From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ej1-x629.google.com (mail-ej1-x629.google.com [IPv6:2a00:1450:4864:20::629]) by sourceware.org (Postfix) with ESMTPS id C9D153853543 for ; Thu, 6 Oct 2022 07:55:44 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org C9D153853543 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-ej1-x629.google.com with SMTP id b2so2744084eja.6 for ; Thu, 06 Oct 2022 00:55:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date; bh=tvWTcRf33AQiv7aE8WejJGyJY9FhiS5TsCJF8lJmT84=; b=qYS4CC0L3ViAEXH5m5jysBO4+Rl0/YqBF10mTlvhhni9V+0mqa0QqqlY6zv7+Yjz+6 cy6hgrhax5FM0VMP3OsQ2cf5kshdK0YY0sATIIK3JivjIlC39hhr4wD9ACGAihFAdB4O +p2XUyWH1NAQA9+G2MsydmnALYQFek+f89HrrqF4zMyXpXnZOpMGUofBGanoVVhWmhUo Krjl2Cd4yeUwNQz60zlSIYfeHbknGBPd9f2iOFI4lts/MHyksGE6hSYjif+mG9qpWZ20 12HoGcSuYJX6uuoBxb8otRlwofRK1yOL6geHIIfZv+k/jyFv0MaEOf85wTFVINmbDGrY JWWw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date; bh=tvWTcRf33AQiv7aE8WejJGyJY9FhiS5TsCJF8lJmT84=; b=ioVrSEGX96G9HnM2/grj+61YW4BeOELUgXnANXhy3F+z0WJj1GW32a3A0Vk3yLNXW4 azstCKOnN8eCDO90hySxNhMBMyB0U5w/gf96fTY9qpggZgiyskcFAVri8kcxGGuzu7R+ hW/Pz3N0C/ZqGzlV5H9Hia18VU/E7I32ws4tRFBaVmapHVu9q6sP7CUQGHHKd85NPdRv TeBhywRDFaSmDh1e8Fu1wVBKVYFckRcu2Ogedzn4bzBkkuYgBXrjkqyeHFevxmTH1nTq YREFnmYR5svsi5hP0W6x3w6vLbZIHl9W8FX7BmvMhK6tuFoLTjQkF4aAIj3xAtZwWk8A Z7vQ== X-Gm-Message-State: ACrzQf3JI7NVHLVweUx/FeXGlYJ9dJRX+ZoQKnSWHLwogbaqQ/j/CAhC Rrsr5uT57ngL9fnQ/T/5QD8TrxTYxwfMmSen8gE= X-Google-Smtp-Source: AMsMyM4GY4bMSki339UnLPz4Zf+nWJ+7TdtJXBhDEeZ4LF/oWL9P8DaD3r9ZZbvsEbLlIg5TM9ffBJecmrMCzqgMtzw= X-Received: by 2002:a17:907:1de1:b0:78d:20bb:3934 with SMTP id og33-20020a1709071de100b0078d20bb3934mr3061306ejc.29.1665042943241; Thu, 06 Oct 2022 00:55:43 -0700 (PDT) MIME-Version: 1.0 References: <70a6f006-7e51-f962-ab5b-a6d785b936ea@lambda.is> <863fd013-4f3a-da86-0e24-c5d4250a919d@lambda.is> In-Reply-To: <863fd013-4f3a-da86-0e24-c5d4250a919d@lambda.is> From: Richard Biener Date: Thu, 6 Oct 2022 09:55:29 +0200 Message-ID: Subject: Re: Surprising CFG construction with goto from then to else To: =?UTF-8?Q?J=C3=B8rgen_Kvalsvik?= Cc: GCC Development , jh@suse.cz, mliska@suse.cz Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-8.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP,WEIRD_PORT autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Mon, Oct 3, 2022 at 2:22 PM J=C3=B8rgen Kvalsvik wrote: > > > On 9/8/22 12:30, J=C3=B8rgen Kvalsvik wrote: > > On 02/09/2022 14:22, Richard Biener wrote: > >> On Fri, Sep 2, 2022 at 11:50 AM J=C3=B8rgen Kvalsvik wro= te: > >>> > >>> > >>> Hello, > >>> > >>> I played some more with odd programs and the effect on control flow > >>> graph construction (as a part of condition coverage support [1]) and > >>> came across this: > >>> > >>> int fn (int a, int b, int c) { > >>> int x =3D 0; > >>> if (a && b) { > >>> if (c) { > >>> goto a_; > >>> } else { > >>> x =3D a; > >>> } > >>> } else { > >>> a_: > >>> x =3D (a - b); > >>> } > >>> > >>> return x; > >>> } > >>> > >>> Run through gcov --conditions I get: > >>> > >>> 4: 5: if (a && b) { > >>> condition outcomes covered 2/2 > >>> condition outcomes covered 2/2 > >>> 2: 6: if (c) { > >>> condition outcomes covered 2/2 > >>> > >>> Which is clearly not correct. So I started digging into why and dump = the > >>> CFG as the coverage profiling sees it https://i.imgur.com/d0q72rA.png > >>> [2]. I apologize for the labeling, but A2 =3D a, A3 =3D b, A5 =3D c a= nd A9 the > >>> else block. The problem, which is what confuses the algorithm, is tha= t a > >>> and b don't share A9 as a successor (on false) as I would expect. > >>> > >>> If I add some operation before the label the problem disappears and a > >>> and b share false-destination again https://i.imgur.com/PSrfaLC.png [= 3]. > >>> > >>> } else { > >>> x++; > >>> a_: > >>> x =3D (a - b); > >>> } > >>> > >>> 4: 5: if (a && b) { > >>> condition outcomes covered 4/4 > >>> 2: 6: if (c) { > >>> condition outcomes covered 2/2 > >>> > >>> > >>> When dumping the cfg in the former case with -fdump-tree-cfg-graph I = get > >>> a CFG without the split destinations in a and b > >>> https://i.imgur.com/05MCjzp.png [3]. I would assume from this that th= e > >>> graph dump happens after _more_ CFG transformations than the branch > >>> profiling. > >>> > >>> So my questions are: > >>> > >>> 1. Is the control flow graph expected to be constructed as such where= a > >>> and b don't share outcome, or is it to be considered a bug? > >>> 2. If yes, would it be problematic to push the branch coverage and > >>> condition profiling to a later stage where the cfg has been fixed? > >> > >> I would say you should only see more nodes merged. It's a bit hard to > >> follow > >> what you say with the namings - I usually run cc1 in gdb, breaking at > >> execute_build_cfg where you can do, after build_gimple_cfg finished > >> (and before cleanup_tree_cfg ()) do a 'dot-fn' in gdb which produces a > >> nice > >> picture of the CFG and code with graphviz. > >> > >> It looks like I would have expected, in particular we do not force a > >> new basic-block to be generated for a_: after the D.1991: artificial > >> label we have for the else. That might be premature optimization > >> for your case (but the cleanup_tree_cfg () would immediately do > >> that as well). > >> > >> Richard. > > > > I did some more digging into this and have isolated the problem to edge > > splitting inside the branch_prob () function itself. > > > > gcc/profile.cc:1248 > > > > if (last > > && gimple_has_location (last) > > && !RESERVED_LOCATION_P (e->goto_locus) > > && !single_succ_p (bb) > > && (LOCATION_FILE (e->goto_locus) > > !=3D LOCATION_FILE (gimple_location (last)) > > || (LOCATION_LINE (e->goto_locus) > > !=3D LOCATION_LINE (gimple_location (last))))) > > { > > basic_block new_bb =3D split_edge (e); > > edge ne =3D single_succ_edge (new_bb); > > ne->goto_locus =3D e->goto_locus; > > } > > > > Based on the cleaned-up cfg that gcc dumps later it looks like this > > split only lives through the branch coverage/profiling phase (it may > > bleed slightly later but it shouldn't be of significance). > > > > Out of curiosity I removed the splitting altogether and no tests failed > > when running make check-gcc check-g++ RUNTESTFLAGS=3D"gcov.exp". Either= it > > was not covered by tests in the first place, or whatever behaviour this > > check is meant to fix is resolved elsewhere. I have to admit I don't > > really see a difference with/without this patch, but I don't know what > > to look for. > > > > The check was first introduced in 2005 by Jan (cc): > > > > commit d783b2a2dc91e1d2c1fea78cac2b6c6c73b3680d > > Author: Jan Hubicka > > Date: Thu Aug 4 00:10:54 2005 +0200 > > > > profile.c (branch_prob): Split edges with goto locus on them to ge= t > > proper line counts. > > > > > > * profile.c (branch_prob): Split edges with goto locus on = them > > to get proper line counts. > > * tree-cfg.c (make_cond_expr_edges): Record user goto > > locuses, if any. > > > > * gcov-1.C: Fix switch counts. > > * gcov-4b.c: Likewise. > > > > What stands out to me in the check is that it uses location-file and > > location-line to decide if to split the edge. I added a few prints to > > see when the file/line is set: > > > > 2 int goto1 (int a) { > > > > > > 3 if (a) > > 4 goto end; > > 5 > > 6 return 1; > > 7 end: > > 8 x +=3D a; > > 9 return 0; > > 10 } > > > > if (a_5(D) !=3D 0) > > > > > > edge (true) > > last goto2.c:3 > > goto (null):0 > > > > if (a_5(D) !=3D 0) > > edge (false) > > last goto2.c:3 > > goto (null):0 > > > > // predicted unlikely by goto predictor. > > edge (fallthru) > > last goto2.c:4 > > goto goto2.c:4 > > > > The goto statement is the only with with a location for both the basic > > block and the edge. > > > > 12 int goto2 (int a) { > > > > > > 13 if (a) { goto end; } > > 14 else { label: a++; } > > 15 > > 16 return 1; > > 17 end: > > 18 x +=3D a; > > 19 return 0; > > 20 } > > > > if (a_5(D) !=3D 0) > > edge (true) > > last goto2.c:13 > > goto (null):0 > > > > if (a_5(D) !=3D 0) > > edge (false) > > last goto2.c:13 > > goto goto2.c:14 > > > > // predicted unlikely by goto predictor. > > edge (fallthru) > > last goto2.c:13 > > goto goto2.c:13 > > > > Now the else block has two locations as well, with the edge label > > e->goto_locus being inside the else block. Note that this label is > > _unrelated_ to the edge jump a (false) -> else. > > > > Now a function without gotos: > > > > 22 int goto3 (int a, int b) { > > 23 if (a && b) { > > 24 x +=3D a * b; > > 25 } else { > > 26 x -=3D 1; > > 27 } > > 28 return 0; > > 29 } > > > > if (a_7(D) !=3D 0) > > edge (true) > > last goto2.c:23 > > goto (null):0 > > > > if (a_7(D) !=3D 0) > > edge (false) > > last goto2.c:23 > > goto (null):0 > > > > if (b_8(D) !=3D 0) > > edge (true) > > last goto2.c:23 > > goto (null):0 > > > > if (b_8(D) !=3D 0) > > edge (false) > > last goto2.c:23 > > goto (null):0 > > > > x =3D _3; > > edge (fallthru) > > last goto2.c:24 > > goto goto2.c:24 > > > > x =3D _5; > > edge (fallthru) > > last goto2.c:26 > > goto (null):0 > > > > Now the checks if (a) and (b) don't have goto_locus on the edges. For > > completeness I included the implied jumps in the then/else blocks which > > _do_ have edge locus in the then case. > > > > Finally, the case that expose the problem for me: > > > > 31 int goto4 (int a, int b) { > > 32 if (a) { > > 33 if (b) goto elseblock; > > 34 else a++; > > 35 } else { > > 36 elseblock: > > 37 a--; > > 38 } > > 39 > > 40 return a; > > 41 } > > > > if (a_2(D) !=3D 0) > > edge (true) > > last goto2.c:32 > > goto (null):0 > > > > if (a_2(D) !=3D 0) > > edge (false) > > last goto2.c:32 > > goto goto2.c:36 > > > > if (b_3(D) !=3D 0) > > edge (true) > > last goto2.c:33 > > goto (null):0 > > > > if (b_3(D) !=3D 0) > > edge (false) > > last goto2.c:33 > > goto (null):0 > > > > Again, a (false) has a goto_locus because there is an unrelated label a= t > > the top of the else block. For completeness, it also applies happens > > when there's a label on top of then: > > > > 43 int goto5 (int a, int b) { > > 44 if (a) { > > 45 then: > > 46 a++; > > 47 } else { > > 48 a--; > > > > > > 49 } > > 50 > > 51 return a; > > 52 } > > > > if (a_2(D) !=3D 0) > > edge (true) > > last goto2.c:44 > > goto goto2.c:45 > > > > if (a_2(D) !=3D 0) > > edge (false) > > last goto2.c:44 > > goto (null):0 > > > > This causes the edge to split which probably isn't a problem for the > > branch coverage, but it is problematic for my condition coverage > > algorithm. So how do we solve this? > > > > 1. Is the edge splitting necessary? I didn't find a test that covers > > this (there might be one, any idea?). If the edge splitting is not > > necessary anymore then removing it should be fine for my coverage needs= . > > 2. Assuming the edge split is necessary making a decision on source fil= e > > + line seems vulnerable to source formatting: > > > > 54 int goto6 (int a) { > > 55 if (a) label: goto end; > > > > > > 56 return 0; > > 57 end: > > 58 return 1; > > 59 > > 60 } > > > > if (a_2(D) !=3D 0) > > edge (true) > > last goto2.c:55 > > goto goto2.c:55 > > > > if (a_2(D) !=3D 0) > > edge (false) > > last goto2.c:55 > > goto (null):0 > > > > // predicted unlikely by goto predictor. > > edge (fallthru) > > last goto2.c:55 > > goto goto2.c:55 > > > > Now unrelated gotos all have the same file/line signature. Since the > > edge to the goto_locus is unrelated to the label itself. > > 3. Assuming the test is fine and necessary I *could* work around the > > problem by recording the original edge somewhere (it is very important > > that all conditions in an expression share the same then/else basic > > blocks), or enough metadata to make a virtual edge, but that is a hack = I > > hope I don't have to do. > > > > Anyway, the problem is not with the cfg construction itself, but an edg= e > > splitting that happens specifically for profiling. > > > > CC Martin, maybe you have any idea? > > > > Thanks, > > J=C3=B8rgen. > > So I finally found some time to tinker with this and made some good > progress I think. > > Like I wrote before, the problem is the edge split. I tried removing it > but could not produce any test failures (or so I thought), but it turns > out not much is missing for it work. I started by running the test suite > twice and compare the .gcov files before/after applying this patch: > > > diff --git a/gcc/profile.cc b/gcc/profile.cc > index a4751279571..ff9f6ef2df2 100644 > --- a/gcc/profile.cc > +++ b/gcc/profile.cc > @@ -1244,19 +1244,7 @@ branch_prob (bool thunk) > Don't do that when the locuses match, so > if (blah) goto something; > is not computed twice. */ > - if (last > - && gimple_has_location (last) > - && !RESERVED_LOCATION_P (e->goto_locus) > - && !single_succ_p (bb) > - && (LOCATION_FILE (e->goto_locus) > - !=3D LOCATION_FILE (gimple_location (last)) > - || (LOCATION_LINE (e->goto_locus) > - !=3D LOCATION_LINE (gimple_location (last))))) > - { > - basic_block new_bb =3D split_edge (e); > - edge ne =3D single_succ_edge (new_bb); > - ne->goto_locus =3D e->goto_locus; > - } > + > if ((e->flags & (EDGE_ABNORMAL | EDGE_ABNORMAL_CALL)) > && e->dest !=3D EXIT_BLOCK_PTR_FOR_FN (cfun)) > need_exit_edge =3D 1; > > --- a/gcc/testsuite/lib/gcov.exp > +++ b/gcc/testsuite/lib/gcov.exp > @@ -42,7 +42,7 @@ proc clean-gcov { testcase } { > clean-gcov-file $testcase "gcno" > clean-gcov-file $testcase "gcda" > clean-gcov-file $testcase "h.gcov" > - remote_file host delete "$testcase.gcov" > + #remote_file host delete "$testcase.gcov" > } > > > Run the tests and diff (filtered and most relevant section included): > > $ make check-gcc RUNTESTFLAGS=3Dgcov.exp > $ diff -r sans-split-edge with-split-edge | grep -C 2 -E "^[<>]\s\s" > diff -r sans-split-edge/gcc/gcov-4.c.gcov with-split-edge/gcc/gcov-4.c.gc= ov > 228c228 > < -: 224: break; > --- > > 1: 224: break; > 231c231 > < -: 227: break; > --- > > #####: 227: break; > 237c237 > < -: 233: break; > --- > > 2: 233: break; > > Ok, so there are some breaks that are counted with the edge and not > without. I add the count (N) and lo and behold it fails predictably: > > FAIL: gcc.misc-tests/gcov-4.c line 224: is -:should be 1 > FAIL: gcc.misc-tests/gcov-4.c line 233: is -:should be 2 > > These tests pass when the splitting is re-enabled. > > Ok, so the splitting is necessary (or at the very least a more > substantial change), but check is based on locus. I wrote a few test > programs and printed some diagnostics (file:line:col) when the edge split= s. > > This program breaks condition coverage because of the edge split: > > 1 int fn (int a, int b, int c) { > 2 int x =3D 0; > 3 if (a && b) { > 4 x =3D a; > 5 } else { > 6 a_: > 7 x =3D (a - b); > 8 } > 9 > 10 return x; > 11 } > > loc (src): t.c: 3:10 -> if (a_3(D) !=3D 0) > e->goto: t.c: 6: 1 -> a_: > loc (dst): t.c: 6: 1 -> a_: > loc (src): t.c: 3:13 -> if (b_4(D) !=3D 0) > e->goto: t.c: 6: 1 -> a_: > loc (dst): t.c: 6: 1 -> a_: > > > and this is extract from gcov-4b.c: > > 205 int > 206 test_switch (int i, int j) > 207 { > 208 int result =3D 0; > 209 > 210 switch (i) /* branch(80 25) */ > 211 /* branch(end) */ > 212 { > 213 case 1: > 214 result =3D do_something (2); > 215 break; > 216 case 2: > 217 result =3D do_something (1024); > 218 break; > 219 case 3: > 220 case 4: > 221 if (j =3D=3D 2) /* branch(67) */ > 222 /* branch(end) */ > 223 return do_something (4); > 224 result =3D do_something (8); > 225 break; > 226 default: > 227 result =3D do_something (32); > 228 switch_m++; > > loc (src): gcov-4b.c:214:18 -> result_18 =3D do_something (2); > e->goto: gcov-4b.c:215: 9 -> _22 =3D result_3; > loc (dst): gcov-4b.c:231:10 -> _22 =3D result_3; > loc (src): gcov-4b.c:217:18 -> result_16 =3D do_something (1024); > e->goto: gcov-4b.c:218: 9 -> _22 =3D result_3; > loc (dst): gcov-4b.c:231:10 -> _22 =3D result_3; > loc (src): gcov-4b.c:224:18 -> result_12 =3D do_something (8); > e->goto: gcov-4b.c:225: 9 -> _22 =3D result_3; > loc (dst): gcov-4b.c:231:10 -> _22 =3D result_3; > > The former is a "bad" split and the latter a good one. Notice that for > the former the goto (edge) locus is identical to the first statement in > the destination block, contrary to good split where the goto and dst > locus are different. > > Which leads me to think the problem can be fixed by either extending the > check to also consider the locus of the destination block, or maybe > simpler by comparing the e->locus to first_stmt (e->dest)->locus. > > In other words, by applying something like this patch we can have > condition coverage and still have the test suite pass for all other > cases. I don't think this will be a regression. > > diff --git a/gcc/profile.cc b/gcc/profile.cc > index a4751279571..c1b1028599a 100644 > --- a/gcc/profile.cc > +++ b/gcc/profile.cc > @@ -1239,19 +1240,28 @@ branch_prob (bool thunk) > break; > } > > + gimple *fst =3D nullptr; > + for (gsi =3D gsi_start_nondebug_bb (e->dest); > + !gsi_end_p (gsi); > + gsi_next_nondebug (&gsi)) > + { > + fst =3D gsi_stmt (gsi); > + if (!RESERVED_LOCATION_P (gimple_location (fst))) > + break; > + } > /* Edge with goto locus might get wrong coverage info unle= ss > it is the only edge out of BB. > Don't do that when the locuses match, so > if (blah) goto something; > is not computed twice. */ > - if (last > - && gimple_has_location (last) > + if (fst > + && gimple_has_location (fst) > && !RESERVED_LOCATION_P (e->goto_locus) > && !single_succ_p (bb) > && (LOCATION_FILE (e->goto_locus) > - !=3D LOCATION_FILE (gimple_location (last)) > + !=3D LOCATION_FILE (gimple_location (fst)) > || (LOCATION_LINE (e->goto_locus) > - !=3D LOCATION_LINE (gimple_location (last))))) > + !=3D LOCATION_LINE (gimple_location (fst))))) > { > basic_block new_bb =3D split_edge (e); > edge ne =3D single_succ_edge (new_bb); > > If this sounds reasonably I will clean up my tree and submit the patches > and tests. What do you think? I think the above heuristics only work if they match up with the heuristics where to insert the edge counters to? Your patch above also can cause extra edge splits in case the location of 'last' was the same as the goto location but the location of 'fst' not. So I can't see how this is an unconditional win? Richard. > > Thanks, > J=C3=B8rgen