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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