[Bug lto/106328] Build doesn't respect -j N flag

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106328

--- Comment #10 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 10 Aug 2022, marxin at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106328
> 
> --- Comment #9 from Martin Liška <marxin at gcc dot gnu.org> ---
> > Magically only with recent GNU make, otherwise needs proper prefixed
> > rules in the lto-wrapper generated makefile which I don't think we do.
> 
> Wait, the cooperation works with older GNU make if a Makefile uses prefixed (+)
> rules. WPA does not email any artificial Makefile for WPA streaming. It's a
> Makefile we emit for LTRANS run, e.g.:
> 
> marxin@marxinbox:/dev/shm/objdir> cat /tmp/ccuhgkQs.mk
> ./a.ltrans0.ltrans.o:
>         @g++  '-xlto' '-c' '-fno-openmp' '-fno-openacc' '-fno-pie'
> '-fcf-protection=none' '-mtune=generic' '-march=x86-64' '-O2' '-save-temps'
> '-v' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'
> '-dumpbase' './a.ltrans0.ltrans' '-fltrans' '-o' './a.ltrans0.ltrans.o'
> './a.ltrans0.o'
> ./a.ltrans1.ltrans.o:
>         @g++  '-xlto' '-c' '-fno-openmp' '-fno-openacc' '-fno-pie'
> '-fcf-protection=none' '-mtune=generic' '-march=x86-64' '-O2' '-save-temps'
> '-v' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'
> '-dumpbase' './a.ltrans1.ltrans' '-fltrans' '-o' './a.ltrans1.ltrans.o'
> './a.ltrans1.o'
> ...
> 
> So what can be miss is jobserver detection on BSD that can fail for some
> reason, but it should work fine apart from that. Or do I miss something?

Ah, indeed.  That still leaves the question whether we execute the
WPA stage with the FDs open - I suppose you checked?  And whether
pex_* "properly" does this for all host OSs (how does make jobserver
work on mingw/cygwin?).  I wonder because Honza once said he didn't
implement jobserver support because it would require more fiddling
to get it actually work.

And IIRC BSD 'make' is not GNU make but I think gmake is available
from the ports repo.  The documentation about -flto=jobserver mentions
that already.