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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/105329] Bogus restrict warning when assigning
 1-char string literal to std::string
Date: Thu, 21 Apr 2022 11:15:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105329

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |amacleod at redhat dot com
          Component|c++                         |tree-optimization
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |diagnostic,
                   |                            |missed-optimization,
                   |                            |needs-bisection
   Last reconfirmed|                            |2022-04-21

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
It warns about

__builtin_memcpy (_140, _29, _28);

in

<bb 15> [local count: 14831835]:
__nleft_49 =3D (const size_type) _48;
__builtin_memcpy (_22, "5", __nleft_49);
_28 =3D 1 - __nleft_49;
_29 =3D _22 + 1;
_140 =3D _22 + __nleft_49;
__builtin_memcpy (_140, _29, _28);
pretmp_145 =3D MEM[(const struct basic_string *)s_2(D)]._M_dataplus._M_p;
goto <bb 19>; [100.00%]

the block is guarded with

if (_22 >=3D &MEM <const char[2]> [(void *)"5" + 1B])
  goto <bb 10>; [50.00%]
else
  goto <bb 11>; [50.00%]

if (_22 <=3D "5")
  goto <bb 12>; [50.00%]
else
  goto <bb 13>; [50.00%]

<bb 13> [local count: 22472477]:
_48 =3D _22 - "5";
if (_48 =3D=3D 1)
  goto <bb 14>; [34.00%]
else
  goto <bb 15>; [66.00%]

so we're taking the path where "5" is not one character in size.  We should
have somehow simplified this.  It looks like a missed jump-threading,
not sure if we were able to catch this with the forward threader or the
old VRP.=