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From: "lin1.hu at intel dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/114700] New: Front-end optimization generates wrong code with
 -ftrapv.
Date: Fri, 12 Apr 2024 02:22:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114700

            Bug ID: 114700
           Summary: Front-end optimization generates wrong code with
                    -ftrapv.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lin1.hu at intel dot com
  Target Milestone: ---

We test GCC vs Clang with -ftrapv, and test is=20
z =3D c  - y  - c + a  + y - b;
https://godbolt.org/z/EW1xTsazG

We think the clang is right, the overflow judgment should be performed after
each operation. But the front-end generates a - b directly, looks like ther=
e's
a bug in the front-end's handling of -ftrapv.

-ftrapv
This option generates traps for signed overflow on addition, subtraction,
multiplication operations. The options -ftrapv and -fwrapv override each ot=
her,
so using -ftrapv -fwrapv on the command-line results in -fwrapv being
effective. Note that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being effective.


We have another question, we found the front-end won't optimize z =3D c - y=
 + a -
c, while z =3D c - y - c + a will, is this for any particular reason or is =
it a
bug?=