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From: "kristian.klausen at scoutdi dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/98932] New: Wrong output with -O3 on aarch64
Date: Tue, 02 Feb 2021 13:20:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98932

            Bug ID: 98932
           Summary: Wrong output with -O3 on aarch64
           Product: gcc
           Version: 8.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kristian.klausen at scoutdi dot com
  Target Milestone: ---

Hi,=20

I have been struggling with a vectorization issue, and have managed to
re-create to a minimal example as attached.=20

When compiled with `-O2` the correct output is produced, however when compi=
led
with `-O3` the output is bogus when compiled with gcc-8 and gcc-9. Gcc-7 gi=
ves
the correct output.=20

Tested with the following versions under qemu for aarch64:=20

```
g++-7 --version
g++-7 (Ubuntu/Linaro 7.5.0-6ubuntu2) 7.5.0
g++-8 --version
g++-8 (Ubuntu/Linaro 8.4.0-3ubuntu2) 8.4.0
g++-9 --version
g++-9 (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
```

Output:=20
```
:~# g++-9 -O3 a.cpp && ./a.out
Length is: 26348240
:~# g++-8 -O3 a.cpp && ./a.out
Length is: 26348240
:~# g++-7 -O3 a.cpp && ./a.out
Length is: 294
```

Also gave it a go with the suggested flags before filing the report:=20

```
g++-8 -O3  -Wall -Wextra -fno-strict-aliasing -fwrapv a.cpp && ./a.out
Length is: 26348240
```

The code gives the correct output with gcc-10:=20

```
g++-10 -O3 a.cpp && ./a.out
Length is: 294
g++-10 --version
g++-10 (Ubuntu 10.2.0-5ubuntu1~20.04) 10.2.0
```

Let me know if I can provide more information.=20

Best,
Kristian=