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From: "dan at stahlke dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102970] New: stable_sort uninitialized value with
 -funroll-loops -fno-tree-vectorize
Date: Wed, 27 Oct 2021 19:07:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102970

            Bug ID: 102970
           Summary: stable_sort uninitialized value with -funroll-loops
                    -fno-tree-vectorize
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dan at stahlke dot org
  Target Milestone: ---

Created attachment 51682
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51682&action=3Dedit
demonstration source code

The attached file, compiled with 11.2.0, using the command line in the comm=
ent,
causes std::stable_sort to access uninitialized memory.  In fact, it already
goes wrong by the time it passes the inputs to the first comparison operati=
on.=20
The problem goes away if the copy constructor of the box struct is defaulted
(see the "#if").

It only happens with "-funroll-loops -fno-tree-vectorize".

Here is a Godbolt: https://godbolt.org/z/6PsdPj6q3=