https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102958
Bug ID: 102958
Summary: std::u8string suboptimal compared to std::string,
triggers warnings
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
The two functions below are equivalent and would ideally both result in
optimally efficient code, but as the dump shows, only the one with std::string
does. The other one that uses uchar8_t doesn't. In addition, the latter also
triggers a bogus warning. This was discussed in
https://gcc.gnu.org/pipermail/gcc-patches/2021-October/582416.html and is the
reason for the -Wstringop-overread suppression in
libstdc++-v3/testsuite/27_io/filesystem/path/factory/u8path-char8_t.cc.
$ cat x.C && gcc -O2 -S -std=c++17 -fchar8_t -fdump-tree-optimized=/dev/stdout
x.C
int f ()
{
std::string s = "123456789";
std::string t = s;
return t.length ();
}
int g ()
{
std::u8string s = u8"123456789";
std::u8string t = s;
return t.length ();
}
;; Function f (_Z1fv, funcdef_no=1222, decl_uid=34814, cgraph_uid=307,
symbol_order=346)
int f ()
{
<bb 2> [local count: 1073741824]:
return 9;
}
;; Function g (_Z1gv, funcdef_no=1223, decl_uid=34822, cgraph_uid=308,
symbol_order=347)
int g ()
{
void * D.41167;
size_t __i;
struct u8string t;
struct u8string s;
int _7;
char8_t * _19;
char8_t _23;
char8_t * _34;
char8_t * _41;
char8_t * _51;
char8_t * _52;
char8_t * _61;
char8_t _64;
char8_t * _65;
long unsigned int _67;
long unsigned int _68;
long unsigned int _71;
long unsigned int _72;
char8_t * _73;
long unsigned int _75;
long unsigned int _76;
char8_t * _83;
long unsigned int _85;
char8_t * _102;
void * _129;
char8_t * _130;
char8_t * _134;
long unsigned int _135;
<bb 2> [local count: 1073741824]:
MEM[(struct basic_string *)&s] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&s] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&s]._M_p = &s.D.29743._M_local_buf;
<bb 3> [local count: 8687547547]:
# __i_35 = PHI <__i_21(3), 0(2)>
__i_21 = __i_35 + 1;
_23 = MEM[(const char_type &)"123456789" + __i_21 * 1];
if (_23 != 0)
goto <bb 3>; [89.00%]
else
goto <bb 4>; [11.00%]
<bb 4> [local count: 1073741824]:
if (__i_21 > 15)
goto <bb 5>; [33.00%]
else
goto <bb 8>; [67.00%]
<bb 5> [local count: 354334802]:
if (__i_21 > 4611686018427387903)
goto <bb 6>; [0.04%]
else
goto <bb 7>; [99.96%]
<bb 6> [local count: 141736]:
std::__throw_length_error ("basic_string::_M_create");
<bb 7> [local count: 354193066]:
_85 = __i_35 + 2;
_41 = operator new (_85);
s._M_dataplus._M_p = _41;
s.D.29743._M_allocated_capacity = __i_21;
__builtin_memcpy (_41, "123456789", __i_21);
s._M_string_length = __i_21;
_19 = s._M_dataplus._M_p;
_130 = _19 + __i_21;
MEM[(char_type &)_130] = 0;
MEM[(struct basic_string *)&t] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&t] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&t]._M_p = &t.D.29743._M_local_buf;
if (_19 == 0B)
goto <bb 10>; [30.00%]
else
goto <bb 11>; [70.00%]
<bb 8> [local count: 703255365]:
if (__i_21 == 1)
goto <bb 9>; [50.19%]
else
goto <bb 22>; [49.81%]
<bb 9> [local count: 352981469]:
s._M_string_length = 1;
MEM <unsigned short> [(char_type &)&s + 16] = 49;
MEM[(struct basic_string *)&t] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&t] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&t]._M_p = &t.D.29743._M_local_buf;
_64 = MEM[(const char_type &)&s + 16];
MEM[(char_type &)&t + 16] = _64;
goto <bb 14>; [100.00%]
<bb 10> [local count: 124582]:
std::__throw_logic_error ("basic_string::_M_construct null not valid");
<bb 11> [local count: 342421520]:
_135 = __i_35 + 2;
_61 = operator new (_135);
<bb 12> [local count: 308179374]:
t._M_dataplus._M_p = _61;
t.D.29743._M_allocated_capacity = __i_21;
<bb 13> [local count: 323794238]:
# _34 = PHI <&t.D.29743._M_local_buf(22), _61(12)>
# _134 = PHI <_102(22), _19(12)>
__builtin_memcpy (_34, _134, __i_21);
<bb 14> [local count: 1003677033]:
t._M_string_length = __i_21;
_51 = t._M_dataplus._M_p;
_52 = _51 + __i_21;
MEM[(char_type &)_52] = 0;
_7 = (int) __i_21;
if (&t.D.29743._M_local_buf != _51)
goto <bb 15>; [53.47%]
else
goto <bb 16>; [46.53%]
<bb 15> [local count: 536666103]:
_71 = t.D.29743._M_allocated_capacity;
_72 = _71 + 1;
operator delete (_51, _72);
<bb 16> [local count: 1003677033]:
t ={v} {CLOBBER};
_65 = s._M_dataplus._M_p;
if (&s.D.29743._M_local_buf != _65)
goto <bb 17>; [53.47%]
else
goto <bb 18>; [46.53%]
<bb 17> [local count: 536666103]:
_67 = s.D.29743._M_allocated_capacity;
_68 = _67 + 1;
operator delete (_65, _68);
<bb 18> [local count: 1003677033]:
s ={v} {CLOBBER};
s ={v} {CLOBBER};
t ={v} {CLOBBER};
return _7;
<bb 19> [count: 0]:
<L6>:
_73 = s._M_dataplus._M_p;
if (&s.D.29743._M_local_buf != _73)
goto <bb 20>; [0.00%]
else
goto <bb 21>; [0.00%]
<bb 20> [count: 0]:
_75 = s.D.29743._M_allocated_capacity;
_76 = _75 + 1;
operator delete (_73, _76);
<bb 21> [count: 0]:
s ={v} {CLOBBER};
_129 = __builtin_eh_pointer (7);
__builtin_unwind_resume (_129);
<bb 22> [local count: 0]:
__builtin_memcpy (&s.D.29743._M_local_buf, "123456789", __i_21);
s._M_string_length = __i_21;
_102 = s._M_dataplus._M_p;
_83 = _102 + __i_21;
MEM[(char_type &)_83] = 0;
MEM[(struct basic_string *)&t] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&t] ={v} {CLOBBER};
MEM[(struct _Alloc_hider *)&t]._M_p = &t.D.29743._M_local_buf;
if (_102 == 0B)
goto <bb 10>; [30.00%]
else
goto <bb 13>; [70.00%]
}
In file included from
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/string:40,
from x.C:1:
In static member function ‘static std::char_traits<char>::char_type*
std::char_traits<char>::copy(std::char_traits<char>::char_type*, const
std::char_traits<char>::char_type*, std::size_t)’,
inlined from ‘static void std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::_S_copy(_CharT*, const _CharT*, std::__cxx11::basic_string<_CharT,
_Traits, _Alloc>::size_type) [with _CharT = char; _Traits =
std::char_traits<char>; _Alloc = std::allocator<char>]’ at
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.h:361:21,
inlined from ‘static void std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::_S_copy(_CharT*, const _CharT*, std::__cxx11::basic_string<_CharT,
_Traits, _Alloc>::size_type) [with _CharT = char8_t; _Traits =
std::char_traits<char8_t>; _Alloc = std::allocator<char8_t>]’ at
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.h:356:7,
inlined from ‘static void std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::_S_copy_chars(_CharT*, const _CharT*, const _CharT*) [with _CharT =
char8_t; _Traits = std::char_traits<char8_t>; _Alloc =
std::allocator<char8_t>]’ at
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.h:408:16,
inlined from ‘void std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::_M_construct(_InIterator, _InIterator, std::forward_iterator_tag)
[with _FwdIterator = const char8_t*; _CharT = char8_t; _Traits =
std::char_traits<char8_t>; _Alloc = std::allocator<char8_t>]’ at
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.tcc:225:25,
inlined from ‘std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::basic_string(const _CharT*, const _Alloc&) [with
<template-parameter-2-1> = std::allocator<char8_t>; _CharT = char8_t; _Traits =
std::char_traits<char8_t>; _Alloc = std::allocator<char8_t>]’ at
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.h:541:14,
inlined from ‘int g()’ at x.C:12:21:
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/char_traits.h:355:56:
warning: ‘void* __builtin_memcpy(void*, const void*, long unsigned int)’
reading between 16 and 4611686018427387903 bytes from a region of size 10
[-Wstringop-overread]
355 | return static_cast<char_type*>(__builtin_memcpy(__s1, __s2,
__n));
|
~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
