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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109717] -Warray-bound error with gnu++20 and fmt
 library
Date: Thu, 04 May 2023 11:11:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109717

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jwakely.gcc at gmail dot c=
om,
                   |                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
          Component|tree-optimization           |libstdc++
   Last reconfirmed|                            |2023-05-04

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we see

<bb 6> [local count: 14986075]:
_60 =3D operator new (4);
_43 =3D MEM[(const struct buffer *)this_10(D)].size_;
_44 =3D _43 * 4;
_63 =3D (signed long) _44;
if (_63 > 4)
  goto <bb 7>; [90.00%]
else
  goto <bb 8>; [10.00%]

<bb 7> [local count: 12138721]:
__builtin_memmove (_60, pretmp_118, _44);
goto <bb 10>; [100.00%]

and if _63 > 4 then the memmove will write out of bounds of the allocated
storage.  The stringop-overflow diagnostic is about the same thing.

There's eventually a missed optimization that we do not discover this
path as not taken, but I do not see any write to size_ in the function.
There's

<bb 5> [local count: 272474101]:
*pretmp_118 =3D 1;
if (pretmp_154 =3D=3D 0)
  goto <bb 6>; [50.00%]
else
  goto <bb 12>; [50.00%]

and

<bb 4> [local count: 148083751]:
pretmp_118 =3D MEM[(struct buffer *)this_10(D)].ptr_;
pretmp_154 =3D MEM[(struct buffer *)this_10(D)].capacity_;
if (exp_6(D) =3D=3D 0)
  goto <bb 5>; [20.24%]
else
  goto <bb 13>; [79.76%]

but given capacity is zero on this path size_ is probably zero as well
(but that's not visible here).

An assertion in the library that size_ <=3D capacity_ would maybe help.

To sum up, there's nothing the diagnostic code can do here - it simply
doesn't have sufficient information to prune this diagnostic.=