[ADT] Add `llvm::range_size` function for generic ranges

[kuhar]

Summary

This function follows std::ranges::size from C++20. It is intended
mainly for generic code that does not know the exact range type.
I did not modify the existing llvm::size function because it has a strict
guarantee of O(1) running time, and we cannot guarantee that when we delegate
size check to user-defined size functions.

Use range_size to optimize size checks in zip* and enumerate
functions. Before that, we would have to perform linear scans for ranges
without random access iterators.

This is the last change I have planned in the series that overhauls
zip* and enumerate.

[dblaikie]

Would std::distance suffice?

[kuhar]

In D146231#4199846, @dblaikie wrote:

Would std::distance suffice?

It does work and this is what we use in the assertions now. The issue is that this degrades to a linear scan over some iterator types (e.g., with sets/maps)

[dblaikie]

llvm/include/llvm/ADT/STLExtras.h

1786–1800

Rather than detecting both member and non-member, could we standardize on non-member, like adl_begin/adl_end works? Could expose adl_size to match adl_begin/end too, in addition to this wrapper that has the fallback to distance.

1809–1810

I'd guess this can be trivially optimized from the generic std::distance call? Maybe not worth the special case?

[kuhar]

llvm/include/llvm/ADT/STLExtras.h

1786–1800

Good idea. I wonder why std::ranges::size doesn't do it too.

1809–1810

Once we go through std::size it should cover it too. This will make this diverge from std::ranges::size but I don't think anyone will notice the difference, and our implementation will become simpler.

[kuhar]

Use adl_size to simplify the implementation and reduce branching.

[zero9178]

LGTM with tiny note

llvm/include/llvm/ADT/STLExtras.h

2424–2425

Guessing you still need to rebase this ontop of https://reviews.llvm.org/D146340?
Does that happen to also fix the compiler bug from that revision or will we still require the workaround using the intializer list?

[kuhar]

llvm/include/llvm/ADT/STLExtras.h

2424–2425

Yes, this needs rebasing. I've just checked and the workaround is still necessary.

[kuhar]

Rebase

[dblaikie]

Looks good, thanks!