From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 0C9EA386F001; Wed, 18 Nov 2020 19:05:48 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 0C9EA386F001 From: "rguenther at suse dot de" To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/85315] missed range optimisation opportunity for derefences where index must be 0 or otherwise constrained Date: Wed, 18 Nov 2020 19:05:48 +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: 8.0.1 X-Bugzilla-Keywords: missed-optimization X-Bugzilla-Severity: normal X-Bugzilla-Who: rguenther at suse dot de 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: In-Reply-To: References: 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 List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 18 Nov 2020 19:05:49 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D85315 --- Comment #15 from rguenther at suse dot de --- On November 18, 2020 3:55:44 PM GMT+01:00, amacleod at redhat dot com wrote: >https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D85315 > >--- Comment #12 from Andrew Macleod --- >Maybe I'm a little dense. > >if we are presuming that=20=20 > &x + (a + b)=20 >implies a + b =3D=3D 0, then we also should assume that > > &x + a implies a =3D=3D 0 > >and if we can make those assumptions, then >&x + 1 is garbage because we can assume 1 =3D=3D 0. > >And if a and b are both unsigned, then I guess we can also assume a =3D=3D >b =3D=3D >MAX_UINT / 2 ? > > >Now, if we decided to actually do this... I see IL: > > : > x.0_1 =3D x; > y =3D x.0_1; > a.1_2 =3D a; > b.2_3 =3D b; > _4 =3D a.1_2 + b.2_3; > _5 =3D (long unsigned int) _4; > _6 =3D _5 * 4; > _7 =3D &y + _6; > >The clear implications is that _6 =3D=3D 0 in this expression? > >If we implemented that in the operator_pointer_plus::op1_range routine, >and >then were to back substitute, we'd get >(_6)[0,0] =3D _5 * 4 -> _5 =3D [0,0] >(_5)[0,0] =3D (long unsigned int) _4; -> _4 =3D=3D [0,0] >(_4)[0,0] =3D a.1_2 + b.2_3 which gives us nothing additional... Other >than a >potential relationship to track I suppose a.1_2 =3D=3D -B.2_3 for signed, >but it >would record that _4 is [0,0] when we calculate an outgoing range. > >but regardless, its seems that another straightforward place to do this >would >be in statement folding? Isn't the basic assumption: > >_7 =3D &y + _6; >implies _6 is always 0, which would enable us to fold this to >_7 =3D &y >then _6 is unused and the other statements would ultimately just go >away. > >So why not make folding simply throw away the "+ _6" part because it is >now >being forced to be 0? We can't really assume that it is [0,0], but >then not >use that information to optimize? Well, clearly the zero case is degenerate but it extends to sth like int a[= 2] ; and &a + n. I guess you're already handling ARRAY_REF indices.=