[AArch64 ELF ABI] Vector calls and lazy binding on AArch64

Florian Weimer fweimer@redhat.com
Wed May 22 15:06:00 GMT 2019


* Szabolcs Nagy:

> AAELF64: in the Symbol Table section add
>
>  st_other Values
>      The  st_other  member  of  a symbol table entry specifies the symbol's
>      visibility in the lowest 2 bits.  The top 6 bits  are  unused  in  the
>      generic  ELF ABI [SCO-ELF], and while there are no values reserved for
>      processor-specific semantics, many other architectures have used these
>      bits.
>
>      The  defined  processor-specific  st_other  flag  values are listed in
>      Table 4-5-1.
>
>  Table 4-5-1, Processor specific st_other flags
>              +------------------------+------+---------------------+
>              |Name                    | Mask | Comment             |
>              +------------------------+------+---------------------+
>              |STO_AARCH64_VARIANT_PCS | 0x80 | The        function |
>              |                        |      | associated with the |
>              |                        |      | symbol may follow a |
>              |                        |      | variant   procedure |
>              |                        |      | call  standard with |
>              |                        |      | different  register |
>              |                        |      | usage convention.   |
>              +------------------------+------+---------------------+
>
>      A  symbol  table entry that is marked with the STO_AARCH64_VARIANT_PCS
>      flag set in its st_other field may be associated with a function  that
>      follows  a  variant  procedure  call  standard with different register
>      usage convention from the one  defined  in  the  base  procedure  call
>      standard  for  the  list  of  argument,  caller-saved and callee-saved
>      registers [AAPCS64].  The rules  in  the  Call  and  Jump  relocations
>      section  still  apply to such functions, and if a subroutine is called
>      via a symbol reference that  is  marked  with  STO_AARCH64_VARIANT_PCS
>      then  code that runs between the calling routine and called subroutine
>      must preserve the contents of all registers except IP0,  IP1  and  the
>      condition code flags [AAPCS64].

Can you clarify if there has to be a valid stack at this point which can
be used during the call transfer?  What about the stack alignment
requirement?

Thanks,
Florian



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