
>
>
> We'd also need something like this extra piece, to ensure we don't
> leak exceptions. Currently __gxx_exception_cleanup assumes that if
> ATOMIC_INT_LOCK_FREE < 2 the referenceCount can never be greater than
> 1, because there are not exception_ptr objects that could increase it.
>
> If we enable exception_ptr unconditionally then that assumption
> doesn't hold. This patch uses the exception_ptr code to do the
> cleanup, under the same mutex as any other increments and decrements
> of the reference count.
>
> It's a bit of a hack though.
>
Should I have applied this one on top of the other?

I ran a validation with it alone, and
arm-none-eabi with default mode, cpu, and fpu does not build:
In file included from
/tmp/9260164_29.tmpdir/aci-gcc-fsf/sources/gcc-fsf/gccsrc/libstdc++-v3/libsupc++/eh_throw.cc:27:0:
/tmp/9260164_29.tmpdir/aci-gcc-fsf/sources/gcc-fsf/gccsrc/libstdc++-v3/libsupc++/exception_ptr.h:43:4:
error: #error This platform does
not support exception propagation.
 #  error This platform does not support exception propagation.
    ^~~~~
make[4]: *** [eh_throw.lo] Error 1


In addition, on arm-none-eabi --with-mode=thumb --with-cpu=cortex-a9,
I've noticed a regression in c++
  - PASS now FAIL             [PASS => FAIL]:

  g++.dg/opt/pr36449.C  -std=gnu++11 execution test
  g++.dg/opt/pr36449.C  -std=gnu++14 execution test
  g++.dg/opt/pr36449.C  -std=gnu++98 execution test

My logs show:
qemu: uncaught target signal 11 (Segmentation fault) - core dumped

The validation of the other patch is still running: I had to re-run it
because the
patch didn't apply because of the ChangeLog entry.