Re: [Boost-users] [Boost.Container] Performance of static_vector with default_init_t

Subject: Re: [Boost-users] [Boost.Container] Performance of static_vector with default_init_t
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2016-12-04 17:35:55

On 01/12/2016 14:09, dariomt_at_[hidden] wrote:
> Hi,
>
> I'm constructing a static_vector<double,N>(n, default_init).
>
> I was expecting the constructor call to vanish into just setting the
> start pointer and the size & capacity members (and verifying n<=N).
>
> However, the profiler is showing some time spent in the loop
> inside uninitialized_default_init_alloc_n
> (boost/container/detail/utilities.hpp:678)
>
> For a builtin type and default_init_t, why do we even need to invoke the
> allocator? Can't we just leave the memory uninitialized?
>
> This is with Boost 1.55 and VS2008
>
> Thanks in advance

It should be a no-op, but maybe it is not correctly inlined. With some 
compilers I still see some calls to uninitialized_default_init_alloc_n 
that disappear when the exception handling part is commented. It seems 
that the compiler can't simplify that function enough to make it a no-op.

Could you please try with a more recent Boost version to see if the 
problem has improved a bit from your version?

In any case please report a bug to see if something can be done, at 
least for POD types.

Ion