[Bug tree-optimization/102216] New: False positive warray-bounds with -O2

[vries at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102216

            Bug ID: 102216
           Summary: False positive warray-bounds with -O2
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

With a gcc build from commit 637dfcf43cf, I run into an incorrect Warray-bounds
(which causes a buildbreaker when building gdb, as reported here:
https://sourceware.org/pipermail/gdb/2021-September/049687.html ).

Reproducer minimized from gdb/language.c:
...
$ cat language.c
#include <algorithm>

static inline bool
compare_cstrings (const char *str1, const char *str2)
{
  return str1 < str2;
}

void
add_set_language_command ()
{
  static const char **language_names;

  language_names = new const char *[6];

  language_names[0] = "auto";
  language_names[1] = "local";
  language_names[2] = "unknown";

  const char **language_names_p = language_names;
  /* language_names_p == &language_names[0].  */
  language_names_p++;
  /* language_names_p == &language_names[1].  */
  language_names_p++;
  /* language_names_p == &language_names[2].  */
  language_names_p++;
  /* language_names_p == &language_names[3].  */

  const char **sort_begin;

  if (0)
    sort_begin = &language_names[3];
  else
    sort_begin = language_names_p;

  language_names[3] = "";
  language_names[4] = "";
  language_names[5] = nullptr;

  std::sort (sort_begin, &language_names[5], compare_cstrings);
}
...

First with gcc-11:
...
$ g++-11 -x c++ -Werror -Wall -O2 -S language.c -Warray-bounds=1
$ 
...

Now with trunk:
...
$ g++ -x c++ -Werror -Wall -O2 -S language.c -Warray-bounds=1 
In file included from
/home/vries/gcc_versions/devel/install/include/c++/12.0.0/algorithm:61,
                 from language.c:1:
In function ‘void std::__final_insertion_sort(_RandomAccessIterator,
_RandomAccessIterator, _Compare) [with _RandomAccessIterator = const char**;
_Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (*)(const char*, const
char*)>]’,
    inlined from ‘void std::__sort(_RandomAccessIterator,
_RandomAccessIterator, _Compare) [with _RandomAccessIterator = const char**;
_Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (*)(const char*, const
char*)>]’ at
/home/vries/gcc_versions/devel/install/include/c++/12.0.0/bits/stl_algo.h:1960:31,
    inlined from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter =
const char**; _Compare = bool (*)(const char*, const char*)]’ at
/home/vries/gcc_versions/devel/install/include/c++/12.0.0/bits/stl_algo.h:4868:18,
    inlined from ‘void add_set_language_command()’ at language.c:40:13:
/home/vries/gcc_versions/devel/install/include/c++/12.0.0/bits/stl_algo.h:1869:32:
error: array subscript 19 is outside array bounds of ‘void [48]’
[-Werror=array-bounds]
 1869 |           std::__insertion_sort(__first, __first + int(_S_threshold),
__comp);
      |          
~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
language.c: In function ‘void add_set_language_command()’:
language.c:14:38: note: at offset 152 into object of size 48 allocated by
‘operator new []’
   14 |   language_names = new const char *[6];
      |                                      ^
cc1plus: all warnings being treated as errors
...

Now set 'if (0)' to 'if (1)':
...
$ g++ -x c++ -Werror -Wall -O2 -S language.c -Warray-bounds=1 
$
...

So either the warning is incorrect (for this fairly trivial example), or
there's a compiler/std::sort bug and the warning is letting us known.