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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101501] [11/12 Regression] wrong code at -O3
 on x86_64-linux-gnu
Date: Thu, 22 Jul 2021 07:19:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101501

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-07-22
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot =
gnu.org
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
niter analysis computes

Analyzing # of iterations of loop 2
  exit condition [a_lsm.5_10 * 52, + , 204] !=3D 0
  bounds on difference of bases: -255 ... 0
  result:
    # of iterations (a_lsm.5_10 * 52) / 52, bounded by 63

but then

Loop 2 iterates at most 14 times.
Loop 2 likely iterates at most 14 times.

because we have nb_iterations_upper_bound set on the loop.  That upper bound
is set by the vectorizer which vectorizes the loop but then it's immediately
removed again because the runtime profitability check optimizes to 1 !=3D 0=
?!
With -fno-vect-cost-model things don't fare any better but -fno-tree-vector=
ize
fixes the issue.=