[Bug other/94330] New: No warning if jobserver not available

[rdiezmail-gcc at yahoo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94330

            Bug ID: 94330
           Summary: No warning if jobserver not available
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rdiezmail-gcc at yahoo dot de
  Target Milestone: ---

If you pass -flto=jobserver , and the jobserver file descriptors are not
actually available, you get no warning whatsoever from GCC.

GNU Make does try to help in this scenario with the following warning:

make[1]: warning: jobserver unavailable: using -j1.  Add `+' to parent make
rule.

Without such a warning, it is really easy to miss the opportunity to
parallelise the build. In fact, GCC seems to use 2 threads in this scenario,
which misleads you into thinking that the option is working correctly, when in
fact is not using all other CPU cores.

More context on this issue is here:

https://lists.gnu.org/archive/html/help-make/2020-02/msg00000.html

https://gcc.gnu.org/legacy-ml/gcc-help/2020-02/msg00069.html