[LLVMdev] Is there any tool can generate MIPS ELF file?

[Daniel Sanders]

> There are a lot of MIPS ABIs.

Yes, and we've discovered that there seem to be incompatible extensions to some of these ABI's too.

> I'm pretty sure Imagination Technologies working up a new abi right now.

Not exactly. We're not working on any completely new ABI's but we are fixing a compatibility flaw between the O32 ABI and a (currently) unsupported extension that allows the efficient use of FR=1 mode (FPU with 64-bit registers). This is going to be important since MIPS32r6/MIPS64r6 will not have direct support for FR=0 mode (FPU with 32-bit registers). At the moment, this extension is available in all the tools and is enabled with -mfp64. Unfortunately, it is not possible to inter-link O32 and O32+fp64 code since they require the FPU to be in different modes. To fix the compatibility flaw, we are adding an O32 extension called fpxx (enabled with -mfpxx) which operates correctly in both FPU modes and is essentially O32 with some minor restrictions. O32+fpxx is ABI-compatible with O32 and O32+fp64 and can inter-link with either but not both at the same time (because they conflict with eachother). The intention is that as code is re-compiled, O32+fpxx will replace O32 and we will end up the mode requirements being either 'FR=1' or "don't care".

In case you're curious, the fpxx restrictions are:
* Use of mtc1, mfc1, lwc1, and swc1 on the upper 32-bits of a double precision value is not permitted.
* Some callee-saved registers are also treated as caller-saved.

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Eric Christopher
> Sent: 18 June 2014 18:48
> To: Matheus Almeida
> Cc: LLVM Developers Mailing List
> Subject: Re: [LLVMdev] Is there any tool can generate MIPS ELF file?
> 
> On Wed, Jun 18, 2014 at 2:03 AM, Matheus Almeida
> <Matheus.Almeida at imgtec.com> wrote:
> >> Why Imagination Technologies do not offer the latest MIPS ABI document
> download link just like the ISA docs?
> > It's something we're considering to do and the documents should be
> available at some point in the [hopefully] not too distant future.
> >
> >> then why GCC disagree with some MIPS ABI, it should be freely designed
> by MIPS ABI designer and compiler backend target implementation.
> > I don't have a very good explanation to this question. The ABIs were
> implemented a long time ago and I can only guess at this point but I'd say that
> there were some misinterpretations of the spec and given the popularity of
> GCC, the implementation became the new standard and it's something that
> LLVM needs to cope with.
> >
> 
> There are a lot of MIPS ABIs.
> 
> o32, n32, n64 - These came from the SGI world for IRIX originally, but were
> also adopted for linux and used in gcc for both.
> o64 - Invented at Cygnus for 64-bit processors eabi - Different one invented
> at Cygnus meabi - Invented with MIPS and Cygnus (and possibly others) in
> the late 90s as a new embedded abi.
> And there's been at least one or two other ABIs since - I'm pretty sure
> Imagination Technologies working up a new abi right now.
> 
> -eric
> 
> > Regards,
> > Matheus
> >
> > -----Original Message-----
> > From: Nancy [mailto:nancydreaming at gmail.com]
> > Sent: 17 June 2014 13:56
> > To: Matheus Almeida
> > Cc: LLVM Developers Mailing List
> > Subject: Re: [LLVMdev] Is there any tool can generate MIPS ELF file?
> >
> > Thank you very much for your information and documents!
> >
> > Why Imagination Technologies do not offer the latest MIPS ABI document
> download link just like the ISA docs? If they thought no much people
> interested in that doc, they had to make greate effort on compiler like
> GCC,LLVM by themself,then why GCC disagree with some MIPS ABI, it
> should be freely designed by MIPS ABI designer and compiler backend target
> implementation. Do I miss something?
> >
> > --
> > Best Regards,
> > Yu Rong Tan
> >
> > On Mon, Jun 16, 2014 at 5:30 PM, Matheus Almeida
> <Matheus.Almeida at imgtec.com> wrote:
> >> Could you be more specific about what ABIs are you after ? The reason
> I'm asking is that there are several ABIs available for Mips and only a few of
> them are supported by LLVM (o32, n32 and n64). There are several ABIs
> defined by GNU with very little documentation that describes them (EABI is
> one example).
> >>
> >> We are aware that the available documentation and GCC disagree
> sometimes and given that GCC is the de-facto standard, LLVM tries to follow
> GCC's behaviour as close as possible.
> >>
> >> As far as I know there isn't any link from Imagination Technologies'
> website that I can point you to w.r.t ABIs but there's documentation out
> there like:
> >> https://dmz-portal.mips.com/mw/images/f/fe/MD00305-2B-ABIDESC-
> SPC-01.03.pdf   // This is probably the best document as it compares the 3
> most important ABIs. Note that it's an old document and needs some
> refactoring given that the implementation in GCC sometimes differs from the
> documentation.
> >> http://math-atlas.sourceforge.net/devel/assembly/mipsabi32.pdf // O32
> >> ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/MIPS-N32-ABI-
> Handbook.
> >> pdf //n32
> >>
> >> Regards,
> >> Matheus
> >>
> >
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