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From: "slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug other/109898] 'make install -j' sometimes corrupts 'dir' file
 for .info files due to parallel 'install-info' calls
Date: Thu, 18 May 2023 19:35:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109898

--- Comment #3 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> (In reply to Sergei Trofimovich from comment #0)
> > --- gcc-12.2.0/gcc/Makefile.in	2022-08-19 10:09:52.280658631 +0200
> > +++ gcc-12.2.0-new/gcc/Makefile.in	2023-05-04 14:35:44.401420184 +0200
> > @@ -3781,6 +3781,11 @@
> >  	  fi; \
> >  	fi
> >=20=20
> > +# We don't care about the order in which the info files are built, but
> > +# install-info doesn't support multiple parallel invocations writing to
> > +# the same `dir-file`, so we have to disable parallelism for that reas=
on:
> > +.NOTPARALLEL: install-info
>=20
> Prerequisites of .NOTPARALLEL are ignored, so doesn't this un-parallelize
> building the entire gcc sub-dir?

When I tested on small example '.NOTPARALLEL: target' serialized exactly
prerequisites of 'target' and nothing more. 'info make' seems to confirm the
behaviour:

"""
'.NOTPARALLEL'

     If '.NOTPARALLEL' is mentioned as a target with no prerequisites,
     all targets in this invocation of 'make' will be run serially, even
     if the '-j' option is given.  Any recursively invoked 'make'
     command will still run recipes in parallel (unless its makefile
     also contains this target).

     If '.NOTPARALLEL' has targets as prerequisites, then all the
     prerequisites of those targets will be run serially.  This
     implicitly adds a '.WAIT' between each prerequisite of the listed
     targets.  *Note Disabling Parallel Execution: Parallel Disable.
"""

Here is the test that confirms it:

$ cat Makefile
all: a b

a: 1 2
        echo a

1:
        sleep 1

2:
        sleep 1

b: 3 4
        echo b

3:
        sleep 1

4:
        sleep 1

.NOTPARALLEL: a

$ make -j | LANG=3DC ts
May 18 20:34:23 sleep 1
May 18 20:34:23 sleep 1
May 18 20:34:23 sleep 1
May 18 20:34:24 sleep 1
May 18 20:34:24 echo b
May 18 20:34:24 b
May 18 20:34:25 echo a
May 18 20:34:25 a

Note how it takes 'b' 1 second to finish (due to parallelism) while 'a' tak=
es 2
seconds (targets are sequential).=