What is the motivation and benefits for changing the calling convention?
I was sure I remembered you posting before sometime on one of the
mailing lists about changing the STM8 calling convention with some
reasons, but I can't find anything now.
Personally, my comments are that I think the proposed new calling
convention will make life difficult when writing assembly code that is
to be called by C code. In my opinion, the current STM8 calling
convention suits that scenario just fine.
Currently, where all arguments are on the stack, this gives more
flexibility to use A/X/Y registers however one wants, and the function
arguments are always there when you need them, without worrying about
overwriting or preserving them. With the new convention, one typically
would have to push A/X onto the stack immediately, and then use the
values from there. If the aim is to reduce stack usage, then it would
seem to be counter-productive in this respect.
I can see a little benefit of having the first 8/16-bit argument passed
in A/X, as you may want to immediately operate on the argument value
anyway (a lot of my assembly immediately does "ld a, (3,sp)" or "ldw x,
(3,sp)" - you'd save that operation), but it's troublesome in that if
you want to refer again to the argument value later, you would have
needed to have saved it yourself. Any benefit will probably only be for
the most trivial of single-argument functions.
SDCC 4.1.0 will likely use a different calling convention from the
current one for stm8.
Did you mean 4.2.0? Obviously, 4.1.0 is already out. :)
This aspect is similar to Raisonance, and to some degree IAR.
Is this a goal, to have some kind of binary compatibility with those
compilers?
For the medium memory model, stack cleanup for stack parameters is
done by the calle for some cases
Typo obscures understanding here: is that supposed to be "caller" or
"callee"?
Regards,
Basil Hussain
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