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