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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101014] [12 Regression] Big compile time hog
 with -O3 since r12-1268-g9858cd1a6827ee7a
Date: Tue, 22 Jun 2021 13:55:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101014

--- Comment #17 from Andrew Macleod <amacleod at redhat dot com> ---
Created attachment 51050
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51050&action=3Dedit
patch to fix the issue

The gift that keeps on giving eh.
OK, this should solve the infinite loop. Give it a try, I'm running it thro=
ugh
testing now.

When I introduced the sparse on-entry cache, it is limited to 15 unique ran=
ges
for any given ssa-name, then  it reverts to varying for any additional valu=
es
to be safe.

The cache propagation engine works by combining incoming ranges and if that=
 is
different than that current on-entry range, stores that  and proceeds to pu=
sh
this new value on outgoing edges.

What was happening here is this new value that was calculated was beyond th=
e 15
allowed. When it was stored, it was stored as VARYING.  This block was in a
cycle feeding back to itself, so when it calculated the on-enty value again=
 and
compared, it though it needed to update again.  Which of course failed agai=
n...
and the endless loop of trying to propagate was born.

This patch checks that the value being stored to the cache is the same as t=
he
value when it is immediately reloaded. If that fails, we stop trying to
propagate that value.

Please check that it solves both this problam, and likely the 101148 proble=
m=