RFC: A new MIPS64 ABI

[David Daney]

On 02/14/2011 12:29 PM, David Daney wrote:
> Background:
>
> Current MIPS 32-bit ABIs (both o32 and n32) are restricted to 2GB of
> user virtual memory space. This is due the way MIPS32 memory space is
> segmented. Only the range from 0..2^31-1 is available. Pointer
> values are always sign extended.
>
> Because there are not already enough MIPS ABIs, I present the ...
>
> Proposal: A new ABI to support 4GB of address space with 32-bit
> pointers.
>
> The proposed new ABI would only be available on MIPS64 platforms. It
> would be identical to the current MIPS n32 ABI *except* that pointers
> would be zero-extended rather than sign-extended when resident in
> registers. In the remainder of this document I will call it
> 'n32-big'. As a result, applications would have access to a full 4GB
> of virtual address space. The operating environment would be
> configured such that the entire lower 4GB of the virtual address space
> was available to the program.
>
>
> At a low level here is how it would work:
>
> 1) Load a pointer to a register from memory:
>
> n32:
> LW $reg, offset($reg)
>
> n32-big:
> LWU $reg, offset($reg)
>
> 2) Load an address constant into a register:
>
> n32:
> LUI $reg, high_part
> ORI $reg, low_part

That is not reality.  Really it is:

LUI $reg, R_MIPS_HI16
ADDIU $reg, R_MIPS_LO16


>
> n32-big:
> ORI $reg, high_part
> DSLL $reg, $reg, 16
> ORI $reg, low_part
>

This one would really be:

ORI $reg, R_MIPS_HI16
DSLL $reg, $reg, 16
ADDIU $reg, R_MIPS_LO16


>
> Q: What would have to change to make this work?
>
> o A new ELF header flag to denote the ABI.
>
> o Linker support to use proper library search paths, and linker scrips
> to set the INTERP program header, etc.
>
> o GCC has to emit code for the new ABI.
>
> o Could all existing n32 relocation types be used? I think so.
>
> o Runtime libraries would have to be placed in a new location
> (/lib32big, /usr/lib32big ...)
>
> o The C library's ld.so would have to use a distinct LD_LIBRARY_PATH
> for n32-big code.
>
> o What would the Linux system call interface be? I would propose
> using the existing Linux n32 system call interface. Most system
> calls would just work. Some, that pass pointers in in-memory
> structures, might require kernel modifications (sigaction() for
> example).
>