http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60295
--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> --- -flto=jobserver is added by bootstrap-lto.mk. There are static void stream_out (char *temp_filename, lto_symtab_encoder_t encoder, bool last) { #ifdef HAVE_WORKING_FORK static int nruns; if (!lto_parallelism || lto_parallelism == 1) { do_stream_out (temp_filename, encoder); return; } /* Do not run more than LTO_PARALLELISM streamings FIXME: we ignore limits on jobserver. */ if (lto_parallelism > 0 && nruns >= lto_parallelism) { wait_for_child (); nruns --; } ... /* TODO: jobserver communicatoin is not supported, yet. */ if (!strcmp (flag_wpa, "jobserver")) lto_parallelism = -1; else { lto_parallelism = atoi (flag_wpa); if (lto_parallelism <= 0) lto_parallelism = 0; } I would assume lto_parallelism == -1.