[LLVMdev] Few questions about stack frame and calling conventions implementation in a backend

Andrew Lenharth andrewl at lenharth.org
Sun Apr 18 07:59:44 PDT 2010

On Fri, Apr 16, 2010 at 7:21 AM, Artur Pietrek <pietreka at gmail.com> wrote:
> Hi Andrew,
> thanks for answering
> On Thu, Apr 15, 2010 at 3:35 PM, Andrew Lenharth <andrewl at lenharth.org>
> wrote:
>> 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.
> There is no specific register needed here. It's just dedicated RA register
> is a special register that could be read/set via a GPR using get/set
> instructions only. So this is why I need a GPR register in
> prologue/epilogue.
> Aren't prologue/epilogue run after register allocation? If I copy to a
> virtual register, wouldn't I have to rerun the RA pass?

I think I was suggesting that you could reserve a specific physical
register for this purpose.  By copying into a virtual you free the
register during the body to be used for other stuff.  But you always
have the physical register for prologue/epilogue (by ensuring the RA
wouldn't use it in the prologue/epilogue).

>> > 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.
> There is no way of doing that in the backend?

The problem is that is is the C ABI that says this, which is currently
handled in the front end.  You can have the front end pass structs
directly (if I remember correctly) and do this yourself, but
specifying it in the front end is all of a line or two of code (this
is a standard ABI thing that the front end handles on several


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