[Bug target/50235] New: Wrong code with volatile bitfields and -Os

["arthur.j.odwyer at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50235

             Bug #: 50235
           Summary: Wrong code with volatile bitfields and -Os
    Classification: Unclassified
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: arthur.j.odwyer@gmail.com


Created attachment 25137
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25137
Output of "ajo-gcc -w -Os test.c -v"

This failure reproduces for me with svn revision 178264 (2011-08-29), but it
goes back at least as far as gcc-4.5. I'm on Ubuntu 10.10, x86-64.

cat >test.c <<EOF
struct S1 {
   unsigned f0, f1;
   unsigned short f2, f3;
   unsigned f4 : 16;
   unsigned f5, f6;
   volatile unsigned f7 : 28;
};
static struct S1 g_76;
static struct S1 g_245 = {0,0,0,0,0,0,0,1};
static signed char g_323 = 0x80;
static void func_1(void) {
    g_245.f7 &= 1;
    for (g_323 = 0; g_323 <= -1; g_323 -= 2) {
        g_76 = g_76;
        g_76.f4 ^= 11;
    }
}
int main() {
    func_1();
    printf("%x %x\n", (unsigned)g_323, (unsigned)g_245.f7);
}
EOF
gcc -w -O3 test.c ; ./a.out
gcc -w -Os test.c ; ./a.out

With -O3, it correctly prints "0 1".  With -Os, it incorrectly prints "7e 1".
In trunk, -fstrict-volatile-bitfields fixes the bug --- but I don't see
anything implementation-defined about this test case. It shouldn't need any
special options to compile correctly.


This test case is reduced from the output of Csmith 2.1.0 (git hash 01aa8b04,
https://github.com/Quuxplusone/csmith/), using the following command line:
csmith --no-paranoid --no-longlong --no-pointers --arrays --jumps --no-consts
--volatiles --checksum --no-divs --no-muls --bitfields --no-packed-struct -s
1163880864