Use SHF_GNU_RETAIN to preserve symbol definitions

Use SHF_GNU_RETAIN to preserve symbol definitions

In assemly code, the section flag 'R' sets the SHF_GNU_RETAIN flag to
indicate that the section must be preserved by the linker.

Add SECTION_RETAIN to indicate a section should be retained by the linker
and set SECTION_RETAIN on section for the preserved symbol if assembler
supports SHF_GNU_RETAIN.  All retained symbols are placed in separate
sections with

	.section .data.rel.local.preserved_symbol,"awR"
preserved_symbol:
...
	.section .data.rel.local,"aw"
not_preserved_symbol:
...

to avoid

	.section .data.rel.local,"awR"
preserved_symbol:
...
not_preserved_symbol:
...

which places not_preserved_symbol definition in the SHF_GNU_RETAIN
section.

gcc/

2020-12-01  H.J. Lu  <hjl.tools@gmail.com>

	* configure.ac (HAVE_GAS_SHF_GNU_RETAIN): New.  Define 1 if
	the assembler supports marking sections with SHF_GNU_RETAIN flag.
	* output.h (SECTION_RETAIN): New.  Defined as 0x4000000.
	(SECTION_MACH_DEP): Changed from 0x4000000 to 0x8000000.
	(default_unique_section): Add a bool argument.
	* varasm.c (get_section): Set SECTION_RETAIN for the preserved
	symbol with HAVE_GAS_SHF_GNU_RETAIN.
	(resolve_unique_section): Used named section for the preserved
	symbol if assembler supports SHF_GNU_RETAIN.
	(get_variable_section): Handle the preserved common symbol with
	HAVE_GAS_SHF_GNU_RETAIN.
	(default_elf_asm_named_section): Require the full declaration and
	use the 'R' flag for SECTION_RETAIN.
	* config.in: Regenerated.
	* configure: Likewise.
	* doc/sourcebuild.texi: Document R_flag_in_section.

gcc/testsuite/

2020-12-01  H.J. Lu  <hjl.tools@gmail.com>
	    Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	* c-c++-common/attr-used.c: Check the 'R' flag.
	* c-c++-common/attr-used-2.c: Likewise.
	* c-c++-common/attr-used-3.c: New test.
	* c-c++-common/attr-used-4.c: Likewise.
	* gcc.c-torture/compile/attr-used-retain-1.c: Likewise.
	* gcc.c-torture/compile/attr-used-retain-2.c: Likewise.
	* lib/target-supports.exp
	(check_effective_target_R_flag_in_section): New proc.