[LLVMdev] [PowerPC] ABI questions

[Hal Finkel]

----- 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.

Thanks again,
Hal

> 
> Bye,
> Ulrich
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory