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From: "wsnyder at wsnyder dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97623] New: Extremely slow O2 compile (>>O(n^2))
Date: Thu, 29 Oct 2020 02:02:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97623

            Bug ID: 97623
           Summary: Extremely slow O2 compile (>>O(n^2))
           Product: gcc
           Version: 9.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wsnyder at wsnyder dot org
  Target Milestone: ---

Created attachment 49461
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D49461&action=3Dedit
slow.cpp

The attached program compiles very slowly with -O2.  It is fast with clang
(FWIW).

This is an shortened version of a routine taking > 10 minutes.

$ gcc --version
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0

time g++ -O2 -c slow.cpp
 6.457  (average)
time g++ -Os -c slow.cpp
 0.236  (average)
time g++ -O2 -DNO_BOTTOM -c slow.cpp
 0.560  (average)
time g++ -O2 -DNO_TOP -c slow.cpp
 0.702  (average)
time g++ -O2 -DNO_FINAL -c slow.cpp
 6.271  (average)
time clang++ -O2 -c slow.cpp
 0.210  (average)

Note Os is much faster.  The defines remove the top half of the function, or
bottom half, note the time is crazy faster, one would expect the whole prog=
ram
to be only slightly slower than the sum of these.  Removing a single statem=
ent
from the bottom (-DNO_FINAL) also makes an enormous 0.21 second difference,=
 so
some algorithm is not scaling correctly.

Would greatly appreciate some indication of 1. a compiler flag to bypass the
bad optimization if possible, and 2. a hint as to what operator or how to a=
void
this as this is generated code so there's some flexibility for a workaround.

Thanks.=