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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103821] [12 Regression] huge compile time
 (jump threading) at -O3 for simple code since
 r12-4790-g4b3a325f07acebf47e82de227ce1d5ba62f5bcae
Date: Mon, 10 Jan 2022 21:12:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103821

--- Comment #3 from Andrew Macleod <amacleod at redhat dot com> ---
Interesting.  This isn't caused by jump threading, just exposed by it.

We end up unrolling this loop, and the pattern of code creates a set of
cascading multiplies for which we can precisely evaluate them with subrange=
s.

For instance, we calculate:

_38 =3D int [8192, 8192][24576, 24576][40960, 40960][57344, 57344]

so _38 has 4 subranges, and then we calculate:

_39 =3D _38 * _38;

we do 16 multiplications and end up with:  int [67108864, 67108864][2013265=
92,
201326592][335544320, 335544320][469762048, 469762048][603979776,
603979776][1006632960, 1006632960][1409286144, 1409286144][1677721600,
1677721600][+INF, +INF]

This feeds other multiplies and progresses rapidly to blow up the number of
subranges, which are then propagated via PHIs and other operations.

Folding of subranges is an O(n*m) process. We perform the operation on each
pair of subranges and union them.   Values like _38 * _38 that continue fee=
ding
each other quickly become exponential.

Then combining that with union (an inherently linear operation over the num=
ber
of subranges) at each step of the way adds an additional quadratic operatio=
n on
top of the exponential factor.=20

I will adjust the wi_fold routine to recognize when the calculation is movi=
ng
in an exponential direction, simply produce a summary a result instead of a
precise one.  Longer term, we could consider merging some of the subranges =
to
prevent the exponential growth, but still retain some precision.=