From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 86C8F3858C36; Fri, 23 Jun 2023 18:06:33 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 86C8F3858C36
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1687543593;
	bh=j4MiEwr28TypDUwNKeBLgrvf79zYDxTRoObFWIVAJOw=;
	h=From:To:Subject:Date:From;
	b=BkucUhKA7CkAAXT/tXPXTAN0KMm1/tui0ntz65pn+qUokbIVNaIMiCntUSDaZvOk4
	 nkQD7MQoUuakqHlojAaXcU0q04zRxnGbwGcSoJr1OslYW+iGnoArKW6+9XYJMpMrJB
	 rFbJiy3ntEcvlVsaghJcqQmopvoiPh2AusahLGyc=
From: "lennox.ho at intel dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110381] New: Incorrect loop unrolling for structs of
 floating point types
Date: Fri, 23 Jun 2023 18:06:33 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.1.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: lennox.ho at intel dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-110381-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110381

            Bug ID: 110381
           Summary: Incorrect loop unrolling for structs of floating point
                    types
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lennox.ho at intel dot com
  Target Milestone: ---

We believe gcc is incorrectly unrolling loops while performing summation of
structs with floating point members:

Here's a minimal example:

```
#include <iostream>

using value_type =3D double;

struct FOO {
   value_type a =3D 0;
   value_type b =3D 0;
   value_type c =3D 0;
};

value_type sum_8_foos(const FOO* foos) {
    value_type sum =3D 0;

    for (int i =3D 0; i < 8; ++i) {
        auto foo =3D foos[i];

        sum +=3D foo.c;
        sum +=3D foo.b;
        sum +=3D foo.a;
    }

    return sum;
}

int main() {
    FOO foos[8];
    foos[0].b =3D 5;

    std::cout << sum_8_foos(foos) << '\n';
    return 0;
}
```
With -O1, we get 5.
With -O2, we get 10.

godbolt link: https://godbolt.org/z/7cxeb3Gsv

Slightly reorganising the assembly output for the loop,
```
.L2
        add     rdi, 48

        addsd   sum, QWORD PTR [rdi-48] // c
        addsd   sum, QWORD PTR [rdi-40] // b
        addsd   sum, QWORD PTR [rdi-32] // a

        addsd   sum, QWORD PTR [rdi-24] // c
        addsd   sum, QWORD PTR [rdi-16] // b
        addsd   sum, QWORD PTR [rdi-8]  // a

        add     rax, 24

        addsd   sum, QWORD PTR [rax-16] // b
        addsd   sum, QWORD PTR [rax-24] // c

        cmp     rdi, end
        jne     .L2
```

There appears to be duplicate additions for the members b and c.

This behaviour appears on gcc 12.1 and is still present in gcc 13.1.=