Re: Default std::vector<bool> default and move constructor

[Marc Glisse <marc dot glisse at inria dot fr>]

On Mon, 15 May 2017, François Dumont wrote:

>    I also added some optimizations. Especially replacement of std::fill with 
> calls to __builtin_memset. Has anyone ever proposed to optimize std::fill in 
> such a way ? It would require a test on the value used to fill the range but 
> it might worth this additional runtime check, no ?

Note that with -O3, gcc recognizes the pattern in std::fill and generates 
a call to memset (there is a bit too much extra code around the memset, 
but a couple match.pd transformations should fix that). That doesn't mean 
we can't save it the work. If you want to save the runtime check, there is 
always __builtin_constant_p...

The __fill_bvector part of the fill overload for vector<bool> could do 
with some improvements as well. Looping is unnecessary, one just needs to 
produce the right mask and and or or with it, that shouldn't take more 
than 4 instructions or so.

There was a time when I suggested overloading std::count and std::find in 
order to use __builtin_popcount, etc. But from what I've seen of committee 
discussions, I expect that there will be specialized algorithms (possibly 
member functions) eventually, making the overload less useful.

--
Marc Glisse