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From: "dartdart26 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/101923] New: std::function's move ctor is slower than
 the copy one for empty source objects
Date: Sun, 15 Aug 2021 13:17:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101923

            Bug ID: 101923
           Summary: std::function's move ctor is slower than the copy one
                    for empty source objects
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dartdart26 at gmail dot com
  Target Milestone: ---

std::function's move constructor calls swap() irrespective of whether the
source object is empty or not. In contrast, the copy constructor first chec=
ks
if the source object is empty and if it is, nothing is being done as the `t=
his`
object is constructed in an empty state by _Function_base().

Calling swap() on an empty source requires more work, because some data nee=
ds
to be copied - for example, the POD data cannot be moved.

Could the move constructor check if the source is empty too, as the copy one
does? Please let me know if I am missing a rule that prevents that.

I have noticed that on version 9.3.0, but I see the code is the same in cur=
rent
master at:
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dblob;f=3Dlibstdc%2B%2B-v3/include/=
bits/std_function.h;hb=3Dc22bcfd2f7dc9bb5ad394720f4a612327dc898ba#l391

I have tested on a MacBook M1 and the copy ctor for empty sources is almost=
 2x
faster than the move ctor:

-----------------------------------------------------
Benchmark           Time             CPU   Iterations
-----------------------------------------------------
copy            0.945 ns        0.945 ns    555789159
move             1.83 ns         1.83 ns    382183169

I have made an YouTube video for describing my findings and the benchmark
results:
https://www.youtube.com/watch?v=3DWA3mKab-tn8=