Re: genesi and open rd
On Monday 08 February 2010, Martin Guy wrote:
> On 2/8/10, Karsten König <remur@gmx.net> wrote:
> > OT: Are there plans to raise this to ARMv5 or even higher after Squeeze?
> > Or are the benefits not worth dropping the older devices?
>
> I hope not. armv4t is pretty widespread in single-board computers,
> while the v5 benefits are vanishingly small: one extra instruction
> that only really helps the softfloat division assembly routine.
It's a bit more than that. Specifically:
- Interworking sucks less. Many/most v4t functions require an extra
instruction in the return sequence. The EABI required code be interworking
safe, so you pay for this even if you're not actually using Thumb code. In
Thumb mode the overhead can be significantly worse.
- More efficient doubleword load/store. Can make a fair difference for heavy
64bit arithmetic, though in practice this is not so important for general
purpose code.
- A couple of bit-counting instructions you mentioned above. Helps division
(float and integer) and a few other specific algorithms.
N.B. the last two are v5te rather than v5t. However v5t hardware is fairly
rare (a few old arm10 cores) so we can probably ignore it. i.e. once we decide
v4t is dead go straight to v5te.
Paul
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