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From: "maxim.yegorushkin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/94184] New: Global variable inline constructor elision.
Date: Sun, 15 Mar 2020 19:36:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94184

            Bug ID: 94184
           Summary: Global variable inline constructor elision.
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: maxim.yegorushkin at gmail dot com
  Target Milestone: ---

The following code:

    struct A {=20
        int a_;=20
        A(int a) : a_(a) {}
    };

    struct B {=20
        int a_;=20
        constexpr B(int a) : a_(a) {}
    };

    A a{1};
    B b{1};

When compiled with `gcc-9.2 -O3 -march=3Dskylake -mtune=3Dskylake -std=3Dgn=
u++17`
generates a call to `A` constructor for variable `a`:

    _GLOBAL__sub_I_a:
        mov    DWORD PTR [rip+0x200bd6],0x1        # 601030 <__TMC_END__>
        ret=20=20=20=20

And no call for `B` constructor for variable `b`, as expected of `constexpr=
`.

`clang-9.0 -O3 -march=3Dskylake -mtune=3Dskylake -std=3Dgnu++17`, however, =
elides the
constructor calls for both `a` and `b` global variables, regardless of
`constexpr`.

Why can't `gcc` elide the call to `A` constructor for `a`, since `A` constu=
ctor
is an inline function with a clear side-effect?

https://gcc.godbolt.org/z/5y3cD5=