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