Re: [PATCH] add initial support for J2 core to sh target

[Oleg Endo <oleg.endo@t-online.de>]

On 02 Sep 2015, at 02:08, Rich Felker <dalias@libc.org> wrote:

> On Wed, Sep 02, 2015 at 01:24:55AM +0900, Oleg Endo wrote:
>>> I'm not sure what the best way to achieve multiple goals is, but the
>>> current behavior makes it so you need --isa=any (and a final binary
>>> with weird ABI tag) to have a binary that supports atomic operations
>>> on any SH model. musl libc already has such support (except the new J2
>>> CAS instruction) and I would like to eventually provide a libatomic
>>> approach for GCC too so that it's possible to use __sync/C11 atomics
>>> and have the binary be safe to run on any model that supports the
>>> baseline ISA & ABI you built for (e.g. all >=SH2 if you used -m2).
>> 
>> I don't know the details of your implementation. The compiler
>> generated atomic sequences are not really compatible. The safest
>> thing is not to enable any atomic model in GCC and let it emit
>> function calls to __atomic*.
> 
> Exactly -- but then, libatomic.a needs to contain J2-specific cas.l
> opcodes and SH4A-specific movli.l/movco.l opcodes and code that
> selects at runtime which to use (or whether to use imask or gusa)
> based on hwcap, etc. The point is that a mix of opcodes for different
> ISA levels end up being in the final binary, which might otherwise be
> targeted for SH-2 baseline so it can run on any of them.
> 
>>> I have a patch for that part, just not expanding the
>>> already-very-complex SH "family-tree" of instruction support. However
>>> it's likely that encoding details will change (the draft encoding
>>> overlaps with something used by SH2A IIRC, and the intent was not to
>>> have such overlap)
>> 
>> Yeah, it overlaps with the first 16 bit word of the 32 bit SH2A
>> load/store insns.
>> 
>>> so I'm holding off on submitting it until the
>>> hardware side works out this issue.
>> 
>> Sounds reasonable.
> 
> In the mean time, do you have any suggestsions on how the ISA level
> stuff should be done to add J2 on the binutils side?

Let's continue this topic on the binutils list
( https://sourceware.org/ml/binutils/2015-09/msg00031.html )