https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86276
--- Comment #2 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
With -O3 we now do quite well.
_Z4goodv:
.LFB1248:
.cfi_startproc
ret
.cfi_endproc
.LFE1248:
.size _Z4goodv, .-_Z4goodv
.p2align 4
.globl _Z3badv
.type _Z3badv, @function
_Z3badv:
.LFB1260:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
movq %rdi, %rbx
movl $22, %edi
call _Znwm
movb $2, (%rax)
leaq 22(%rax), %rdx
movq %rax, (%rbx)
addq $1, %rax
movq %rax, 8(%rbx)
movq %rbx, %rax
movq %rdx, 16(%rbx)
popq %rbx
.cfi_def_cfa_offset 8
ret
good is optimized to empty function since we now have __builtin_operator_new.
Bad is pretty straighforward construction of the vector:
struct vector bad ()
{
unsigned char * __result;
unsigned char * _26;
unsigned char * _28;
<bb 2> [local count: 1073741824]:
_26 = operator new (22);
something_3(D)->D.25721._M_impl.D.25032._M_start = _26;
_28 = _26 + 22;
something_3(D)->D.25721._M_impl.D.25032._M_end_of_storage = _28;
MEM[(char * {ref-all})_26] = 2;
__result_59 = _26 + 1;
something_3(D)->D.25721._M_impl.D.25032._M_finish = __result_59;
return something_3(D);
}
With -O2 however we end up offlining:
;; Function bad (_Z3badv, funcdef_no=1260, decl_uid=25847, cgraph_uid=171,
symbol_order=187)
struct vector bad ()
{
void * D.27512;
const unsigned char D.25850[1];
unsigned char * _33;
unsigned char * _38;
unsigned char * _40;
unsigned char * _41;
long int _42;
long unsigned int _43;
void * _44;
long unsigned int _47;
vector(2) long unsigned int _56;
<bb 2> [local count: 1073741824]:
MEM[(struct _Vector_impl_data *)something_3(D)]._M_start = 0B;
MEM[(struct _Vector_impl_data *)something_3(D)]._M_end_of_storage = 0B;
_38 = operator new (22);
<bb 3> [local count: 1073741824]:
_47 = (long unsigned int) _38;
_56 = {_47, _47};
MEM <vector(2) long unsigned int> [(unsigned char * *)something_3(D)] = _56;
_33 = _38 + 22;
something_3(D)->D.25721._M_impl.D.25032._M_end_of_storage = _33;
D.25850[0] = 2;
std::vector<unsigned char>::_M_assign_aux.isra (something_3(D), &D.25850,
&MEM <const unsigned char[1]> [(void *)&D.25850 + 1B]);
<bb 4> [local count: 1073741824]:
D.25850 ={v} {CLOBBER(eos)};
return something_3(D);
<bb 5> [count: 0]:
<L2>:
D.25850 ={v} {CLOBBER(eos)};
_41 = MEM[(struct _Vector_base *)something_3(D)]._M_impl.D.25032._M_start;
if (_41 != 0B)
goto <bb 6>; [53.47%]
else
goto <bb 7>; [46.53%]
<bb 6> [count: 0]:
_40 = MEM[(struct _Vector_base
*)something_3(D)]._M_impl.D.25032._M_end_of_storage;
_42 = _40 - _41;
_43 = (long unsigned int) _42;
operator delete (_41, _43);
<bb 7> [count: 0]:
_44 = __builtin_eh_pointer (1);
__builtin_unwind_resume (_44);
}
So it first constructs empty vector and then offlines assignment froom C array.
Since it may throw an exception there is EH calling delete which is always
noop.
I think we need to start with ability to IPA propagate that the vector is empty
and thus _M_start==_M_end_of_storage
Curiously codegen for good is even longer:
void good ()
{
void * D.27490;
unsigned char * something$_M_end_of_storage;
unsigned char * something$_M_start;
struct vector something;
const unsigned char D.25837[1];
void * _10;
unsigned char * _36;
unsigned char * _41;
long int _45;
long unsigned int _46;
long int _49;
long unsigned int _50;
vector(2) long unsigned int _53;
long unsigned int _59;
<bb 2> [local count: 1073741824]:
MEM[(struct _Vector_impl_data *)&something] ={v} {CLOBBER(bob)};
_41 = operator new (22);
goto <bb 4>; [100.00%]
<bb 3> [count: 0]:
<L4>:
D.25837 ={v} {CLOBBER(eos)};
goto <bb 10>; [100.00%]
<bb 4> [local count: 1073741824]:
_59 = (long unsigned int) _41;
_53 = {_59, _59};
_36 = _41 + 22;
D.25837[0] = 2;
MEM <vector(2) long unsigned int> [(void *)&something] = _53;
MEM <unsigned char *> [(struct vector *)&something + 16B] = _36;
std::vector<unsigned char>::_M_assign_aux.isra (&something, &D.25837, &MEM
<const unsigned char[1]> [(void *)&D.25837 + 1B]);
<bb 5> [local count: 1073741824]:
something$_M_start_20 = MEM <unsigned char *> [(struct vector *)&something];
something$_M_end_of_storage_21 = MEM <unsigned char *> [(struct vector
*)&something + 16B];
D.25837 ={v} {CLOBBER(eos)};
if (something$_M_start_20 != 0B)
goto <bb 6>; [53.47%]
else
goto <bb 7>; [46.53%]
<bb 6> [local count: 574129752]:
_45 = something$_M_end_of_storage_21 - something$_M_start_20;
_46 = (long unsigned int) _45;
operator delete (something$_M_start_20, _46); [tail call]
<bb 7> [local count: 1073741824]:
something ={v} {CLOBBER(eob)};
something ={v} {CLOBBER(eos)};
return;
<bb 8> [count: 0]:
<L3>:
something$_M_start_22 = MEM <unsigned char *> [(struct vector *)&something];
something$_M_end_of_storage_23 = MEM <unsigned char *> [(struct vector
*)&something + 16B];
D.25837 ={v} {CLOBBER(eos)};
if (something$_M_start_22 != 0B)
goto <bb 9>; [53.47%]
else
goto <bb 10>; [46.53%]
<bb 9> [count: 0]:
_49 = something$_M_end_of_storage_23 - something$_M_start_22;
_50 = (long unsigned int) _49;
operator delete (something$_M_start_22, _50);
<bb 10> [count: 0]:
something ={v} {CLOBBER(eob)};
_10 = __builtin_eh_pointer (2);
__builtin_unwind_resume (_10);
}
