From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 21719 invoked by alias); 14 Jun 2011 13:39:55 -0000 Received: (qmail 21707 invoked by uid 22791); 14 Jun 2011 13:39:53 -0000 X-SWARE-Spam-Status: No, hits=-2.4 required=5.0 tests=AWL,BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW,RFC_ABUSE_POST X-Spam-Check-By: sourceware.org Received: from mail-wy0-f175.google.com (HELO mail-wy0-f175.google.com) (74.125.82.175) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Tue, 14 Jun 2011 13:39:38 +0000 Received: by wye20 with SMTP id 20so4643169wye.20 for ; Tue, 14 Jun 2011 06:39:37 -0700 (PDT) MIME-Version: 1.0 Received: by 10.227.163.76 with SMTP id z12mr6429884wbx.75.1308058776509; Tue, 14 Jun 2011 06:39:36 -0700 (PDT) Received: by 10.227.28.69 with HTTP; Tue, 14 Jun 2011 06:39:36 -0700 (PDT) In-Reply-To: <1307718680.2592.35.camel@gnopaine> References: <1307383631.4798.11.camel@L3G5336.ibm.com> <1307456077.4798.39.camel@L3G5336.ibm.com> <1307718680.2592.35.camel@gnopaine> Date: Tue, 14 Jun 2011 13:58:00 -0000 Message-ID: Subject: Re: [Design notes, RFC] Address-lowering prototype design (PR46556) From: Richard Guenther To: "William J. Schmidt" Cc: gcc-patches@gcc.gnu.org, bergner@vnet.ibm.com, dje.gcc@gmail.com, steven@gcc.gnu.org, law@redhat.com Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable X-IsSubscribed: yes Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org X-SW-Source: 2011-06/txt/msg01050.txt.bz2 On Fri, Jun 10, 2011 at 5:11 PM, William J. Schmidt wrote: > On Tue, 2011-06-07 at 16:49 +0200, Richard Guenther wrote: >> On Tue, Jun 7, 2011 at 4:14 PM, William J. Schmidt >> wrote: > > > >> >> > Loss of aliasing information >> >> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D >> >> > The most serious problem I've run into is degraded performance due = to poorer >> >> > instruction scheduling choices. =A0I tracked this down to >> >> > alias.c:nonoverlapping_component_refs_p. >> >> > >> >> > This code proves that two memory accesses don't overlap by attempti= ng to prove >> >> > that they access different fields of the same structure. =A0This is= done using >> >> > the MEM_EXPRs of the two rtx's, which record the expression trees t= hat were >> >> > translated into the rtx's during expand. =A0When address lowering i= s not >> >> > present, a simple COMPONENT_REF will appear in the MEM_EXPR: =A0x.a= , for >> >> > example. =A0However, address lowering changes the simple COMPONENT_= REF into a >> >> > [TARGET_]MEM_REF that is no longer necessarily identifiable as a fi= eld >> >> > reference. =A0Thus the aliasing machinery can no longer prove that = two such >> >> > field references are disjoint. >> >> > >> >> > This has severe consequences for performance, and has to be dealt w= ith if >> >> > address lowering is to be successful. >> >> > >> >> > I've worked around this with an admittedly fragile solution; I'll d= iscuss the >> >> > drawbacks below. =A0The idea is to construct a mapping from replace= ment mem_refs >> >> > to the original expressions that they replaced. =A0When a MEM_EXPR = is being set >> >> > during expand, we first look up the mem_ref in the mapping. =A0If p= resent, the >> >> > MEM_EXPR is set to the original expression, rather than to the mem_= ref. =A0This >> >> > essentially duplicates the behavior in the absence of address lower= ing. >> >> >> >> Ick. =A0We had this in the past via TMR_ORIGINAL which caused all sor= ts >> >> of problems. =A0Removing it didn't cause much degradation because we = now >> >> preserve points-to information. >> >> >> >> Originally I played with lowering all memory accesses to MEM_REFs >> >> (see the old mem-ref branch), and the loss of type-based alias >> >> disambiguation was indeed an issue. >> >> >> >> But - I definitely do not like the idea of preserving something simil= ar >> >> to TMR_ORIGINAL. =A0Instead we can try preserving some information >> >> we derive from it. =A0We keep the original access type that we can use >> >> for TBAA but do not retain knowledge on whether the type of the >> >> MEM_REF is valid for TBAA or if it is view-converted. >> > >> > Yes, I really don't like what I have at the moment, either. =A0I put i= t in >> > place as a stopgap to let me proceed to look for other performance >> > problems. >> > >> > The question is how we can infer useful information for TBAA from the >> > MEM_REFs and TMRs. =A0I poked at trying to identify types and offsets = from >> > the MEM_EXPRs, but this ended up being useless; I had to constrain too >> > many cases to maintain correctness, and couldn't prove the type >> > information for the important cases in SPEC I was trying to address. >> > >> > Unfortunately, the whole design goes down the drain if we can't find a >> > way to solve the TBAA issue. =A0The performance degradations are too >> > costly. >> >> If you look at what basic TBAA the alias oracle performs then it boils >> down to the fact that get_alias_set for a.b.c might end up using the >> alias-set of the type of C but for MEM[&a + 4] it will use the alias set >> of the type of a. =A0The tree alias-oracle extracts both alias sets, that >> of the outermost valid type and that of the innermost as both are >> equally useful. =A0But the MEM_REF (or TARGET_MEM_REF) tree >> only have storage for one such alias-set. =A0Thus my idea at some point >> was to store the other one as well in some form. =A0It will not be >> the full information (after all, the complete access path does provide >> some extra information - see aliasing_component_refs_p). > > This is what concerns me. =A0TBAA information for the outer and inner > components doesn't seem sufficient to provide what > nonoverlapping_component_refs_p is currently able to prove. =A0The latter > searches for a common RECORD_TYPE somewhere along the two access paths, > and then disambiguates if the two associated referenced fields differ. > For a simple case like "struct x { int a; int b; };", a and b have the > same type and alias-set, so the alias-set information doesn't add > anything. =A0It isn't sufficient alone for the disambiguation of x1.a =3D > MEM_REF[&x1, 0] and x2.b =3D MEM_REF[&x2, 4]. > > Obviously the offset is sufficient to disambiguate for this simple case > with a common base type, but when the shared record types aren't at the > outermost level, we can't detect whether it is. > > At the moment I don't see how we can avoid degradation unless we keep > the full access path around somewhere, for [TARGET_]MEM_REFs built from > COMPONENT_REFs. =A0I hope I'm wrong. You are not wrong. But the question is, does it make a difference? Richard.