From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 374373858002; Mon, 27 Jun 2022 13:58:06 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 374373858002 From: "marxin at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/105740] missed optimization switch transformation for conditions with duplicate conditions Date: Mon, 27 Jun 2022 13:58:05 +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: 12.1.0 X-Bugzilla-Keywords: missed-optimization X-Bugzilla-Severity: normal X-Bugzilla-Who: marxin 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: 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, 27 Jun 2022 13:58:06 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105740 --- Comment #9 from Martin Li=C5=A1ka --- (In reply to luoxhu from comment #8) > (In reply to rguenther@suse.de from comment #6) > > On Tue, 21 Jun 2022, jakub at gcc dot gnu.org wrote: > >=20 > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105740 > > >=20 > > > --- Comment #5 from Jakub Jelinek --- > > > The problem with switch-conversion done multiple times is that when i= t is done > > > early, it can do worse job than when it is done late, e.g. we can hav= e better > > > range information later which allows (unfortunately switch-conversion= doesn't > > > use that yet, there is a PR about it) to ignore some never reachable = values > > > etc. > > > So ideally we either need to be able to undo switch-conversion and re= do it if > > > things have changed, or do it only late and for e.g. inlining costs p= erform it > > > only in analysis mode and record somewhere what kind of lowering woul= d be done > > > and how much it would cost. > > > With multiple if-to-switch, don't we risk that we turn some ifs into = switch, > > > then > > > switch-conversion lowers it back to ifs and then another if-to-switch= matches > > > it again and again lowers it? > >=20 > > Yeah, I think ideally switch conversion would be done as part of switch > > lowering (plus maybe an extra if-to-switch). The issue might be what > > I said - some passes don't like switches, but they probably need to be > > taught. As of inline cost yes, doing likely-switch-converted analysis > > would probably work. >=20 > git diff > diff --git a/gcc/passes.def b/gcc/passes.def > index b257307e085..1376e7cb28d 100644 > --- a/gcc/passes.def > +++ b/gcc/passes.def > @@ -243,8 +243,6 @@ along with GCC; see the file COPYING3. If not see > Clean them up. Failure to do so well can lead to false > positives from warnings for erroneous code. */ > NEXT_PASS (pass_copy_prop); > /* Identify paths that should never be executed in a conforming > program and isolate those paths. */ > NEXT_PASS (pass_isolate_erroneous_paths); > @@ -329,6 +327,7 @@ along with GCC; see the file COPYING3. If not see > POP_INSERT_PASSES () > NEXT_PASS (pass_simduid_cleanup); > NEXT_PASS (pass_lower_vector_ssa); > + NEXT_PASS (pass_if_to_switch); > NEXT_PASS (pass_lower_switch); > NEXT_PASS (pass_cse_reciprocals); > NEXT_PASS (pass_reassoc, false /* early_p */); >=20 > Tried this to add the second if_to_switch before lower_switch, but switch > lowering doesn't work same as switch_conversion: Note the lowering expand to a decision tree where node of such tree can be jump-tables, bit-tests or simple comparisons. >=20 > ;; Function test2 (test2, funcdef_no=3D0, decl_uid=3D1982, cgraph_uid=3D1, > symbol_order=3D0) >=20 > beginning to process the following SWITCH statement ((null):0) : ------- > switch (_2) [INV], case 1: [INV], case 2: [IN= V], > case 3: [INV], case 4: 3> [INV], case 5: [INV], case 6: [INV]> >=20 > ;; GIMPLE switch case clusters: JT(values:6 comparisons:6 range:6 density: > 100.00%):1-6 So jump-table is selected. Where do you see this GIMPLE representation? ... >=20 > ASM still contains indirect jump table like -fno-switch-conversion: >=20 > Is this bug of lower_switch or expected? What bug do you mean?=20 > From the code, they have different > purpose as switch_conversion turns switch to single if-else while No switch_conversion expands a switch statement to a series of assignment based on CSWITCH[index] arrays. > lower_switch expand CLUSTERS as a decision tree.=