llvm dev - Apr 2010 - [LLVMdev] Few questions about stack frame and calling conventions implementation in a backend

Hi all,
I've been working for some time now on a backend for our CPU. However I
couldn't figure out how to implement some stuff.
I'd appreciate your help with these.

First thing is return address
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Hi all
Ups, I'm really sorry for that previous message, I've sent it by
mistake.

So let me write it once more.

I've been working for some time now on a backend for our CPU. However I
couldn't figure out how to implement some stuff.
I'd appreciate your help with these.

First thing is return address saving. To do that, first I have to copy it to
a general purpose register. I have no idea how to find an unused gpr
register in emitPrologue/emitEpilogue. I've noticed that in other backends
RegScavenger is used for that purpose, but not from inside of those methods.
So my question is how could I get an unused register from inside of these
methods?

And my second problem is with returning structures by value. ABI says that
aggregates up to 32bytes should be returned directly in register. Any advice
how this could be done?

Thanks
Artur
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On Thu, Apr 15, 2010 at 3:40 AM, Artur Pietrek <pietreka at gmail.com>
wrote:
> Hi all
> Ups, I'm really sorry for that previous message, I've sent it by
mistake.
>
> So let me write it once more.
>
> I've been working for some time now on a backend for our CPU. However I
> couldn't figure out how to implement some stuff.
> I'd appreciate your help with these.
>
> First thing is return address saving. To do that, first I have to copy it
to
> a general purpose register. I have no idea how to find an unused gpr
> register in emitPrologue/emitEpilogue. I've noticed that in other
backends
> RegScavenger is used for that purpose, but not from inside of those
methods.
> So my question is how could I get an unused register from inside of these
> methods?
Do you have an API specified register where the variable is passed?
If so, it is a matter of copying it to a virtual register in the
prologue which is then a live in to the first basic block.  If you
must have it in a specific register, you can make your ret inst take
it as a use and copy the virtual register back to the physical one for
the ret.
> And my second problem is with returning structures by value. ABI says that
> aggregates up to 32bytes should be returned directly in register. Any
advice
> how this could be done?
This has to be done in the front end, I think.

Andrew
>
> Thanks
> Artur
>
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