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Re: [PATCH] Micro-optimization to avoid creating temporary path


On Thu, Dec 20, 2018 at 1:12 AM François Dumont <frs.dumont@gmail.com> wrote:
>
> On 12/18/18 5:38 PM, Jonathan Wakely wrote:
> > On 18/12/18 15:52 +0000, Jonathan Wakely wrote:
> >> Now that path::operator/=(basic_string_view<value_type>) works directly
> >> from the string argument, instead of constructing a temporary path from
> >> the string, it's potentially more efficient to do 'path(x) /= s' instead
> >> of 'x / s'. This changes the only relevant place in the library.
> >>
> >>     * src/filesystem/std-dir.cc (filesystem::_Dir::advance): Append
> >>     string to lvalue to avoid creating temporary path.
> >
> > This is only an optimization if it doesn't introduce a new copy! Fixed
> > by the attached patch.
> >
> > Tested x86_64-linux, committed to trunk.
> >
> >
> Isn't it just equivalent to the original code:
>
> -       entry = fs::directory_entry{path / entp->d_name, get_file_type(*entp)};
>
> The temporary un-named instance was already moved, no ?

The saving is from avoiding converting entp->d_name to a path
(operator/ is only defined for when both sides are paths).


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