From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 5F938385840B; Mon, 4 Oct 2021 06:36:22 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 5F938385840B From: "rguenther at suse dot de" To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/102540] [12 Regression] Dead Code Elimination Regression at -O3 since r12-476-gd846f225c25c5885 Date: Mon, 04 Oct 2021 06:36:22 +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: 12.0 X-Bugzilla-Keywords: 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: 12.0 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: Mon, 04 Oct 2021 06:36:22 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102540 --- Comment #5 from rguenther at suse dot de --- On Fri, 1 Oct 2021, amacleod at redhat dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102540 >=20 > --- Comment #4 from Andrew Macleod --- >=20 >=20 > (In reply to Richard Biener from comment #2) > > FRE1 has the following difference, simplifying the (unsigned int) trunc= ation. > >=20 > > : > > a.0_1 =3D a; > > _2 =3D (unsigned int) a.0_1; > > b =3D _2; > > - c_10 =3D (long int) _2; > > + _6 =3D a.0_1 & 4294967295; > > + c_10 =3D _6; > > if (c_10 !=3D 0) > > goto ; [INV] > > else > >=20 >=20 > Why does FRE make this transformation/simplification? It's a match.pd transform that transforms a zero-extend from a smaller precision via two NOP_EXPRs to a single BIT_AND_EXPR which is better and more canonical on GIMPLE. > It removes a > relationship between c_10 and _2. The reason ranger no longer can fold _2= =3D=3D 0 > is because the sequence is now: >=20 > a.0_1 =3D a; > _2 =3D (unsigned int) a.0_1; > b =3D _2; > _6 =3D a.0_1 & 4294967295; > c_10 =3D _6; > if (c_10 !=3D 0) > goto ; [INV] >=20 > We do not find _2 is non-zero on the outgoing edge because _2 is not rela= ted to > the calculation in the condition. (ie c_10 no longer has a dependency on= _2) >=20 > We do recalculate _2 based on the outgoing range of a.0_1, but with it be= ing a > 64 bit value and _2 being 32 bits, we only know the outgoing range of a.0= _1 is > non-zero.. we dont track any of the upper bits...=20 > 2->3 (T) a.0_1 : long int [-INF, -1][1, +INF] > And when we recalculate _2 using that value, we still get varying because > 0xFFFF0000 in not zero, but can still produce a zero in _2. >=20 > The problem is that the condition c_10 !=3D 0 no longer related to the va= lue of > _2 in the IL... so ranger never sees it. and we cant represent the 2^16 > subranges that end in [1,0xFFFF]. >=20 > Before that transformation,=20 > _2 =3D (unsigned int) a.0_1; > b =3D _2; > c_10 =3D (long int) _2; > The relationship is obvious, and ranger would relate the c_10 !=3D 0 to _= 2 no > problem. I see - too bad. Note the transform made the dependence chain of _6 one instruction shorter without increasing the number of instructions so it's a profitable transform. Btw, the relation is still there but only indirectly via a.0_1. The old (E)VRP had this find_asserts(?) that produced assertions based on the definitions - sth that now range-ops does(?), so it would eventually have built assertions for a.0_1 for both conditions and allow relations based on that? I can't seem to find my way around the VRP code now - pieces moved all over the place and so my mind fails me on the searching task :/=