vector<bool> _M_start and 0 offset

[Marc Glisse]

On Mon, 17 Sep 2018, Jonathan Wakely wrote:

> On 15/09/18 14:27 +0200, Marc Glisse wrote:
>> Hello,
>> 
>> as explained in the PR, the implementation of vector<bool> is weirdly 
>> wasteful. Preserving the ABI prevents from changing much for now, but this 
>> small tweak can help the compiler remove quite a bit of dead code.
>> 
>> I think most other direct uses of _M_start are in constructors where the 
>> offset has just been initialized to 0, so the compiler should already know 
>> enough there, but I may have missed a few relevant places where the same 
>> idea could be used.
>> 
>> I used C++11 syntax because I find it nicer, and the compiler accepts it in 
>> C++98 mode with just a warning, suppressed in a standard header.

^^^^^^^^^^

>> Bootstrap+regtest on powerpc64le-unknown-linux-gnu.
>> 
>> 2018-09-15  Marc Glisse  <marc.glisse@inria.fr>
>>
>>        PR libstdc++/87258
>> 	* include/bits/stl_bvector.h (vector::begin(), vector::cbegin()):
>> 	Rebuild _M_start with an explicit 0 offset.
>> 
>> -- 
>> Marc Glisse
>
>> Index: include/bits/stl_bvector.h
>> ===================================================================
>> --- include/bits/stl_bvector.h	(revision 264178)
>> +++ include/bits/stl_bvector.h	(working copy)
>> @@ -802,25 +802,25 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>> #endif
>> 
>> #if __cplusplus >= 201103L
>>       void
>>       assign(initializer_list<bool> __l)
>>       { _M_assign_aux(__l.begin(), __l.end(), 
>> random_access_iterator_tag()); }
>> #endif
>>
>>       iterator
>>       begin() _GLIBCXX_NOEXCEPT
>> -      { return this->_M_impl._M_start; }
>> +      { return { this->_M_impl._M_start._M_p, 0 }; }
>>
>>       const_iterator
>>       begin() const _GLIBCXX_NOEXCEPT
>> -      { return this->_M_impl._M_start; }
>> +      { return { this->_M_impl._M_start._M_p, 0 }; }
>
> Won't this fail to compile in C++98 mode?

"I used C++11 syntax because I find it nicer, and the compiler accepts it 
in C++98 mode with just a warning, suppressed in a standard header."

Even with -Wsystem-headers I don't get a warning, I have to precompile 
with -P -E then compile the result to get "warning: extended initializer 
lists only available with -std=c++11 or -std=gnu++11".

-- 
Marc Glisse