* c-opts.c (c_common_handle_option): Handle -fpch-preprocess.
* c-common.h (flag_pch_preprocess): Declare.
(c_common_pch_pragma): Likewise.
* c-common.c (flag_pch_preprocess): New.
* c-pch.c (c_common_read_pch): Support -fpreprocess-only.
(c_common_pch_pragma): New.
* c-ppoutput.c (cb_read_pch): New.
(init_pp_output): Support -fpch-preprocess.
* c-pragma.c (init_pragma): Support #pragma GNUC pch_preprocess.
* c.opt (fpch-preprocess): New.
* gcc.c (cpp_options): When save-temps, pass -fpch-preprocess.
* doc/cppopts.texi: Document -fpch-preprocess.
* doc/invoke.texi (Precompiled Headers): Mention that
-fpreprocessed is safe for PCH. Mention that if an option is
listed as safe that doesn't mean it does what you expect.