Re: [boost] [MPL][vector] Is there interest in mpl::vector using variadic templates?

On 03/02/2017 01:54 PM, Bruno Dutra via Boost wrote:

On Thu, Mar 2, 2017 at 7:42 PM, Larry Evans via Boost

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wrote:

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On 03/02/2017 11:54 AM, Larry Evans via Boost wrote:

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On 03/02/2017 11:27 AM, Peter Dimov via Boost wrote:

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Larry Evans wrote:

One problem with the above cppljevans mpl is there's no at.hpp.

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Instead, the non-variadic boost/mpl/at.hpp was used. The reason no variadic at was created was because, AFAICT, there was no non-recursive method for picking the I-th element from T... , and, IIUC, recursive templates cause compile-time slow downs.

Have you read

http://pdimov.com/cpp2/simple_cxx11_metaprogramming_2.html

?

Nope. Thanks *very much* for the link. I'm impressed (especially with the way you actually cited the parts of the standard to guide your search for the best method!).

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Search for mp_at.

Thanks for that tip.

I'm a bit surprised that the mp_map_from_list was fastest. I would have thought that large template classes mean slow compile times, but I guess not.

Since you seem to be interested on benchmarking compilation times, check out http://metaben.ch.

The at benchmark there indicates mpl.vector is the fastest in the, I guess you'd call it, the "performance at given number_of_elements pop-up list", when the cursor is moved over to the rhs at number_of_elements=500. Hmmm. Now I see there's actually no mpl.vector graph at that number_of_elements. OOPS. I see that that's because mpl.vector was not run for number_of_elements > 50, but that's only obvious when the subtract_baseline radio button is turned-off. To avoid this confusion, the graph should give some indication, in the "performance at given number_of_elements pop-up list", which methods were actually run at the given number_of_elements.

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This:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4235.htm

claims:

template instantiations use memory that is never freed during the compilation process

so, maybe mp_map_from_list should be used with caution. If many mp_map_from_list instantiations are used, I'm guessing the compiler might become pressed for memory. Maybe a benchmark showing compile time vs number of instantiations would show this. For example, instead of N=800, see what happens when 2 instantiations with N=400 happen, and compare with a similar test for mp_repeat_c.

This is actually not the case, in fact the exact opposite is true.

So n4235.htm was wrong or maybe I've misinterpreted it?

It turns out mp_map_from_list is actually very cheap to the compiler in terms of memory allocation, because it takes advantage of template memoization, so if you retrieve every single element of a list using this trick, the compiler needs to instantiate it only once. On the other hand, the trick employing void pointers can't be expressed in a way that take advantage of memoization, which means that, for every element retrieved from the list, the entire pack of void pointers must be instantiated again.

So how do you resolve what n4235 is saying and what you say above? I'm guessing that 4235 was actually referring to this memoization when it said: template instantiations use memory that is never freed The memoizations do use memory which, IIUC, is never freed until the compiler is finished. OTOH, when you say:

for every element retrieved from the list, the entire pack of void pointers must be instantiated again.

That means more compile time is needed for the repeated instantiations; however, no memory is *permanently* used as is the case for memoization. IOW, this is a classic case of trading memory for speed: https://en.wikipedia.org/wiki/Space%E2%80%93time_tradeoff OTOH, my point was that as these memoization's increase, the compiler has less memory to use; hence, might slow down; whereas, since, the memory used by the vec_of_void* method is reused, memory pressure is less, and, although it's slower for 1 instance of a pack, it might become faster as many packs are used. I hope that makes more sense ;) -regards, Larry [snip]