Re: Introducing a nanoMIPS port for Binutils, GDB and GOLD

[Joseph Myers <joseph at codesourcery dot com>]

On Tue, 1 May 2018, Matthew Fortune wrote:

> [2] Codescape GNU tools for nanoMIPS: ELF ABI Supplement,
>     https://codescape.mips.com/components/toolchain/nanomips/2018.04-02/docs/MIPS_nanoMIPS_ABI_supplement_01_02_DN00179.pdf

There seems to be a lot that is incomplete or inconsistent in this 
document (or inconsistent between it and other nanoMIPS documents).

To start with, do you have floating point in the architecture and ABI or 
not?  This ABI says it's purely a soft-float ABI.  But the p32 Porting 
Guide says there are 32 floating-point registers, of which 8 are argument 
registers.  Then in the ISA manual, I see no mention of floating point 
beyond avoiding it between LL and SC if it might trap.

Then your argument passing ABI says structs are treated as a sequence of 
32-bit chunks, either all in registers or all on the stack.  But then you 
give an example marking a struct as passed by reference, when the earlier 
description of argument passing made no mention of that.  I also see 
nothing about whether later arguments could go in registers after a struct 
was forced to the stack because it wouldn't all fit there.  I also see 
nothing about how non-struct/union arguments larger than 64 bits (e.g. 
_Complex double) are passed.  What's the argument promotion for an _Bool 
argument (i.e. as a byte it has to be 0 or 1, but are the high 24 bits 
defined as 0 when passed?)?  What about _Complex float when there's only 
room for one 32-bit value in a register - is it all on the stack, or split 
between registers and stack? (it's not 64-bit aligned in memory, so I'd 
expect it not to need 64-bit alignment when passed as an argument - the 
32-bit powerpc ABI *does* 64-bit align it in register arguments, but 
that's an artifact of documenting accidental existing practice, not 
something you want when designing a new ABI).

Then you list [signed] long twice in the table of types, as both 32-bit 
and 64-bit, presumably the latter needs to be long long.  long double is 
described as quad precision, but listed as 64-bit, which contradict each 
other (and it's not C99 only).

I see you're using GCC 6 at present, but when using GCC 7 or later you'll 
have _FloatN and _FloatNx types (TS 18661-3) as well, so it would be a 
good idea to document the ABIs for those.

Does the p32 ABI use PLTs (like most non-MIPS architectures) or not?

-- 
Joseph S. Myers
joseph@codesourcery.com