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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113441] [14 Regression] Fail to fold the last
 element with multiple loop
Date: Mon, 22 Jan 2024 16:16:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113441

--- Comment #9 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
So on SVE the change is cost modelling.

Bisect landed on g:33c2b70dbabc02788caabcbc66b7baeafeb95bcf which changed t=
he
compiler's defaults to using the new throughput matched cost modelling used=
 be
newer cores.

It looks like this changes which mode the compiler picks for when using a f=
ixed
register size.

This is because the new cost model (correctly) models the costs for FMAs and
promotions.

Before:

array1[0][_1] 1 times scalar_load costs 1 in prologue
int) _2 1 times scalar_stmt costs 1 in prologue

after:

array1[0][_1] 1 times scalar_load costs 1 in prologue=20
(int) _2 1 times scalar_stmt costs 0 in prologue=20

and the cost goes from:

Vector inside of loop cost: 125

to

Vector inside of loop cost: 83=20

so far, nothing sticks out, and in fact the profitability for VNx4QI drops =
from

Calculated minimum iters for profitability: 5

to

Calculated minimum iters for profitability: 3

This causes a clash, as this is now exactly the same cost as VNx2QI which u=
sed
to be what it preferred before.

Which then leads it to pick the higher VF.

In the end smaller VF shows:

;; Guessed iterations of loop 4 is 0.500488. New upper bound 1.

and now we get:

Vectorization factor 16 seems too large for profile prevoiusly believed to =
be
consistent; reducing.=20=20
;; Guessed iterations of loop 4 is 0.500488. New upper bound 0.
;; Scaling loop 4 with scale 66.6% (guessed) to reach upper bound 0

which I guess is the big difference.

There is a weird costing going on in the PHI nodes though:

m_108 =3D PHI <m_92(16), m_111(5)> 1 times vector_stmt costs 0 in body=20
m_108 =3D PHI <m_92(16), m_111(5)> 2 times scalar_to_vec costs 0 in prologue

they have collapsed to 0. which can't be right..=