https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65145
Bug ID: 65145
Summary: size of atomic object is not correct
Product: gcc
Version: 4.9.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: alexey.lapshin at oracle dot com
The size of atomic object does not match with documentation -
https://gcc.gnu.org/wiki/Atomic/GCCMM/UnalignedPolicy specifically :
"GCC 4.9 will provide an atomic type attribute which can be set on objects.
This attribute will force a specific alignment and size on the object which may
be different than the original data type. The alignment and size will attempt
to provide lock free operations, if they exist. ... An object will be promoted
up to the next lock-free size in order to enable lock free operations, as long
as it isn't already a documented lock free size. "
~/atomic_test$ cat non_power_2_atomic.c
#include <stdatomic.h>
#include <stdio.h>
typedef struct {
char c [5];
} non_power_to_obj;
int main ( void ) {
non_power_to_obj obj;
_Atomic non_power_to_obj aobj;
printf("\n Size and Alignment usual object ");
printf(" : sizeof(obj) %d __alignof__(obj) %d ", sizeof(obj),
__alignof__(obj) );
printf("\n Size and Alignment of atomic object ");
printf(" : sizeof(aobj) %d __alignof__(aobj) %d \n", sizeof(aobj),
__alignof__(aobj) );
return 0;
}
~/atomic_test$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/opt/gcc/libexec/gcc/i386-pc-solaris2.11/4.9.2/lto-wrapper
Target: i386-pc-solaris2.11
Configured with: ./configure --prefix=/opt/gcc/
Thread model: posix
gcc version 4.9.2 (GCC)
~/atomic_test$ gcc -O -latomic -std=c11 non_power_2_atomic.c
~/atomic_test$ ./a.out
Size and Alignment usual object : sizeof(obj) 5 __alignof__(obj) 1
Size and Alignment of atomic object : sizeof(aobj) 5 __alignof__(aobj) 1
According to the https://gcc.gnu.org/wiki/Atomic/GCCMM/UnalignedPolicy
the size and alignment of "aobj" should be 8.
The bug us found on Solaris x86, but it could be on other platforms
also(SPARC/Linux). This bug is also exist in g++.
