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 B202C3858D28; Tue, 11 Apr 2023 10:42:51 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org B202C3858D28
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1681209771;
	bh=loMMdiNGOD1DBO+HEmt9mq6AnkAppRlhSi5xxYkp9R0=;
	h=From:To:Subject:Date:From;
	b=ARjDZxV+CZSyXIBVunuAGNdmwFzYLS84Ib42Q+ZzBMZ9iMb21aEkc3XyVGGPkYz55
	 fpn0C0QjJBcewgl2zs2vsWQ00c8szsUPLZHPNvhGB3knyj2sdauiwKHh8xKxsaxmJB
	 PiqHN/5VBTQMLDfcbpDSPhS/bferKmU7VJXbyfd4=
From: "stefano.d at posteo dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109471] New: Missing loop unrolling for small std::vector?
Date: Tue, 11 Apr 2023 10:42:51 +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: 12.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: stefano.d at posteo dot de
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-109471-4@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=3D109471

            Bug ID: 109471
           Summary: Missing loop unrolling for small std::vector?
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefano.d at posteo dot de
  Target Milestone: ---

Hi,
I did some benchmarks where I wanted to bench a switch-case statement again=
st a
small std::vector. In GCC 12.2 the result was as expected: The switch-case
statement was faster than the linear search in std::vector. But when I swit=
ched
to clang 13 or above, the std::vector implementation was much faster than t=
he
switch-case statement!

You can the results here:
https://quick-bench.com/q/9DEDS7rQm0MnIFxwZt3A2iD86G0

Please switch here between GCC and clang.

Here an assembly output:

https://godbolt.org/z/j8oenP8x9

Kind regards
Stefano=