[PATCH] Add a builtin for indexing into parameter packs

[David Majnemer via cfe-commits]

Would you mind uploading the clang-side of this patch to phabricator? It
will make it a little easier for me to perform a review.  See
http://llvm.org/docs/Phabricator.html for more details.

On Mon, Nov 30, 2015 at 9:13 AM, Louis Dionne via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> Hi,
>
> I wrote a patch that adds a `__nth_element` builtin that allows fetching
> the
> n-th type of a parameter pack with very little compile-time overhead. The
> patch
> was inspired by r252036 and r252115 by David Majnemer, which add a similar
> `__make_integer_seq` builtin for efficiently creating a
> `std::integer_sequence`.
> At its core, the builtin makes the following specialization
>
> __nth_element<i, T1, ..., Tn>
>
> equivalent to Ti. Further analysis of the problem this patch is trying to
> solve
> is available on a blog post I just wrote [1]. The post also present
> benchmarks
> showing the compile-time improvement over existing techniques.
>
> The patch is currently in a very rough state. It is my first time
> contributing
> to Clang, and I encountered some issues that I hacked around just to get a
> working solution. If there is interest in this patch, I will gladly improve
> it until it is in an acceptable state, but I will most likely need some
> pointers to do so. Things that are hacky right now:
>
> 1. I wasn't able to create a non-type template parameter from a builtin
> type.
>    Hence, `__nth_element` must currently be used as
>      __nth_element<TypeOfTheIndex, Index, Ts...>
>    instead of just `__nth_element<Index, Ts...>`, while forcing `Index` to
> be
>    something like `unsigned long`. This way, I was able to copy Majnemer's
>    code that creates non-type template parameters. This is probably trivial
>    to change, but I couldn't figure it out immediately.
>
> 2. I wasn't sure how to retrieve an actual integer from an `APSInt`, so
> right
>    now I use `getExtValue()`. This is probably wrong, as I didn't really
>    understand what I was doing.
>
> There are certainly other issues that a code review would address, if we
> make
> it that far. Also, the name `__nth_element` is badly chosen, since it seems
> to refer to the `nth_element` algorithm of the standard library. We should
> settle on something else, but I have no preference.
>
> I also wrote a corresponding patch for libc++ to use the builtin to
> implement
> `std::tuple_element`. You can find both patches attached to this message.
> Also,
> these patches were initially posted as bug reports [2] and [3],
> respectively.
>
> Please let me know if there is any interest for going forward with these
> patches.
>
> Regards,
> Louis Dionne
>
> [1]: http://ldionne.com/2015/11/29/efficient-parameter-pack-indexing
> [2]: https://llvm.org/bugs/show_bug.cgi?id=25670
> [3]: https://llvm.org/bugs/show_bug.cgi?id=25671
>
>
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