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 6B025388C024; Sun, 21 Jun 2020 18:03:59 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6B025388C024
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1592762639;
 bh=TzvQH2THYCcLbIA27VBOuxG+gzd+JIUaQmzVQT0cWfE=;
 h=From:To:Subject:Date:In-Reply-To:References:From;
 b=tZkLOpbQq8woYn1Oke5IDyYXLUfB3XBj99YfJhuoFeQSVQ+whjudBG5W6H7k/9+Ur
 1dkgUEpNq5aFhaChyHowCPil2hhiWe73bF9j+Ro8hkcs4D7ckejBzPLk85gLQCn7JL
 P4yeQjNifsrNVBwv7H7PdMYTbyVtg0i5Rqapy4ng=
From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/90436] Redundant size checking in vector
Date: Sun, 21 Jun 2020 18:03:59 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 10.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: glisse at gcc dot gnu.org
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: 
Message-ID: <bug-90436-4-3Ggh4rZEVL@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-90436-4@http.gcc.gnu.org/bugzilla/>
References: <bug-90436-4@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
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <http://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Sun, 21 Jun 2020 18:03:59 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D90436

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
(side note not related to the redundant size checking)
It is surprising how, in the code from comment 2, adding v.reserve(1000) do=
es
not help, it even slows the program down slightly here (yes, that's rather =
hard
to believe). To reap the benefits, I also need to add in the loop:
if(v.size()=3D=3Dv.capacity())__builtin_abort();
which enables the compiler to remove the reallocation code, and once that c=
ode
is removed it can actually prove that size never reaches capacity and remove
the call to abort! We don't even need __builtin_unreachable there. And once=
 all
that dead code is removed, it can finally vectorize.=