[LLVMdev] [PowerPC] ABI questions

[Bill Schmidt]

On Thu, 2014-07-31 at 07:48 -0500, Hal Finkel wrote:
> ----- Original Message -----
> > From: "Ulrich Weigand" <Ulrich.Weigand at de.ibm.com>
> > To: "David Wiberg" <dwiberg at gmail.com>
> > Cc: llvmdev at cs.uiuc.edu
> > Sent: Wednesday, July 30, 2014 2:29:22 PM
> > Subject: Re: [LLVMdev] [PowerPC] ABI questions
> > 
> > Hi David,
> > 
> > > I'm trying to understand which ABIs are supported in the PowerPC
> > > backend and I'm getting a bit confused. Here's what I've gathered
> > > so
> > > far alongside with some questions.
> > 
> > Sorry for the confusion, this all should probably be cleaned up a
> > bit.
> > In general, LLVM today supports the following ABIs:
> > - The Darwin ABI (32-bit and 64-bit flavors)
> > - The 32-bit SVR4 ABI
> > - Two variants of the 64-bit SVR4 ABI: ELFv1 and ELFv2
> > 
> > The ELFv1 ABI is used on 64-bit big-endian Linux and AIX.
> > The ELFv2 ABI is used on 64-bit little-endian Linux.
> > 
> > Unfortunately, the term "SVR4" is quite overloaded, and applies
> > both to the 32-bit and the 64-bit ABIs, even though they are
> > quite different.  "ELF ABI" is used interchangably with "SVR4 ABI".
> > 
> > ELFv1 is a recently introduced term to refer to what was previously
> > just called the ELF ABI (or equivalently SVR4 ABI); it is now used
> > to refer to the *old* variant of the ELF ABI as opposed to the new
> > one (ELFv2).
> > 
> > > - In PPCSubtarget.h there's DarwinABI, SVR4ABI and ELFv2ABI.
> > > - The CodeGenerator documentation claims that the AIX PowerPC ABI
> > > is
> > > followed (with some deviations). Is this refering to the DarwinABI?
> > 
> > No, that's the ELFv1 ABI.
> > 
> > > - In a recent commit a TargetABI value and enumeration was added to
> > > PPCSubtarget which contains PPC_ABI_UNKNOWN, PPC_ABI_ELFv1 and
> > > PPC_ABI_ELFv2. For 64-bit non-Darwin the TargetABI value is set to
> > > either ELF-variant in resetSubtargetFeatures(...) based on
> > > endianess
> > > (unless explicitly set previously). As I understand it ELFv2 is a
> > > new
> > > ABI but which ABI does ELFv1 map to?
> > 
> > I hope the above makes it clearer.
> > 
> > > - Since isSVR4ABI() is defined as !isDarwin() it currently is
> > > possible
> > > for SVR4 and ELFv2 to be true at the same time which doesn't seem
> > > correct to me.
> > 
> > isSVR4ABI is true for either ELFv1 or ELFv2, and also for the 32-bit
> > SVR4 ABI.  This should probably be cleaned up by having three
> > separate
> > predicates ...
> > 
> > > - My understanding of SVR4ABI have been that it is the 32-bit PPC
> > > processor supplement from Sun. Is this correct? If so, which ABI is
> > > used for 64-bit non-Darwin?
> > 
> > SVR4 has always been used for either the 32-bit or 64-bit ELF ABIs
> > (*both* are processor supplement documents based on the original
> > Sun ELF/SVR4 ABI).
> > 
> > > I'm trying to implement the PPC EABI (which is based on the
> > > SVR4ABI)
> > > for an out-of-tree subtarget which means I need to understand the
> > > current status a bit better.
> > 
> > It's up to Hal, but I'd suggest to split up the isSVR4ABI predicate
> > into three separate options, e.g. isSVR4ABI (or maybe isSVR4_32ABI),
> > isELFv1ABI, and isELFv2ABI; and then add another one for your new
> > ABI.
> 
> I'd certainly welcome this. The current state of affairs seems to be the result of evolution more than design, and cleaning this up seems like a good idea.

I may have asked this before a long time ago, but:  How would people
feel about eradicating the use of the term SVR4 and replacing it with
ELF throughout?  I really don't like the dated SVR4 terminology very
much.

Thanks,
Bill

> 
> Thanks again,
> Hal
> 
> > 
> > Bye,
> > Ulrich
> > 
> > 
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