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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110760] slp introduces new overflow
 arithmetic
Date: Fri, 21 Jul 2023 07:03:22 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110760

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot =
gnu.org
   Last reconfirmed|                            |2023-07-21
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
           Keywords|                            |wrong-code

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Vector types follow the same rules as scalar types because we eventually lo=
wer
them to scalar ops.  So yes, I think this is a bug.

Now, it will be difficult to exploit since the values are not actually used.

One trick would be to CSE with a later


  tem =3D (unsigned)b[1] - (unsigned)c[1];

where for scalar code FRE would eliminate the above with the earlier a[2]
doing

  tem =3D (unsigned)a[2];

but it currently doesn't do that for vectors and as the vectorizer introduc=
es
the wrong behavior we are not going to decompose that to scalars either.

The particular case in question should be easy to fix though.=