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: unassig...@gcc.gnu.org
ReportedBy: arthur.j.odw...@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
