https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65449
Bug ID: 65449
Summary: -fstrict-volatile-bitfields affects volatile pointer
dereference and produce wrong codes
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: ma.jiang at zte dot com.cn
Hi,all.
It seems that -fstrict-volatile-bitfields can affect volatile pointer
dereference. However, the gcc manual said this option should only affect
accesse to bit-fields or structure fields.
Compiling the test case:
char mt[20];
void main()
{
void *mm=&(mt[1]);
*((volatile int *)mm)=4;
}
with -O2 -mstrict-align -fstrict-volatile-bitfields on PPC. We can see that
"*((volatile int *)mm)=4 " is done by a single stw. Beware that -mstrict-align
means a non-aligned memory access is disallowed, and &(mt[1]) is obviously not
a address aligned to 4-bytes boundary. The compiler should have no reasons to
produce a unaligned stw when mstric-align is on.
Further more,compiling with -O2 -mstrict-align -fno-strict-volatile-bitfields,
the compiler will produce four lbz/stb pairs for "*((volatile int *)mm)=4;".
This is ridiculous as the C standard does not require the read, and surely no
performance benefits could grain from these lbz.
