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 6505C3857344; Thu, 18 May 2023 05:34:47 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6505C3857344
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1684388087;
	bh=APWTY6zHaNurlBt3ToGqdArVI6F3l1SKNtJlduosh7A=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=aQxoQK+6L5uswOjaRcVHK0IqlfOEoWMoLCaB3JRJSZKiekrj0Nk4ATEPYXBv92KaT
	 ppkbYUl6WractuXMac5YmwQRRuUsqRHIwLwUchbfJDq3E4+WTaFlxoBeVQshQomn13
	 tOWr5ScC0UQpYh9grmE6cDMPMvMbxwu+u8LQr2Yw=
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109441] missed optimization when all elements
 of vector are known
Date: Thu, 18 May 2023 05:34:46 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: rguenth at gcc dot gnu.org
X-Bugzilla-Status: NEW
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-109441-4-DX4SYOgWSx@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-109441-4@http.gcc.gnu.org/bugzilla/>
References: <bug-109441-4@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=3D109441

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to AK from comment #3)
> > But IMHO it's academic, right?
>=20
> yes. i was just messing with vector codegen. But in case all the elements=
 of
> a vector/array are same, maybe the loop can be replaced with equivalent
> computation?

Yes.  GCC doesn't currently have the ability to constant propagate or
value-number defs defined by cycles [that actually iterate].  In general
doing that is computationally expensive and only in very few cases you
can short-cut simulating all iterations (final value replacement does this,
but the case in this PR is already too complicated because it involves
a memory load).

For the case in this PR when simplified to not require removal of all
the C++ abstraction via inlining we'd need to handle a loopy memory
definition and a loopy memory use.  The loopy memory def we can probably
pattern match to a memset and the loopy memory use could be (but isn't
currently) identified to be always zero.  Pass ordering still stands in the
way then though.=