https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70309
Bug ID: 70309
Summary: wrong code at -O2 and -O3 in both 32-bit and 64-bit
modes on x86_64-linux-gnu
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: chengniansun at gmail dot com
Target Milestone: ---
The following code is miscompiled by the trunk at -O2 and -O3 in both 32-bit
and 64-bit modes on x86_64-linux-gnu.
This bug also affects gcc-4.8 and later versions. gcc-4.7 is okay.
$: gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-source-trunk/configure --enable-languages=c,c++,lto
--prefix=/usr/local/gcc-trunk --disable-bootstrap
Thread model: posix
gcc version 6.0.0 20160319 (experimental) [trunk revision 234350] (GCC)
$:
$: gcc-trunk -O2 small.c ; ./a.out ; echo $?
1
$: gcc-trunk -O1 small.c ; ./a.out ; echo $?
0
$: gcc-4.8 -O3 small.c ; ./a.out ; echo $?
1
$: cat small.c
typedef struct {
unsigned b0:1;
unsigned b1:1;
unsigned b2:1;
unsigned b3:1;
unsigned b4:1;
unsigned b5:1;
unsigned b6:1;
unsigned b7:1;
unsigned b8:1;
unsigned b9:1;
unsigned b10:1;
unsigned b11:1;
unsigned b12:1;
unsigned b13:1;
unsigned b14:1;
unsigned b15:1;
} BitField;
BitField BA;
unsigned short *pA = (unsigned short *) &BA;
BitField BB;
unsigned short *pB = (unsigned short *) &BB;
char * fn(void)
{
if (BA.b11) {
if (BB.b11)
return "F";
else
return "S";
} else
return "F";
}
int
main (void)
{
*pA = 0xedcb;
*pB = 0x1234;
return fn ()[0] == 'F';
}
$:
