From 465520e3eb45d83ad18394aa537150bfa6bdf117 Mon Sep 17 00:00:00 2001 From: Marc Glisse Date: Fri, 19 Jun 2020 13:03:45 +0100 Subject: [PATCH] libstdc++: std::includes performance tweak A small tweak to the implementation of __includes, which in my application saves 20% of the running time. I noticed it because using range-v3 was giving unexpected performance gains. Some of the gain comes from pulling the 2 calls ++__first1 out of the condition so there is just one call. And most of the gain comes from replacing the resulting if (__comp(__first1, __first2)) ; else ++__first2; with if (!__comp(__first1, __first2)) ++__first2; I was very surprised that the code ended up being so different for such a change, and I still don't really understand where the extra time is going... Anyway, while I blame the compiler for not generating very good code with the current implementation, I believe the change can be seen as a simplification. libstdc++-v3/ChangeLog: * include/bits/stl_algo.h (__includes): Simplify the code. --- libstdc++-v3/include/bits/stl_algo.h | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h index fd6edd0d5f4..550a15f2b3b 100644 --- a/libstdc++-v3/include/bits/stl_algo.h +++ b/libstdc++-v3/include/bits/stl_algo.h @@ -2783,15 +2783,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION _Compare __comp) { while (__first1 != __last1 && __first2 != __last2) - if (__comp(__first2, __first1)) - return false; - else if (__comp(__first1, __first2)) - ++__first1; - else - { - ++__first1; + { + if (__comp(__first2, __first1)) + return false; + if (!__comp(__first1, __first2)) ++__first2; - } + ++__first1; + } return __first2 == __last2; } -- 2.30.2