vector<bool> _M_start and 0 offset

[Marc Glisse]

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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: vbool.patch
Type: text/x-diff
Size: 1692 bytes
Desc: 
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20180915/d4464390/attachment.bin>