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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/104165] [12 Regression] -Warray-bounds for
 unreachable code inlined from std::sort()
Date: Fri, 21 Jan 2022 17:16:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104165

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |msebor at gcc dot gnu.org
   Last reconfirmed|                            |2022-01-21
             Blocks|                            |56456
            Summary|[12 Regression] Incorrectly |[12 Regression]
                   |identifying array bounds    |-Warray-bounds for
                   |with -O2                    |unreachable code inlined
                   |-Werror=3Darray-bounds        |from std::sort()
     Ever confirmed|0                           |1

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning triggers for the out of bounds pointer addition in &f.148 + 64 =
for
an array of 16 chars:

int foo (int n)
{
  unsigned char f.148[16];
  ...
  <bb 8> [local count: 116203857]:
  _25 =3D ASSERT_EXPR <_15, _15 !=3D &f.148>;
  _6 =3D (signed long) _14;
  _19 =3D _6 /[ex] 4;
  __n.3_20 =3D (long unsigned int) _19;
  _21 =3D __builtin_clzl (__n.3_20);
  _22 =3D 63 - _21;
  _23 =3D (long int) _22;
  _24 =3D _23 * 2;
  std::__introsort_loop<int*, long int,
__gnu_cxx::__ops::_Iter_comp_iter<bar(int, int)::<lambda(int, int)> > >
(&f.148, _25, _24, __comp);
  if (_14 > 64)
    goto <bb 9>; [50.00%]
  else
    goto <bb 15>; [50.00%]

  <bb 9> [local count: 58101929]:
  std::__insertion_sort.constprop (&f.148, &MEM <int[0:D.48109]> [(void
*)&f.148 + 64B]);   <<< -Warray-bounds
  goto <bb 14>; [100.00%]

The basic block is removed later, in CCP3, as unreachable:

Removing basic block 8
;; basic block 8, loop depth 0
;;  pred:=20=20=20=20=20=20
std::__insertion_sort.constprop (&f.148, &MEM <int[0:D.48109]> [(void *)&f.=
148
+ 64B]);
if (&MEM <int[0:D.48109]> [(void *)&f.148 + 64B] !=3D _15)
  goto <bb 9>; [89.00%]
else
  goto <bb 13>; [11.00%]
;;  succ:       9
;;              13


The warning runs too early to benefit from the subsequent simplification.  =
This
is one instance of a false positive that something like __builtin_warning()
would cure.

GCC 11 emits (and eliminates) the same invalid expression but the warning is
suppressed because in GCC 11 -Warray-bounds wasn't issued code inlined from
system headers.  That's a GCC 12 change.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D56456
[Bug 56456] [meta-bug] bogus/missing -Warray-bounds=