[gcc/devel/c++-modules] libstdc++: std::includes performance tweak

[gcc/devel/c++-modules] libstdc++: std::includes performance tweak

[Nathan Sidwell]

https://gcc.gnu.org/g:465520e3eb45d83ad18394aa537150bfa6bdf117

commit 465520e3eb45d83ad18394aa537150bfa6bdf117
Author: Marc Glisse <marc.glisse@inria.fr>
Date:   Fri Jun 19 13:03:45 2020 +0100

    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.

Diff:
---
 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;
     }