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Date: Mon, 17 Sep 2018 16:29:00 -0000
From: Jonathan Wakely <jwakely@redhat.com>
To: Marc Glisse <marc.glisse@inria.fr>
Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: Re: vector<bool> _M_start and 0 offset
Message-ID: <20180917154554.GP23172@redhat.com>
References: <alpine.DEB.2.02.1809151309440.6584@stedding.saclay.inria.fr>
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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?