[LLVMdev] [PowerPC] ABI questions

[Ulrich Weigand]

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.

Bye,
Ulrich