[RFC] RISC-V: Support STO_RISCV_VARIANT_CC and DT_RISCV_VARIANT_CC.

[Fangrui Song]

On 2021-08-11, Nelson Chu wrote:
>This is the original discussion,
>https://github.com/riscv/riscv-elf-psabi-doc/pull/190
>
>And here is the glibc part,
>https://sourceware.org/pipermail/libc-alpha/2021-August/129931.html
>
>For binutils part, we need to support a new direcitve: .variant_cc.
>The function symbol marked by .variant_cc means it need to be resolved
>directly without resolver for dynamic linker.  We also add a new dynamic
>entry, STO_RISCV_VARIANT_CC, to indicate there are symbols with the
>special attribute in the dynamic symbol table of the object.
>
>This patch just passed the toolchain regressions.  I have not integrated
>the llvm and glibc parts, and tested them together.  Maybe someone can help
>to put them together for testing, that would be very great.
>
>bfd/
>	* elfnn-riscv.c (riscv_elf_link_hash_table): Added variant_cc
>	flag. It is used to check if relocations for variant CC symbols
>	may be present.
>	(allocate_dynrelocs): If the symbol has STO_RISCV_VARIANT_CC
>	flag, then raise the variant_cc flag of riscv_elf_link_hash_table.
>	(riscv_elf_size_dynamic_sections): Added dynamic entry for
>	variant_cc.
>	(riscv_elf_merge_symbol_attribute): New function, used to merge
>	non-visibility st_other attributes, including STO_RISCV_VARIANT_CC.
>binutils/
>	* readelf.c (get_riscv_dynamic_type): New function.
>	(get_dynamic_type): Called get_riscv_dynamic_type for riscv targets.
>	(get_riscv_symbol_other): New function.
>	(get_symbol_other): Called get_riscv_symbol_other for riscv targets.
>gas/
>	* config/tc-riscv.c (s_variant_cc): Marked symbol that it follows a
>	variant CC convention.
>	(riscv_elf_copy_symbol_attributes): Same as elf_copy_symbol_attributes,
>	but without copying st_other.  If a function symbol has special st_other
>	value set via directives, then attaching an IFUNC resolver to that symbol
>	should not override the st_other setting.
>	(riscv_pseudo_table): Support variant_cc diretive.
>	* config/tc-riscv.h (OBJ_COPY_SYMBOL_ATTRIBUTES): Defined.
>	* testsuite/gas/riscv/variant_cc-set.d: New testcase.
>	* testsuite/gas/riscv/variant_cc-set.s: Likewise.
>	* testsuite/gas/riscv/variant_cc.d: Likewise.
>	* testsuite/gas/riscv/variant_cc.s: Likewise.

Quick comment:

For LLD's STO_RISCV_VARIANT_CC patch, I asked what to do with
STT_GNU_IFUNC. The reply was that nothing is needed.
Does RISC-V do special things with IFUNC? I haven't looked at the patch.
https://reviews.llvm.org/D93045