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From: "pascal_cuoq at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/109956] New: GCC reserves 9 bytes for struct s { int a; char
 b; char t[]; } x = {1, 2, 3};
Date: Wed, 24 May 2023 16:09:33 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 14.0
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X-Bugzilla-Who: pascal_cuoq at hotmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109956

            Bug ID: 109956
           Summary: GCC reserves 9 bytes for struct s { int a; char b;
                    char t[]; } x =3D {1, 2, 3};
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pascal_cuoq at hotmail dot com
  Target Milestone: ---

Static-lifetime variables of type =E2=80=9Cstruct with FAM=E2=80=9D (flexib=
le array member)
with an initializer for the FAM are a GCC extension.

As of GCC 13.1 and Compiler Explorer =E2=80=9Ctrunk=E2=80=9D, targeting x86=
, the definition
=E2=80=9Cstruct s { int a; char b; char t[]; } x =3D {1, 2, 3};=E2=80=9D re=
serves 9 bytes for x,
and in fact, with various initializers, the trailing padding for variables =
of
type =E2=80=9Cstruct s=E2=80=9D is always 3, as if the size to reserve for =
the variable was
computed as =E2=80=9Csizeof (struct s) + n * sizeof(element)=E2=80=9D.

Input file:
struct s { int a; char b; char t[]; } x =3D {1, 2, 3};

Command:
gcc -S fam_init.c

Result (with Ubuntu 9.4.0-1ubuntu1~20.04.1 which exhibits the same behavior=
 as
the recent versions on Compiler Explorer):
        .align 8
        .type   x, @object
        .size   x, 9
x:
        .long   1
        .byte   2
        .byte   3
        .zero   3


Clang up to version 14 used to round up the size of the variable to a multi=
ple
of the alignment of the struct, but even this is not necessary. It is only
necessary that the size reserved for a variable of type t is at least
=E2=80=9Csizeof(t)=E2=80=9D bytes, and also to reserve enough space for the=
 initializer. Clang
15 and later uses the optimal formula:

max(sizeof (struct s), offsetof(struct s, t[n]))

Compiler Explorer link: https://gcc.godbolt.org/z/5W7h4KWT1

This ticket is to suggest that GCC uses the same optimal formula as Clang 15
and later.=