Re: [v3] Fix Werror breakage (maintainer-mode)

["Richard Guenther" <richard dot guenther at gmail dot com>]

On Sun, Jul 6, 2008 at 12:08 PM, Paolo Carlini <paolo.carlini@oracle.com> wrote:
>> Can somebody enlighten me as to a proper locale_init.cc fix?
>
> Yes, the warning is new and I'd like to understand whether the warning is worrisome or not, or even bogus. That's not clear to me, at the moment. As a LAST resort, the warning could be suppressed via the "usual" trick, something like:
>
>    const char* __p = reinterpret_cast<const char*>(c_locale);
>    return *reinterpret_cast<const locale*>(__p);
>
> which I would consider a tad better than disabling -Werror selectively.

The code does

  typedef char fake_locale[sizeof(locale)]
  __attribute__ ((aligned(__alignof__(locale))));
  fake_locale c_locale;

  const locale&
  locale::classic()
  {
    _S_initialize();
    return reinterpret_cast<const locale&>(c_locale);
  }

which violates C aliasing rules.  But - I understand that possibly

  void
  locale::_S_initialize_once()
  {
    // 2 references.
    // One reference for _S_classic, one for _S_global
    _S_classic = new (&c_locale_impl) _Impl(2);
    _S_global = _S_classic;
    new (&c_locale) locale(_S_classic);
  }

is always called before the above, so the dynamic type of c_locale
will be locale, so this _doesn't_ violate the C++ aliasing rules.

(of course some of us don't agree on the interpretation of the C++
standard wording with regards to whether it is allowed to change
the dynamic type of storage with a declared type, c_locale in this case)

A miscompilation is very unlikely (even though GCC still may have bugs
with honoring the C++ memory model), as reads/writes of type c_locale
always conflict with reads/writes of type char, which is the declared type
of c_clocale.

For "correctness" and code documentation purposes in this case it would
be nice to have a way to declare un-typed memory which cannot be
accessed through the declaration identifier or a pointer of its "type", like

  typedef void fake_locale
__attribute__((aligned(__alignof__(locale)),size(__sizeof__(locale)));
  fake_locale c_locale;

or some similar way of associating the identifier c_locale with some memory
(basically a "static" malloc).

Richard.