[LLVMdev] Request for merge: GHC/ARM calling convention.

[David Terei]

On 15 October 2011 00:31, Duncan Sands <baldrick at free.fr> wrote:
> Hi David,
>
>> The code above is needed as the GHC calling convention redefines what
>> registers are considered callee save. No one else rummages in to the
>> original function as all the other calling conventions use the same
>> set of callee and caller save registers, so GHC is the only one that
>> needs to differentiate.
>
> shouldn't the caller also know what registers are callee saved so that it
> can avoid saving the contents of those registers to the stack?

I assume so, I'm not an expert on LLVM or this area at all really.
What's the follow on logic / question here?

>
>>>> Based on knowledge of calling convention we return different set of
>>>> callee saved
>>>> registers. For example on ARM and when GHC CC is used, we return empty
>>>> set.
>>>
>>> yeah, but here MF is (presumably) the callee.  The callee shouldn't be
>>> relevant
>>> to how arguments are marshalled at the call-site.  Especially as there
>>> might not
>>> even be a callee function (case of an indirect call).  Since calls
>>> themselves
>>> are annotated with the calling convention to use, I guess the calling
>>> convention
>>> should be grabbed from the call instruction itself somehow.
>>
>> This isn't about the call-site.
>
> I see, thanks for explaining.  But then can the MachineFunction really be
> null?

Yes. For the X86 equiv of this code (X86RegisterInfo.cpp) the 'if
(MF)' lines already existed before my patch. So I assume the existing
code was correct. A quick grep for calls to 'getCalleeSavedRegs' also
shows some calls in some optimisation passes that don't pass in an
argument and the default argument is null.

Hope this all helps. Is there someone who 'owns' this section of the
code that we can rope into the discussion though as I'm not that
qualified.

Cheers,
David