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From: "albin at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug other/102942] New: Implicit copy constructor not elided
Date: Tue, 26 Oct 2021 08:49:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102942

            Bug ID: 102942
           Summary: Implicit copy constructor not elided
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: albin at yahoo dot com
  Target Milestone: ---

This is seen on every GCC version. The copy-constructor for a class with a =
data
buffer only gets elided if the copy-constructor is user provided. So if I h=
ave
a copy-constructor that explicitly calls memcpy, the compiler will elide it
when it can (and thus NOT call memcpy), but if I do not declare any such
constructor, the compiler *will* call mempcy even if the call could be elid=
ed.

https://godbolt.org/z/5YvWTE847

#include <cstring>

struct StructWithImplicitCpyCtr
{
    StructWithImplicitCpyCtr();
    float data[5000];
};

struct StructWithExplicitMemCpyCpyCtr
{
    StructWithExplicitMemCpyCpyCtr();
    StructWithExplicitMemCpyCpyCtr(StructWithExplicitMemCpyCpyCtr const& ot=
her)
    { std::memcpy(data, other.data, sizeof(data)); }
    float data[5000];
};

void take(StructWithImplicitCpyCtr);
void take(StructWithExplicitMemCpyCpyCtr);

void foobar()
{
    // The copy constructor explicitly calls memcpy, but copy elision remov=
es
it
    take(StructWithExplicitMemCpyCpyCtr());

    // Here there is no explicit copy constructor to elide, we should expec=
t to
see
    // similar code generated, instead we see a call to memcpy!!!
    take(StructWithImplicitCpyCtr());
}=