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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/97623] [9/10/11 Regression] Extremely slow O2
 compile (>>O(n^2))
Date: Tue, 03 Nov 2020 14:16:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97623

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Wilson Snyder from comment #7)
> Thanks for the quick service.  I can't easily try GCC trunk, but:
>=20
> -O2
>   compile 98.61s
>   runtime 25.76s
>=20
> -O2 -fno-code-hoisting
>   compile 40.13s
>   runtime 26.40s (+2.5%)

Hmm, OK.  Can you provide the output of -O2 -fno-code-hoisting -ftime-repor=
t?

I have a patch limiting hoist insertions to 3 and that makes the reduced
testcase drop from 13s to 2.5s and drop PRE into reasonable territory:

 tree PRE                           :   0.44 ( 18%)   0.00 (  0%)   0.45 ( =
18%)
 1177k (  5%)

and with -fno-code-hoisting

 tree PRE                           :   0.37 ( 15%)   0.00 (  0%)   0.38 ( =
15%)
  477k (  2%)

where it takes only two insert iterations (the last iteration is always
a no-op, so that's perfect).  Before limiting but after the already
committed patch it was

 tree PRE                           :  10.41 ( 81%)   0.01 ( 33%)  10.48 ( =
81%)
   11M ( 33%)

The function is quite big and ANTIC_IN compute is costly and more
importantly AVAIL_OUT is quadratic in size (ugh).

> -Os
>   compile 4.25s
>   runtime 23.42s (-10%)
>=20
> That -Os runs faster is expected as this program is generally
> instruction-fetch limited.  I'd have expected -fno-code-hoisting to help
> more, compile time wise.

Me, too.

> Am I correct in understanding that the slowdown is roughly correlated to =
the
> number of "if" or "?:" statements?

Yeah, that's the case.=