https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65149
Bug ID: 65149
Summary: unaligned atomic object causes Bus Error on SPARC
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
Following test case caused Bus Error on SPARC :
$ cat bus_error.cpp
#include <atomic>
#include <stdio.h>
typedef struct {
char c[8];
} struct8;
typedef struct {
char c[1];
std::atomic< struct8 > as;
} obj_t;
obj_t obj;
int main( void ) {
printf("\n sizeof(obj.as) %d alignof(obj.as) %d", sizeof(obj.as),
alignof(obj.as) );
printf("\n\n atomic_is_lock_free(&obj.as) %d
__atomic_always_lock_free(sizeof(obj.as), &(obj.as)) %d",
atomic_is_lock_free(&obj.as), __atomic_always_lock_free(sizeof(obj.as),
&(obj.as)), atomic_is_lock_free(&obj.as) );
printf("\n\n");
return obj.as.load().c[0];
}
$ g++ version is - 4.9.2 (GCC)
$ g++ bus_error.cpp -latomic -m32 -std=c++11
$ ./a.out
sizeof(obj.as) 8 alignof(obj.as) 1
atomic_is_lock_free(&obj.as) 1 __atomic_always_lock_free(sizeof(obj.as),
&(obj.as)) 0
Bus Error (core dumped) <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
The reason for the failure is that __atomic_always_lock_free() reported 0,
but compiler inserts lock-free implementation :
_ZNKSt6atomicI7struct8E4loadESt12memory_order:
.LLFB359:
save %sp, -104, %sp
.LLCFI4:
ld [%fp+64], %i5
st %i0, [%fp+68]
st %i1, [%fp+72]
ld [%fp+68], %g1
membar 2
ldd [%g1], %g2
std %g2, [%fp-8]
mov %i5, %g3
add %fp, -8, %g2
mov 8, %g1
mov %g3, %o0
mov %g2, %o1
mov %g1, %o2
call memcpy, 0
nop
nop
mov %i5, %i0
return %i7+12
nop
.LLFE359:
This lock-free implementation could not work correctly with unaligned data.
The bug was found on Solaris SPARC, but it could exist on other platforms.
