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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/97832] AoSoA complex caxpy-like loops: AVX2+FMA -Ofast 7 times slower than -O3 Date: Tue, 17 Nov 2020 09:21:11 +0000 [thread overview] Message-ID: <bug-97832-4-IIX5Y1ewqL@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-97832-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97832 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Component|target |tree-optimization --- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> --- Ah, thanks - that helps. So we're re-associating from *_89 = (((*_89) - (f_re_34 * x_re_82)) - (f_im_35 * x_im_88)); *_91 = (((*_91) + (f_im_35 * x_re_82)) - (f_re_34 * x_im_88)); to *_89 = ((*_89) - ((f_re_34 * x_re_82) + (f_im_35 * x_im_88))); *_91 = (((*_91) + (f_im_35 * x_re_82)) - (f_re_34 * x_im_88)); that makes the operations unbalanced. This is (a - b) - c -> a - (b + c) as we're optimizing this as a + -b + -c. Even smaller testcase: double a[1024], b[1024], c[1024]; void foo() { for (int i = 0; i < 256; ++i) { a[2*i] = a[2*i] + b[2*i] - c[2*i]; a[2*i+1] = a[2*i+1] - b[2*i+1] - c[2*i+1]; } } here ranks end up associating the expr as (-b + -c) + a and negate re-propagation goes (-b - c) + a -> -(b + c) + a -> a - (b + c) which is all sensible in isolation. You could say that associating as (-b + -c) + a is worse than (a + -b) + -c in this respect. Ranks are Rank for _8 is 327683 (a) Rank for _13 is 327684 (-b) Rank for _21 is 327684 (-c) where the rank is one more for the negated values because of the negate operation. While heuristically ignoring negates for rank propagation to make all ranks equal helps this new testcase it doesn't help for the larger two. It might still be a generally sound heuristic improvement though. For the effects on vectorization I think we need to do sth in the vectorizer itself, for example linearizing expressions. The first reassoc pass is supposed to do this but then negate re-propagation undoes it in this case - which maybe points to it that needs fixing, somehow associating a not negated operand first.
next prev parent reply other threads:[~2020-11-17 9:21 UTC|newest] Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-11-14 20:44 [Bug target/97832] New: " already5chosen at yahoo dot com 2020-11-16 7:21 ` [Bug target/97832] " rguenth at gcc dot gnu.org 2020-11-16 11:11 ` rguenth at gcc dot gnu.org 2020-11-16 20:11 ` already5chosen at yahoo dot com 2020-11-17 9:21 ` rguenth at gcc dot gnu.org [this message] 2020-11-17 10:18 ` [Bug tree-optimization/97832] " rguenth at gcc dot gnu.org 2020-11-18 8:53 ` rguenth at gcc dot gnu.org 2020-11-18 9:15 ` rguenth at gcc dot gnu.org 2020-11-18 13:23 ` rguenth at gcc dot gnu.org 2020-11-18 13:39 ` rguenth at gcc dot gnu.org 2020-11-19 19:55 ` already5chosen at yahoo dot com 2020-11-20 7:10 ` rguenth at gcc dot gnu.org 2021-06-09 12:41 ` cvs-commit at gcc dot gnu.org 2021-06-09 12:54 ` rguenth at gcc dot gnu.org 2022-01-21 0:16 ` pinskia at gcc dot gnu.org 2022-11-24 23:22 ` already5chosen at yahoo dot com 2022-11-25 8:16 ` rguenth at gcc dot gnu.org 2022-11-25 13:19 ` already5chosen at yahoo dot com 2022-11-25 20:46 ` rguenth at gcc dot gnu.org 2022-11-25 21:27 ` amonakov at gcc dot gnu.org 2022-11-26 18:27 ` already5chosen at yahoo dot com 2022-11-26 18:36 ` already5chosen at yahoo dot com 2022-11-26 19:36 ` amonakov at gcc dot gnu.org 2022-11-26 22:00 ` already5chosen at yahoo dot com 2022-11-28 6:29 ` crazylht at gmail dot com 2022-11-28 6:42 ` crazylht at gmail dot com 2022-11-28 7:21 ` rguenther at suse dot de 2022-11-28 7:24 ` crazylht at gmail dot com
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