pch: Add support for PCH for relocatable executables [PR71934]

pch: Add support for PCH for relocatable executables [PR71934]

So, if we want to make PCH work for PIEs, I'd say we can:
1) add a new GTY option, say callback, which would act like
   skip for non-PCH and for PCH would make us skip it but
   remember for address bias translation
2) drop the skip for tree_translation_unit_decl::language
3) change get_unnamed_section to have const char * as
   last argument instead of const void *, change
   unnamed_section::data also to const char * and update
   everything related to that
4) maybe add a host hook whether it is ok to support binaries
   changing addresses (the only thing I'm worried is if
   some host that uses function descriptors allocates them
   dynamically instead of having them somewhere in the
   executable)
5) maybe add a gengtype warning if it sees in GTY tracked
   structure a function pointer without that new callback
   option

Here is 1), 2), 3) implemented.

Note, on stdc++.h.gch/O2g.gch there are just those 10 relocations without
the second patch, with it a few more, but nothing huge.  And for non-PIEs
there isn't really any extra work on the load side except freading two scalar
values and fseek.

2021-12-03  Jakub Jelinek  <jakub@redhat.com>

	PR pch/71934
gcc/
	* ggc.h (gt_pch_note_callback): Declare.
	* gengtype.h (enum typekind): Add TYPE_CALLBACK.
	(callback_type): Declare.
	* gengtype.c (dbgprint_count_type_at): Handle TYPE_CALLBACK.
	(callback_type): New variable.
	(process_gc_options): Add CALLBACK argument, handle callback
	option.
	(set_gc_used_type): Adjust process_gc_options caller, if callback,
	set type to &callback_type.
	(output_mangled_typename): Handle TYPE_CALLBACK.
	(walk_type): Likewise.  Handle callback option.
	(write_types_process_field): Handle TYPE_CALLBACK.
	(write_types_local_user_process_field): Likewise.
	(write_types_local_process_field): Likewise.
	(write_root): Likewise.
	(dump_typekind): Likewise.
	(dump_type): Likewise.
	* gengtype-state.c (type_lineloc): Handle TYPE_CALLBACK.
	(state_writer::write_state_callback_type): New method.
	(state_writer::write_state_type): Handle TYPE_CALLBACK.
	(read_state_callback_type): New function.
	(read_state_type): Handle TYPE_CALLBACK.
	* ggc-common.c (callback_vec): New variable.
	(gt_pch_note_callback): New function.
	(gt_pch_save): Stream out gt_pch_save function address and relocation
	table.
	(gt_pch_restore): Stream in saved gt_pch_save function address and
	relocation table and apply relocations if needed.
	* doc/gty.texi (callback): Document new GTY option.
	* varasm.c (get_unnamed_section): Change callback argument's type and
	last argument's type from const void * to const char *.
	(output_section_asm_op): Change argument's type from const void *
	to const char *, remove unnecessary cast.
	* tree-core.h (struct tree_translation_unit_decl): Drop GTY((skip))
	from language member.
	* output.h (unnamed_section_callback): Change argument type from
	const void * to const char *.
	(struct unnamed_section): Use GTY((callback)) instead of GTY((skip))
	for callback member.  Change data member type from const void *
	to const char *.
	(struct noswitch_section): Use GTY((callback)) instead of GTY((skip))
	for callback member.
	(get_unnamed_section): Change callback argument's type and
	last argument's type from const void * to const char *.
	(output_section_asm_op): Change argument's type from const void *
	to const char *.
	* config/avr/avr.c (avr_output_progmem_section_asm_op): Likewise.
	Remove unneeded cast.
	* config/darwin.c (output_objc_section_asm_op): Change argument's type
	from const void * to const char *.
	* config/pa/pa.c (som_output_text_section_asm_op): Likewise.
	(som_output_comdat_data_section_asm_op): Likewise.
	* config/rs6000/rs6000.c (rs6000_elf_output_toc_section_asm_op):
	Likewise.
	(rs6000_xcoff_output_readonly_section_asm_op): Likewise.  Instead
	of dereferencing directive hardcode variable names and decide based on
	whether directive is NULL or not.
	(rs6000_xcoff_output_readwrite_section_asm_op): Change argument's type
	from const void * to const char *.
	(rs6000_xcoff_output_tls_section_asm_op): Likewise.  Instead
	of dereferencing directive hardcode variable names and decide based on
	whether directive is NULL or not.
	(rs6000_xcoff_output_toc_section_asm_op): Change argument's type
	from const void * to const char *.
	(rs6000_xcoff_asm_init_sections): Adjust get_unnamed_section callers.
gcc/c-family/
	* c-pch.c (struct c_pch_validity): Remove pch_init member.
	(pch_init): Don't initialize v.pch_init.
	(c_common_valid_pch): Don't warn and punt if .text addresses change.
libcpp/
	* include/line-map.h (class line_maps): Add GTY((callback)) to
	reallocator and round_alloc_size members.