Re: best ABI strategy ?

[Georg-Johann Lay <avr@gjlay.de>]

Aurelien Buhrig wrote:
> Hi,
> 
> I'm trying to optimize our target ABI, and I'm wondering what is the
> best strategy to define it.
> Is there a paper, or any hint, dealing with this topic ?
> For instance,
> - which ratio between call-used-reg vs "static regs"
> - which ratio between arg regs vs call-used-reg
> - what should be return regs
> - ...
> I guess this problem is very target dependent, but there are maybe
> general ideas about this.

If it's about new hardware/silicon/controller, it is a good idea to do a
controller compiler co-design. That way you can get the best out of a new
architecture as you have the most degrees of freedom to find a good point in
the ISA/ABI plane.

This requires of course to start the compiler development early and in a stage
where the ISA is still work in progress, i.e. there is no final silicon but
just an instruction set simulator for the upcoming hardware.

In many cases, you cannot really say what's the best except you actually
implement it and look how smooth it works.  My experience is that ISA designers
just think up to the assembler level, but have no idea how a compiler works and
what it needs and which features it can use and which not.

If the silicon/ISA is already fix and there is no feedback possible, it is
often easy to parametrize different ABI flavours through compiler switches so
you can benchmark, switch back and forth between ABIs and see what their pros
and contras are.

Johann