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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/100104] New: std::transform is 1.5 times faster than
 std::copy with -O3
Date: Thu, 15 Apr 2021 18:06:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100104

            Bug ID: 100104
           Summary: std::transform is 1.5 times faster than std::copy with
                    -O3
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

Consider:

std::vector<double> v1(100, 0);

// copy using copy
std::vector<int> v2;
std::copy(v1.begin(), v1.end(), std::back_inserter(v2));

// copy using transform
std::vector<int> v2;
std::transform(v1.begin(), v1.end(), std::back_inserter(v2), [](auto x) {
return x; });

Those two will generate similar assembly code under -O2, but is very differ=
ent
under -O3/-Ofast, and transform will be 1.5 times faster than the copy. I d=
on=E2=80=99t
know if this is a bug since these two represent the same thing, and correct=
 me
if I am wrong.

quick-bench with -O2:
https://quick-bench.com/q/uKT8QEmPkS1wr153s3P-DRt90eY
quick-bench with -O3:
https://quick-bench.com/q/syuBCQYVtCoVwT2MRtLT25P-MQI
goldbot:=20
https://godbolt.org/z/7ee77cs8W=