[patch, avr, v2] RFC: Support .gnu_attribute 4 (Tag_GNU_AVR_VTABLE_AS) for vtable address spaces

[Georg-Johann Lay]

This is an updated version of the .gnu_attribute Tag_GNU_AVR_VTABLE_AS
proposal

https://sourceware.org/pipermail/binutils/2026-June/149591.html

 > The expected, upcoming support for ISO/IEC TR 18037 "Embedded C"
 > named address spaces in G++ would allow to put C++ virtual tables
 > into such address spaces, see https://gcc.gnu.org/PR69549
 >
 > Currently, vtables are in the generic address space, which wastes
 > RAM on cores that locate .rodata in RAM.
 >
 > Any change of vtables' address space is an ABI change, and
 > therefore it would be nice to have a feature that checks that
 > all object files linked into an app are using the same vtable AS.
 >
 > The patch below introduces .gnu_attribute 4 (Tag_GNU_AVR_VTABLE_AS).
 >
 > This mail is a request for a pre-approval of the patch.
 >
 > I don't intend to apply it until it is clear that it's really
 > of any use and not dead code.  Sadly, PR69549 is only progressing
 > at a glacial pace, so it may take yet another 10 year before
 > G++ can handle named address spaces --- or it may never learn it.
 >
 > The patch is for trunk.

The difference is that GAS doesn't error on unknown attribute values,
preparing for possible future extensions.

Johann

--

AVR: Add support for .gnu_attribute 4 (Tag_GNU_AVR_VTABLE_AS).

The expected, upcoming support for ISO/IEC TR 18037 "Embedded C"
named address spaces in G++ would allow to put C++ virtual tables
into such 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.

Named address spaces in G++ would allow users to chose an address space
that suits their requirements, e.g. by means of a new command-line
option and GCC hooks like TARGET_ADDR_SPACE_FOR_ARTIFICIAL_RODATA.

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) for files that use the generic address space.
2 (Val_GNU_AVR_VTABLE_FLASH) the file uses the 16-bit address space __flash.
3 (Val_GNU_AVR_VTABLE_FLASH1) the file uses the 16-bit address space 
__flash1.
4 (Val_GNU_AVR_VTABLE_FLASH2) the file uses the 16-bit address space 
__flash2.
5 (Val_GNU_AVR_VTABLE_FLASH3) the file uses the 16-bit address space 
__flash3.
6 (Val_GNU_AVR_VTABLE_FLASH4) the file uses the 16-bit address space 
__flash4.
7 (Val_GNU_AVR_VTABLE_FLASH5) the file uses the 16-bit address space 
__flash5.
8 (Val_GNU_AVR_VTABLE_FLASHX) the file uses 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.

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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