https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80331
--- Comment #12 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
I think with my patch to basic_string we should have at least arrived to
something comparable with clang.
With -O2 it is optimized away,
with -O2 -D_GLIBCXX_USE_CXX11_ABI=0 I get:
int sain ()
{
struct allocator_type D.37493;
struct allocator D.33221;
struct _Rep * _18;
_Atomic_word * _29;
char __libc_single_threaded.11_30;
unsigned int _33;
int _34;
<bb 2> [local count: 1073741824]:
_18 = std::basic_string<char>::_Rep::_S_create (1, 0, &D.33221);
MEM[(char_type &)_18 + 24] = 97;
if (_18 != &_S_empty_rep_storage)
goto <bb 3>; [10.00%]
else
goto <bb 8>; [90.00%]
<bb 3> [local count: 107374184]:
_18->D.33079._M_refcount = 0;
_18->D.33079._M_length = 1;
MEM[(char_type &)_18 + 25] = 0;
D.33221 ={v} {CLOBBER(eos)};
__libc_single_threaded.11_30 = __libc_single_threaded;
if (__libc_single_threaded.11_30 != 0)
goto <bb 4>; [67.00%]
else
goto <bb 5>; [33.00%]
<bb 4> [local count: 71940704]:
MEM[(_Atomic_word *)_18 + 16B] = -1;
goto <bb 6>; [100.00%]
<bb 5> [local count: 35433481]:
_29 = &_18->D.33079._M_refcount;
_33 = __atomic_fetch_add_4 (_29, 4294967295, 4);
_34 = (int) _33;
if (_34 <= 0)
goto <bb 6>; [7.88%]
else
goto <bb 7>; [92.12%]
<bb 6> [local count: 27380417]:
std::basic_string<char>::_Rep::_M_destroy (_18, &D.37493);
<bb 7> [local count: 1073741824]:
D.37493 ={v} {CLOBBER(eos)};
return 0;
<bb 8> [local count: 966367640]:
D.33221 ={v} {CLOBBER(eos)};
goto <bb 7>; [100.00%]
}
this is not optimized since _M_create and M_destroy are not inlined. Those
seems to be invisible to compiler, so even if modref was extended to propagate
removable allocations inter-proceduraly, we have problem.
With longer string also both clang and GCC will not optimize the code
completely. We get:
int sain ()
{
size_type __dnew;
const struct string s;
char * _7;
long unsigned int _9;
long unsigned int _10;
char * _17;
long unsigned int __dnew.1_18;
long unsigned int __dnew.2_20;
char * _21;
char * _22;
<bb 2> [local count: 1073741824]:
MEM[(struct basic_string *)&s] ={v} {CLOBBER(bob)};
MEM[(struct _Alloc_hider *)&s] ={v} {CLOBBER(bob)};
MEM[(struct _Alloc_hider *)&s]._M_p = &s.D.25525._M_local_buf;
__dnew = 40;
_17 = std::__cxx11::basic_string<char>::_M_create (&s, &__dnew, 0);
s._M_dataplus._M_p = _17;
__dnew.1_18 = __dnew;
s.D.25525._M_allocated_capacity = __dnew.1_18;
__builtin_memcpy (_17, "this string is longer than 15 characters", 40);
__dnew.2_20 = __dnew;
_21 = s._M_dataplus._M_p;
_22 = _21 + __dnew.2_20;
MEM[(char_type &)_22] = 0;
__dnew ={v} {CLOBBER(eos)};
_7 = s._M_dataplus._M_p;
if (&s.D.25525._M_local_buf == _7)
goto <bb 4>; [18.09%]
else
goto <bb 3>; [81.91%]
<bb 3> [local count: 879501928]:
_9 = s.D.25525._M_allocated_capacity;
_10 = _9 + 1;
operator delete (_7, _10);
<bb 4> [local count: 1073741824]:
s ={v} {CLOBBER(eob)};
s ={v} {CLOBBER(eos)};
return 0;
}
Call to _M_create is problem here. Again it is not seen by modref.
With -std=c++23 we optimize out the code, while clang doesn't
