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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/61082] [x86-64 Itanium ABI] g++ uses wrong return location for class with head padding
Date: Wed, 07 May 2014 10:27:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61082

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
extern "C" int printf(const char*, ...);

struct B { };

struct X : public B { long p; };

struct Y : public B { X q; };

Y foo( Y& y )
{
    Y temp;
    temp = y;
    return temp;
}

int main( void )
{
    printf("%zd %zd\n", sizeof( X ), sizeof( Y ));

    Y y;
    y.q.p = 6L;

    Y yy = foo( y );
    printf("%ld\n", yy.q.p);
}

Clang uses the same registers as GCC:

    .globl    _Z3fooR1Y
    .align    16, 0x90
    .type    _Z3fooR1Y,@function
_Z3fooR1Y:                              # @_Z3fooR1Y
    .cfi_startproc
# BB#0:                                 # %entry
    movq    8(%rdi), %rax
    retq
.Ltmp0:
    .size    _Z3fooR1Y, .Ltmp0-_Z3fooR1Y
    .cfi_endproc