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From: "dhazeghi at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/57303] New: struct miscompiled at -O1 and above
Date: Thu, 16 May 2013 15:59:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 4.9.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: dhazeghi at yahoo dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57303

            Bug ID: 57303
           Summary: struct miscompiled at -O1 and above
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dhazeghi at yahoo dot com

With current gcc trunk, the code below is miscompiled at -O1 and higher
optimization levels on x86_64-linux-gnu, outputting '1' instead of '0' as
expected.  This behavior appears in gcc 4.8.x and 4.7.x as well, but not in
4.6.x or earlier.  It occurs in both 32-bit and 64-bit compiles.

$ gcc-trunk -v
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure
--enable-languages=c,c++,objc,obj-c++,fortran,lto --disable-checking
--with-gmp=/usr/local/gcc-trunk --with-mpfr=/usr/local/gcc-trunk
--with-mpc=/usr/local/gcc-trunk --with-cloog=/usr/local/gcc-trunk
--prefix=/usr/local/gcc-trunk
Thread model: posix
gcc version 4.9.0 20130516 (experimental) [trunk revision 198967] (GCC) 
$ gcc-trunk -O0 small.c
$ ./a.out 
0
$ gcc-4.6 -O1 small.c
$ ./a.out 
0
$ gcc-trunk -O1 small.c
$ ./a.out 
1
$
-----------------------------

int printf(const char *, ...);

struct S0
{
    int f0;
};
struct S1
{
    struct S0 f0;
};

struct S1 x = { {0} };
struct S1 y = { {1} };

static void
foo (struct S0 p)
{
    struct S0 *l = &y.f0;
    *l = x.f0;
    if (p.f0)
        *l = *l;
}

int
main ()
{
    foo(y.f0);
    printf("%d\n", y.f0.f0);
    return 0;
}