https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98465
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |msebor at gcc dot gnu.org
Keywords| |diagnostic,
| |missed-optimization
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Blocks| |97048
Last reconfirmed| |2020-12-28
Known to fail| |10.2.0, 11.0
--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
I'm not sure why I don't see the warning for the test case in my build but I
can reproduce it (as well as a -Warray-bounds) by compiling the translation
unit obtained from the test case and with system header markers removed with
either GCC 10 or 11:
$ g++ -O2 -S -Wall -std=gnu++20 pr98465.C
pr98465.C: In function ‘void f(std::string&)’:
pr98465.C:22316:20: warning: offset ‘2’ outside bounds of constant string
[-Warray-bounds]
22316 | this->_S_copy(__p, __s + __len2 - __len1, __len2);
| ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pr98465.C:22843:12: note: ‘constantString’ declared here
22843 | const char constantString[] = {42, 53};
| ^~~~~~~~~~~~~~
In static member function ‘static constexpr std::char_traits<char>::char_type*
std::char_traits<char>::copy(std::char_traits<char>::char_type*, const
char_type*, std::size_t)’,
inlined from ‘void f(std::string&)’ at pr98465.C:19983:21:
pr98465.C:9051:49: warning: ‘void* __builtin_memcpy(void*, const void*, long
unsigned int)’ reading 2 bytes from a region of size 0 [-Wstringop-overread]
9051 | return static_cast<char_type*>(__builtin_memcpy(__s1, __s2, __n));
| ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
pr98465.C: In function ‘void f(std::string&)’:
pr98465.C:22843:12: note: at offset 2 into source object ‘constantString’ of
size 2
22843 | const char constantString[] = {42, 53};
| ^~~~~~~~~~~~~~
The -Warray-bounds instance comes from a call to c_strlen() made during the
ccp2 pass for the following IL:
void f (struct string & s)
{
_14 = MEM[(const struct basic_string *)s_2(D)]._M_dataplus._M_p;
...
<bb 22> [local count: 179779816]:
if (_14 >= &MEM <const char[2]> [(void *)&constantString + 2B])
goto <bb 23>; [50.00%]
else
goto <bb 24>; [50.00%]
...
<bb 24> [local count: 44944954]:
if (_14 <= &constantString)
goto <bb 25>; [50.00%]
else
goto <bb 26>; [50.00%]
<bb 25> [local count: 22472477]:
__builtin_memcpy (_14, &MEM <const char[2]> [(void *)&constantString + 2B],
2); <<< -Warray-bounds
goto <bb 55>; [100.00%]
...
}
The warning is correct: the call is invalid. It results from the following
transformations:
Folding statement: _45 = 2 - _6;
Queued stmt for removal. Folds to: 2
Folding statement: _46 = &constantString + _45;
Queued stmt for removal. Folds to: &MEM <const char[2]> [(void
*)&constantString + 2B]
Folding statement: __builtin_memcpy (__p_18, _46, 2);
Folded into: __builtin_memcpy (_14, &MEM <const char[2]> [(void
*)&constantString + 2B], 2);
The invalid call survives until expansion which is when the -Wstringop-overread
is issued. I suspect the default -Wno-system-headers setting has something to
do with the warning being masked.
In the IL above, the pointer inequalities that lead to the transformation are
the result of the call to _M_disjunct(__s) in std::string::_M_replace,
presumably done to identify and cope with self-insertion. Since constantString
is a distinct object, the invalid call is unreachable, but without more context
GCC can't figure that out. Annotating the std::string::_M_dataplus._M_p
pointer as one that cannot alias a declared object (other than the _M_local_buf
member) would be one way to avoid the warning (and improve the codegen at the
same time). IIRC, I suggested something like that in the past but no nothing
has materialized yet.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97048
[Bug 97048] [meta-bug] bogus/missing -Wstringop-overread warnings
