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From: "pnarsing at mathworks dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101508] New: Possible undefined behaviour in
 cpp program using "unsigned" type starting from GCC 9
Date: Mon, 19 Jul 2021 10:25:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101508

            Bug ID: 101508
           Summary: Possible undefined behaviour in cpp program using
                    "unsigned" type starting from GCC 9
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pnarsing at mathworks dot com
  Target Milestone: ---

Hi Team,

I'm observing an optimization issue while compiling with GCC 10.3 .
On investigating figured that issue is reproducible in GCC 9 as well .

This can be reproduced by simple test code mentioned below :

------------------------------------------------------

using int_type =3D unsigned int; // works without "unsigned"
int_type constexpr start{0};   // works if >=3D 1
int_type constexpr stop{3};    // works if <=3D 2
int_type constexpr mult{2};    // works if !=3D 2

int main()
{
    int result =3D 0;
    for(int_type aa =3D start; aa < stop; ++aa)
    {
        int_type const bb =3D aa * mult; // works if volatile
        if (aa > bb)
        {
            return 99;
        }
        ++result;
    }
    return result; // should be =3D=3D stop
}

----------------------------------------------------------------

This code snippet returns 99 instead of 3 .

You can check the following link to see the issue reproducibility:
https://godbolt.org/z/zd79PPKfn

Modifying compiler option to -O1 and -Og avoids the issue.=