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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug bootstrap/84402] [meta] GCC build system: parallelism bottleneck
Date: Thu, 09 Jul 2020 11:40:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D84402

--- Comment #37 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Martin Li=C5=A1ka from comment #36)
> (In reply to jojo from comment #35)
> > (In reply to Martin Li=C5=A1ka from comment #30)
> > > A possible solution can be usage of '-flinker-output=3Dnolto-rel -r' =
for huge
> > > files.
> >=20
> > it's useful for splitting huge files ?
>=20
> There's experiment I did:
>=20
> $ time g++ -O2 /tmp/gimple-match.ii -c
>=20
> real    0m35.790s
> user    0m35.490s
> sys    0m0.268s
>=20
> $ time g++ -O2 /tmp/gimple-match.ii -c -flto
>=20
> real    0m8.138s
> user    0m7.915s
> sys    0m0.202s
>=20
> $ time gcc -flto=3Dauto -flinker-output=3Dnolto-rel gimple-match.o  -r -o
> gimple-match2.o
>=20
> real    0m9.087s
> user    1m56.028s
> sys    0m3.292s
>=20
> $ time gcc -flto=3Dauto -flinker-output=3Dnolto-rel gimple-match.o  -r -o
> gimple-match2.o --param lto-partitions=3D8
>=20
> real    0m7.350s
> user    0m48.548s
> sys    0m0.976s
>=20
> $ time gcc -flto=3Dauto -flinker-output=3Dnolto-rel gimple-match.o  -r -o
> gimple-match2.o --param lto-partitions=3D4
>=20
> real    0m9.847s
> user    0m30.462s
> sys    0m0.392s
>=20
> so for N=3D=3D4 we get to 8+10s =3D 18s (compared to the original 36s). A=
nd total
> user time is 30+8, which is comparable
> to the original 36s.

The GSoC parallelism project this year is supposed to replicate this
in a cheaper way and also develop some magic to automatically trigger
it when it seems profitable.=