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

On 2020-10-28, Zack Weinberg <[email protected]> wrote:
> On Wed, Oct 28, 2020 at 2:16 PM Nick Bowler <[email protected]> wrote:
>> On 2020-10-28, H.J. Lu <[email protected]> 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).

Cheers,
  Nick