From 9b4f00dd3f799337d8b8ef5e79f5a682c8059ab9 Mon Sep 17 00:00:00 2001 From: Jonathan Wakely Date: Tue, 3 Mar 2020 11:06:26 +0000 Subject: [PATCH] libstdc++: Micro-optimisations for lexicographical_compare_three_way As noted in LWG 3410 the specification in the C++20 draft performs more iterator comparisons than necessary when the end of either range is reached. Our implementation followed that specification. This removes the redundant comparisons so that we do no unnecessary work as soon as we find that we've reached the end of either range. The odd-looking return statement is because it generates better code than the original version that copied the global constants. * include/bits/stl_algobase.h (lexicographical_compare_three_way): Avoid redundant iterator comparisons (LWG 3410). --- libstdc++-v3/ChangeLog | 5 +++++ libstdc++-v3/include/bits/stl_algobase.h | 7 ++++--- 2 files changed, 9 insertions(+), 3 deletions(-) diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog index 28957dc..b8cf579 100644 --- a/libstdc++-v3/ChangeLog +++ b/libstdc++-v3/ChangeLog @@ -1,3 +1,8 @@ +2020-03-03 Jonathan Wakely + + * include/bits/stl_algobase.h (lexicographical_compare_three_way): + Avoid redundant iterator comparisons (LWG 3410). + 2020-03-02 Jonathan Wakely PR libstdc++/93972 diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h index 7a9d932..4b63086 100644 --- a/libstdc++-v3/include/bits/stl_algobase.h +++ b/libstdc++-v3/include/bits/stl_algobase.h @@ -1711,15 +1711,16 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO return __lencmp; } #endif // is_constant_evaluated - while (__first1 != __last1 && __first2 != __last2) + while (__first1 != __last1) { + if (__first2 == __last2) + return strong_ordering::greater; if (auto __cmp = __comp(*__first1, *__first2); __cmp != 0) return __cmp; ++__first1; ++__first2; } - return __first1 != __last1 ? strong_ordering::greater - : __first2 != __last2 ? strong_ordering::less : strong_ordering::equal; + return (__first2 == __last2) <=> true; // See PR 94006 } template -- 2.7.4