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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/95622] [11 Regression] force_output flag on a
 variable prevents optimization  / regresses
 c-c++-common/goacc/kernels-alias-ipa-pta{-2,-4,}.c
Date: Fri, 12 Jun 2020 13:10:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95622

--- Comment #3 from Tobias Burnus <burnus at gcc dot gnu.org> ---
I think there are three related issues here:

(a) force_output =3D 1 prevents at least one optimization

(b) If not using force_output =3D 1, we need to find another way to
tell the compiler that the variable is actually needed =E2=80=93 even if
unused. (cf. comment 1)

(c) There are missed optimizations related to offloading =E2=80=93 both
before streaming-out the offload table and afterward.

 * * *

(See also PR 92029 for another test case fixed =E2=80=93 but I think we can=
not learn
much from that one as there the optimization happens before the stream out.)

(In reply to Thomas Schwinge from comment #2)
> We'll need to analyze if we need to restore that functionality now, or if
> it's OK to lose that optimization.

Fixing "now" is only possible by reverting the patch. But given that the pa=
tch
fixes a wrong code generation (link errors or ICE) and comes after the first
optimizations have been done (before LTO streamout), I would not undo it. =
=E2=80=93
Hence: not fixing now but the more optimizations the better :-)

[In particular, my impression is that artificial, read-only static variable=
s do
not need to be linked between host and device as one cannot really take the=
ir
address and they cannot be updated or changed.]=