Re: [PATCH] Add --jobserver/jobserver option for GCC -flto=jobserver

[H.J. Lu]

On Wed, Oct 28, 2020 at 11:40 AM Nick Bowler <nbowler@draconx.ca> wrote:
>
> On 2020-10-28, Zack Weinberg <zackw@panix.com> wrote:
> > On Wed, Oct 28, 2020 at 2:16 PM Nick Bowler <nbowler@draconx.ca> wrote:
> >> On 2020-10-28, H.J. Lu <hjl.tools@gmail.com> wrote:
> >> > GCC introduced some time ago option -flto=jobserver in order to use the
> >> > GNU Make jobserver when parallelising LTO builds.  It is actually a
> >> > similar "recursive make".  When doing a recursive make, you need to
> >> > place a '+' character at the beginning of the recipe line in order to
> >> > let GNU Make pass the jobserver file descriptors to the child
> >> > processes.
> >> >
> >> > Add the --jobserver option to add a '+' character to the recipe line in
> >> > program.am and ltlibrary.am.
> > ...
> >> Surely this needs to be a configure-time option, perhaps combined
> >> with some sort of configure test, since otherwise users won't get
> >> this choice, right?  As an automake option the choice made by whomever
> >> prepares the distribution will get baked into distributed Makefile.in
> >> files...
> >
> > I was going to say something very similar: there shouldn't be an
> > option at all.  The decision of whether or not to put + at the
> > beginning of the recipe line should be made _when make is run_, based
> > on whether -flto=jobserver actually appears in $(LDFLAGS) or wherever.
>
> Since this check cannot be done inside a rule, testing LDFLAGS at make
> time is probably impractical to do portably.
>
> However I think I misunderstood the impact of this option on first
> reading.
>
> I am not aware of any portability concerns with including "+" in
> commands, if this is literally the only difference it should be OK
> to just always include it (after ensuring that "make -n" is respected
> portably in the command).
>

"make -n" will execute the recipe with the "+" prefix.   But --jobserver
is off by default.  People who use --jobserver prefer a working GCC
-flto=jobserver over a broken "make -n".


-- 
H.J.