Re: [boost] [GSoC][MPL11] Post C++Now update

Subject: Re: [boost] [GSoC][MPL11] Post C++Now update
From: Larry Evans (cppljevans_at_[hidden])
Date: 2014-05-31 17:54:18

On 05/20/14 15:48, Zach Laine wrote:
> I've seen function template vs. metafunction template instantiations
> profiled before, and I did it myself a few years ago.  Function templates
> have been slower in every profile I've seen.  In my profilings, they were
> ~20% slower.
>
> The linked article appears to get better results not by using constexpr
> alone, but by using it to do numeric computations that allow it to reduce
> template instantiations.

AFAICT, the number of template function instantiations in the linked
article are the same as the number of template class instantiations.

When I run the attached, 1st with:

   defined(CONSTEXPR)

then with:

  !defined(COnSTEXPR)

the same values appear in the:

   (yesno,recurs)

output, indicating the same number of instantiations.

Am I missing something?

BTW, I've asked a similar question on the linked page.

> When the instantiations are close to the same in
> number, constexpr is slower.  In your attachment, it appears that you are
> iterating over types, and so constexpr and metafunction approaches should
> involve the same number of template instantiations.
>
> Zach
>
>
>
> On Tue, May 20, 2014 at 3:20 PM, Larry Evans <cppljevans_at_[hidden]>wrote:
[snip]

>> According to:
>>
>> http://cpptruths.blogspot.com/2011/07/want-speed-use-constexpr-meta.html
>>
>> template function recursion *should* compile faster than template
>> metafunction recursion.
[snip]