tree ad521e3f3ac7d1ec2be2d07036952b89433195c1
parent c651781d2fd64e5b69d7412728b701327ebe2d0d
author Adrian Vogelsgesang <avogelsgesang@tableau.com> 1659651687 -0700
committer Copybara-Service <copybara-worker@google.com> 1676242561 -0800

[libc++][spaceship] Implement `lexicographical_compare_three_way`

The implementation makes use of the freedom added by LWG 3410. We have
two variants of this algorithm:
* a fast path for random access iterators: This fast path computes the
  maximum number of loop iterations up-front and does not compare the
  iterators against their limits on every loop iteration.
* A basic implementation for all other iterators: This implementation
  compares the iterators against their limits in every loop iteration.
  However, it still takes advantage of the freedom added by LWG 3410 to
  avoid unnecessary additional iterator comparisons, as originally
  specified by P1614R2.

https://godbolt.org/z/7xbMEen5e shows the benefit of the fast path:
The hot loop generated of `lexicographical_compare_three_way3` is
more tight than for `lexicographical_compare_three_way1`. The added
benchmark illustrates how this leads to a 30% - 50% performance
improvement on integer vectors.

Implements part of P1614R2 "The Mothership has Landed"

Fixes LWG 3410 and LWG 3350

Differential Revision: https://reviews.llvm.org/D131395

GitOrigin-RevId: 2a06757a200cc8dd4c3aeca98509d50d75bb4a27