https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100366
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |msebor at gcc dot gnu.org
--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning is issued during expansion of the memcpy call. It seems fishy that
although it mentions __builtin_memcpy it points to __builtin_memmove:
error: ‘void* __builtin_memcpy(void*, const void*, long unsigned int)’ writing
1 or more bytes into a region of size 0 overflows the destination
[-Werror=stringop-overflow=]
431 | __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
| ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The IL looks like the warning is justified:
void func (struct vector & vec)
{
const ptrdiff_t _Num;
char * _6;
char * _7;
char * prephitmp_16;
char * _21;
long int _23;
long unsigned int _24;
sizetype prephitmp_31;
char * _36;
char * _38;
char * prephitmp_42;
char * _50;
long unsigned int _Num.10_52;
char * _54;
char * _61;
char * pretmp_67;
long int _69;
char * pretmp_70;
long int _71;
unsigned int pretmp_72;
unsigned int _89;
char * _90;
char * _98;
char * _101;
long unsigned int _112;
char * _116;
long unsigned int _144;
long int _146;
char * _147;
char * _155;
sizetype _157;
char * _158;
<bb 2> [local count: 1073741824]:
_6 = vec_2(D)->D.33449._M_impl.D.32762._M_start;
_7 = vec_2(D)->D.33449._M_impl.D.32762._M_finish;
if (_6 != _7)
goto <bb 4>; [70.00%]
else
goto <bb 3>; [30.00%]
<bb 3> [local count: 322122544]:
_21 = vec_2(D)->D.33449._M_impl.D.32762._M_end_of_storage;
_23 = _21 - _7;
_24 = (long unsigned int) _23;
if (_24 > 3)
goto <bb 5>; [67.00%]
else
goto <bb 13>; [33.00%]
<bb 4> [local count: 751619281]:
vec_2(D)->D.33449._M_impl.D.32762._M_finish = _6;
_147 = vec_2(D)->D.33449._M_impl.D.32762._M_end_of_storage;
_146 = _147 - _6;
_144 = (long unsigned int) _146;
if (_144 > 3)
goto <bb 5>; [67.00%]
else
goto <bb 13>; [33.00%]
<bb 5> [local count: 237404317]:
# prephitmp_16 = PHI <_7(3), _6(4)>
_89 = MEM <unsigned int> [(char * {ref-all})&UTC];
MEM <unsigned int> [(char * {ref-all})prephitmp_16] = _89;
_36 = vec_2(D)->D.33449._M_impl.D.32762._M_finish;
_38 = _36 + 4;
vec_2(D)->D.33449._M_impl.D.32762._M_finish = _38;
goto <bb 12>; [100.00%]
<bb 6> [local count: 108584968]:
__builtin_memmove (_90, pretmp_67, _157);
MEM <unsigned int> [(char * {ref-all})_155] = pretmp_72;
_116 = vec_2(D)->D.33449._M_impl.D.32762._M_finish;
_Num_117 = _116 - prephitmp_42;
if (_Num_117 != 0)
goto <bb 8>; [33.00%] >>> _Num_117 != 0
else
goto <bb 10>; [67.00%]
<bb 7> [local count: 220460391]:
MEM <unsigned int> [(char * {ref-all})_155] = pretmp_72;
_50 = vec_2(D)->D.33449._M_impl.D.32762._M_finish;
_Num_51 = _50 - prephitmp_42;
if (_Num_51 != 0)
goto <bb 8>; [33.00%] >>> _Num_51 != 0
else
goto <bb 9>; [67.00%]
<bb 8> [local count: 108584968]:
# _Num_119 = PHI <_Num_51(7), _Num_117(6)> <<< _Num_119 != 0
_Num.10_52 = (long unsigned int) _Num_119;
__builtin_memcpy (_61, prephitmp_42, _Num.10_52); <<< -Wstringop-overflow
<bb 9> [local count: 256293430]:
# prephitmp_31 = PHI <0(7), _Num.10_52(8)>
_54 = _61 + prephitmp_31;
if (pretmp_67 != 0B)
goto <bb 10>; [40.26%]
else
goto <bb 11>; [59.74%]
<bb 10> [local count: 175940553]:
# _98 = PHI <_54(9), _61(6)>
_71 = pretmp_70 - pretmp_67;
_112 = (long unsigned int) _71;
operator delete (pretmp_67, _112);
<bb 11> [local count: 329045359]:
# _101 = PHI <_54(9), _98(10)>
vec_2(D)->D.33449._M_impl.D.32762._M_start = _90;
vec_2(D)->D.33449._M_impl.D.32762._M_finish = _101;
vec_2(D)->D.33449._M_impl.D.32762._M_end_of_storage = _158;
<bb 12> [local count: 1048452384]:
return;
<bb 13> [local count: 230225493]:
# prephitmp_42 = PHI <_6(4), _7(3)>
_90 = operator new (4); <<< _90 is 4 bytes
pretmp_67 = vec_2(D)->D.33449._M_impl.D.32762._M_start;
_69 = prephitmp_42 - pretmp_67;
_157 = (sizetype) _69;
_158 = _90 + 4;
pretmp_70 = vec_2(D)->D.33449._M_impl.D.32762._M_end_of_storage;
_155 = _90 + _157; <<< _155 is in [_90, _90
+ 4]
_61 = _155 + 4; <<< _61 = _90 + 4
pretmp_72 = MEM <unsigned int> [(char * {ref-all})&UTC];
if (_69 != 0)
goto <bb 6>; [58.89%]
else
goto <bb 7>; [41.11%]
}
In the translation unit the warning is triggered by a call to the __copy_m()
function below:
template<bool _IsMove>
struct __copy_move<_IsMove, true, random_access_iterator_tag>
{
template<typename _Tp>
static _Tp*
__copy_m(const _Tp* __first, const _Tp* __last, _Tp* __result)
{
using __assignable = conditional<_IsMove,
is_move_assignable<_Tp>,
is_copy_assignable<_Tp>>;
static_assert( __assignable::type::value, "type is not assignable" );
const ptrdiff_t _Num = __last - __first;
if (_Num)
__builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
return __result + _Num;
}
};
The test for _Num is what GCC uses to determine that the number of bytes to
copy is nonzero.
