Re: [PATCH] LTO: fallback to -flto=N if -flto=jobserver does not work.

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Apr 22, 2021 at 11:02 AM Martin Liška <mliska@suse.cz> wrote:
>
> On 4/22/21 10:04 AM, Richard Biener wrote:
> > On Wed, Apr 21, 2021 at 3:08 PM Martin Liška <mliska@suse.cz> wrote:
> >>
> >> When -flto=jobserver is used and we cannot detect job server, then we can
> >> still fallbackto -flto=N mode.
> >>
> >> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> >>
> >> Ready to be installed?
> >
> > I think this behavior needs to be documented - it falls back to a less
> > conservative (possibly system overloading) mode - which IMHO is
> > non-obvious and IMHO we shouldn't do.
>
> Sure, I'm sending corresponding patch. Note that it's quite common mistake
> that '+' is missing in Makefile rule. That was motivation for my change.

Sure, but that change won't get this fixed.  IMHO we should eventually
emit diagnostic like

warning: could not find jobserver, compiling N jobs serially

once N > 1 (or 2?).  Likewise if people just use -flto and auto-detection
finds nothing:

warning: using serial compilation of N LTRANS jobs
note: refer to http://.... for how to use parallel compile

using the URL diagnostics to point to -flto=... documentation.

That is, teach users rather than second-guessing and eventually
blowing things up.  IMHO only the jobserver mode is safe to
automatically use.

Richard.

> Martin
>
> >
> > Richard.
> >
> >> Thanks,
> >> Martin
> >>
> >> gcc/ChangeLog:
> >>
> >>         * lto-wrapper.c (run_gcc): When -flto=jobserver is used, but the
> >>         makeserver cannot be detected, then use -flto=N fallback.
> >> ---
> >>  gcc/lto-wrapper.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/gcc/lto-wrapper.c b/gcc/lto-wrapper.c
> >> index 03a5922f8ea..0b626d7c811 100644
> >> --- a/gcc/lto-wrapper.c
> >> +++ b/gcc/lto-wrapper.c
> >> @@ -1585,8 +1585,9 @@ run_gcc (unsigned argc, char *argv[])
> >>        if (jobserver && jobserver_error != NULL)
> >>         {
> >>           warning (0, jobserver_error);
> >> -         parallel = 0;
> >> +         /* Fall back to auto parallelism.  */
> >>           jobserver = 0;
> >> +         auto_parallel = 1;
> >>         }
> >>        else if (!jobserver && jobserver_error == NULL)
> >>         {
> >> --
> >> 2.31.1
> >>
>