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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/103641] [11/12 regression] Severe compile time regression in SLP vectorize step
Date: Sat, 22 Jan 2022 14:30:40 +0000	[thread overview]
Message-ID: <bug-103641-4-KDkJH5deno@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-103641-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103641

--- Comment #20 from Roger Sayle <roger at nextmovesoftware dot com> ---
IMHO, the problem is in tree-vect-patterns.cc's vect_synth_mult_by_constant.
The comment above line 3054 reads:
  /* Use MAX_COST here as we don't want to limit the sequence on rtx costs.
     The vectorizer's benefit analysis will decide whether it's beneficial
     to do this.  */
  bool possible = choose_mult_variant (mode, hwval, &alg, &variant, MAX_COST);

By using MAX_COST here, synth_mult is being allowed to take an unbounded
amount of time, considering all possible permutations/implementations to
find an optimal synthetic multiply sequence.  A more pragmatic bound might
be to compare the target's vector_multiply cost, or failing that use an
arbitrary, but reasonable limit, say COSTS_N_INSNS(8) machine instructions.
In the worst case, if it takes 100 instructions to do a vector multiply,
then the loop probably shouldn't be vectorized.

  parent reply	other threads:[~2022-01-22 14:30 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-10  9:03 [Bug rtl-optimization/103641] New: [aarch64][11 " husseydevin at gmail dot com
2021-12-10  9:25 ` [Bug rtl-optimization/103641] " marxin at gcc dot gnu.org
2021-12-10  9:37 ` [Bug tree-optimization/103641] " pinskia at gcc dot gnu.org
2021-12-10  9:43 ` [Bug tree-optimization/103641] [11/12 " pinskia at gcc dot gnu.org
2021-12-10  9:46 ` pinskia at gcc dot gnu.org
2021-12-10  9:56 ` pinskia at gcc dot gnu.org
2021-12-10 10:01 ` marxin at gcc dot gnu.org
2021-12-10 10:02 ` pinskia at gcc dot gnu.org
2021-12-10 10:03 ` pinskia at gcc dot gnu.org
2021-12-10 10:06 ` marxin at gcc dot gnu.org
2021-12-10 10:08 ` pinskia at gcc dot gnu.org
2021-12-10 10:09 ` pinskia at gcc dot gnu.org
2021-12-10 10:12 ` marxin at gcc dot gnu.org
2021-12-10 10:12 ` pinskia at gcc dot gnu.org
2021-12-10 10:14 ` pinskia at gcc dot gnu.org
2021-12-10 10:15 ` [Bug middle-end/103641] " pinskia at gcc dot gnu.org
2021-12-10 10:24 ` pinskia at gcc dot gnu.org
2021-12-10 10:28 ` pinskia at gcc dot gnu.org
2021-12-10 13:17 ` roger at nextmovesoftware dot com
2021-12-10 13:19 ` husseydevin at gmail dot com
2022-01-18 14:10 ` rguenth at gcc dot gnu.org
2022-01-22 14:30 ` roger at nextmovesoftware dot com [this message]
2022-01-24  8:13 ` rguenther at suse dot de
2022-01-24 16:49 ` roger at nextmovesoftware dot com
2022-01-24 17:02 ` roger at nextmovesoftware dot com
2022-01-25  7:23 ` rguenth at gcc dot gnu.org
2022-01-25  7:52 ` rguenth at gcc dot gnu.org
2022-02-04  7:26 ` rguenth at gcc dot gnu.org
2022-02-04 10:30 ` cvs-commit at gcc dot gnu.org
2022-02-04 10:43 ` rguenth at gcc dot gnu.org
2022-02-04 11:08 ` tnfchris at gcc dot gnu.org
2022-02-07 12:19 ` tnfchris at gcc dot gnu.org
2022-02-07 15:05 ` [Bug middle-end/103641] [11 " rguenth at gcc dot gnu.org
2022-02-08  8:08 ` tnfchris at gcc dot gnu.org
2022-02-08  8:13 ` pinskia at gcc dot gnu.org
2022-02-08  8:15 ` tnfchris at gcc dot gnu.org
2022-03-16  8:22 ` cvs-commit at gcc dot gnu.org
2022-03-16  8:23 ` rguenth at gcc dot gnu.org

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