Re: Fwd: Bug#561891: Please support sh4
On Mon, Dec 21, 2009 at 10:16:40PM +0100, Jens Axboe wrote:
> On Tue, Dec 22 2009, Paul Mundt wrote:
> > SH-4A supports synco, SH-4 does not. SH-4A is based on the SH-4, but has
> > a lot of new instructions and so forth. Folks like to treat them the same
> > so as to keep the same ABI, but there are a lot of architectural
> > differences that are ideally not just glossed over. We have been bitten
> > by these in the past.
> >
> > Linux reports different uname values for all of SH-4, SH-4A, and
> > SH4AL-DSP largely for these reasons, and gcc also has different
> > preprocessor tokens for each. In this case, an __SH4A__ test can be done
> > for synco with the above definitions sufficing for all other variants.
> >
> > Where this silliness started was with FPU-less variants of SH-4 defining
> > __SH3__ because the gcc folks decided that an SH-4 without FPU
> > instructions was "close enough" to the SH-3 ISA, which while generally
> > true at the time, also ignored the fact that they have completely
> > different caches -- SH-4/SH-4A/SH4AL-DSP parts end up having to do
> > I-cache block invalidations from the libc, while SH-3 does not. SH-4 and
> > SH-4A preprocessor tokens on the other hand are sensibly abstracted, so
> > there is no reason to pretend like everything is SH-4.
>
> Thanks for the clarification, Paul. One last question... Will __SH4A__
> cover sh5 as well, or does it need a __SH5__?
>
That will need an __SH5__. SH-4A got things like synco from the SH-5 ISA,
so in most of these cases a test for __SH4A__ or __SH5__ will do. This is
how the kernel does it today at least (although only the instruction
mnemonic is preserved, the actual encoding and instruction size are
completely different).
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