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 876F03858D35; Sat, 10 Jun 2023 12:37:43 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 876F03858D35
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1686400663;
	bh=5KfbPo4zvryTVS0I1rQxGviHkWTGZmXArOzM542YFYo=;
	h=From:To:Subject:Date:From;
	b=qPCJCTj+9Qx7VMXIm87ToZvL/dRVs7RStr9JpFOYsmVUj8UCTTINjXvvacnOLQozz
	 o3nksNKTboGf32SuGFEPlzlwgwHOQEOC7SU/w3MiNwvk9UtZHYIOxVEbTidUihQWld
	 sxPESEBw/iKPEDzHsK7dUViQ0DL9drbT+N8oybog=
From: "llvm at rifkin dot dev" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110203] New: Sum should optimize to closed
 form
Date: Sat, 10 Jun 2023 12:37:43 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.1.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: llvm at rifkin dot dev
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-110203-4@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=3D110203

            Bug ID: 110203
           Summary: Sum should optimize to closed form
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

For both of

int foo(int num) {
    if(num =3D=3D 1) return 1;
    return num + foo(num - 1);
}

int bar(int num) {
    int sum =3D 0;
    for(int i =3D 1; i <=3D num; i++) {
        sum +=3D i;
    }
    return sum;
}


GCC emits loops:

foo(int):
        xor     eax, eax
        cmp     edi, 1
        je      .L8
.L2:
        mov     edx, edi
        sub     edi, 1
        add     eax, edx
        cmp     edi, 1
        jne     .L2
        add     eax, 1
        ret
.L8:
        mov     eax, 1
        ret
bar(int):
        test    edi, edi
        jle     .L12
        add     edi, 1
        mov     eax, 1
        xor     edx, edx
.L11:
        add     edx, eax
        add     eax, 1
        cmp     eax, edi
        jne     .L11
        mov     eax, edx
        ret
.L12:
        xor     edx, edx
        mov     eax, edx
        ret


GCC should optimize this to the closed form.

There is precedent for this transformation, llvm does it
https://godbolt.org/z/Kfffe5aPz.=