[patch] AVR: Add support for .gnu_attribute 4 (Tag_GNU_AVR_VTABLE_AS)

[Georg-Johann Lay]

The upcoming support for ISO/IEC TR 18037 "Embedded C" named
address spaces in G++ would allow to put C++ virtual tables
into such an address spaces, see https://gcc.gnu.org/PR69549.

The current address space used for vtables is the generic
one, which means that RAM is wasted on machines where
.rodata is located in RAM.

The TARGET_ADDR_SPACE_FOR_ARTIFICIAL_RODATA GCC target hook
allows to chose a custom vtable address space. However, in
contrast to current user-selectable address spaces, placing
vtables in a different address space is an ABI change, so
that a means to detect ABI violations is highly appreciated.

The feature of choice is .gnu_attribute 4 (Tag_GNU_AVR_VTABLE_AS)
which would tag an object file with one of the following values:

0 (Val_GNU_AVR_VTABLE_NONE) for files that neither define nor access 
vtables.
1 (Val_GNU_AVR_VTABLE_RAM) use the generic address space.
2 (Val_GNU_AVR_VTABLE_FLASH) use the 16-bit address space __flash.
3 (Val_GNU_AVR_VTABLE_FLASH1) use the 16-bit address space __flash1.
4 (Val_GNU_AVR_VTABLE_FLASH2) use the 16-bit address space __flash2.
5 (Val_GNU_AVR_VTABLE_FLASH3) use the 16-bit address space __flash3.
6 (Val_GNU_AVR_VTABLE_FLASH4) use the 16-bit address space __flash4.
7 (Val_GNU_AVR_VTABLE_FLASH5) use the 16-bit address space __flash5.
8 (Val_GNU_AVR_VTABLE_FLASHX) use the 24-bit address space __flashx.

A tag value of zero is compatible with all other tag values.
Two non-zero tag values are compatible iff they are the same.

Unknown tag values are silently accepted an treated just like
the values above so as to allow for compatibility with future
extensions.

Ok for trunk?

Johann

--

	PR ld/34305
include/
	* elf/avr.h (libiberty.h): Include for ARRAY_SIZE.
	(Tag_GNU_AVR_VTABLE_AS = 4): New enum.
	(Val_GNU_AVR_VTABLE_NONE, Val_GNU_AVR_VTABLE_RAM)
	(Val_GNU_AVR_VTABLE_FLASH, Val_GNU_AVR_VTABLE_FLASH1)
	(Val_GNU_AVR_VTABLE_FLASH2, Val_GNU_AVR_VTABLE_FLASH3)
	(Val_GNU_AVR_VTABLE_FLASH4, Val_GNU_AVR_VTABLE_FLASH5)
	(Val_GNU_AVR_VTABLE_FLASHX, Val_GNU_AVR_VTABLE_Sentinel): New
	enum values for Tag_GNU_AVR_VTABLE_AS.
	(avr_tag_vtable_as_name): New static function.

bfd/
	* elf32-avr.h (bfd_avr_elf_merge_private_bfd_data): New proto.
	* elf32-avr.c (libiberty.h): Include for ARRAY_SIZE.
	(avr_elf_merge_obj_attributes): New static function.
	(bfd_avr_elf_merge_private_bfd_data): New function that calls it.
	(bfd_elf32_bfd_merge_private_bfd_data): Define to
	bfd_avr_elf_merge_private_bfd_data.

gas/
	* doc/as.texi (GNU Object Attributes) [AVR Attributes]: New subsection.

ld/
	* testsuite/ld-avr/attr-gnu-4-1.s: New source.
	* testsuite/ld-avr/attr-gnu-4-2.s: New source.
	* testsuite/ld-avr/attr-gnu-4-1.d: New test.
	* testsuite/ld-avr/attr-gnu-4-2.d: New test.
	* testsuite/ld-avr/attr-gnu-4-1_2.d: New test.

binutils/
	* readelf.c (display_avr_gnu_attribute): New static function.
	(process_arch_specific) <EM_AVR>: Call process_attributes
	with display_avr_gnu_attribute as a callback.
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