[llvm-bugs] [Bug 25670] New: [Feature] Add a builtin for indexing into parameter packs

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25670

            Bug ID: 25670
           Summary: [Feature] Add a builtin for indexing into parameter
                    packs
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ldionne.2 at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15363
  --> https://llvm.org/bugs/attachment.cgi?id=15363&action=edit
Initial version of the proposed patch.

This patch 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.

Louis


[1]: http://ldionne.com/2015/11/29/efficient-parameter-pack-indexing

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