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From: "cyp561 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/51392] New: Wrong code with -Os when __attribute__((__const__)) function returns structure
Date: Sat, 03 Dec 2011 10:09:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51392

             Bug #: 51392
           Summary: Wrong code with -Os when __attribute__((__const__))
                    function returns structure
    Classification: Unclassified
           Product: gcc
           Version: 4.5.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: cyp561@gmail.com
            Target: x86_64-pc-linux-gnu
             Build: gcc version 4.5.3 (Gentoo 4.5.3-r1 p1.0, pie-0.4.5)


Fails with -Os, works with -O2:
4.5.3
Works always:
4.1.2  4.2.4  4.3.6  4.4.6
Fails always:
3.4.6

Only fails if the copy constructor is manually defined.
Works with __attribute__((__pure__)) instead of __attribute__((__const__)).
Still fails with printf and #include <stdio.h> removed.

Maybe it's somehow illegal with __attribute__((__const__)) to return a
structure with a manually defined copy constructor?



Testcase, reduced as much as I could:
/*

$ g++ -O2 -o error error.cpp
$ ./error
(-11 1 4)
$ g++ -Os -o error error.cpp
$ ./error
(1790250 1127603896 -1465067344)
Aborted
$ ./error
(1801140 -1464045896 574146144)
Aborted

*/

extern "C" void abort();

struct Vector3f
{
    Vector3f() {}
    Vector3f(int x, int y, int z) : x(x), y(y), z(z) {}
    Vector3f(Vector3f const &v) : x(v.x), y(v.y), z(v.z) {}  // Essential

    int x, y, z;
};

__attribute__((__const__))
Vector3f pie_SurfaceNormal3fv(Vector3f a, Vector3f b)
{
    return Vector3f(a.y*b.z - a.z*b.y, a.z*b.x - a.x*b.z, a.x*b.y - a.y*b.x);
}

void normalsOnTile(Vector3f *normals)
{
    Vector3f b(3, 5, 7);
    Vector3f c(4, 8, 9);

    *normals = pie_SurfaceNormal3fv(b, c);
}

#include <stdio.h>

int main()
{
    Vector3f normals;
    normalsOnTile(&normals);
    printf("(%d %d %d)\n", normals.x, normals.y, normals.z);
    if (normals.x != -11)
        abort();
}