[llvm-dev] Suggestion / Help regarding new calling convention

[John Criswell via llvm-dev]

On 6/20/16 9:39 AM, vivek pandya via llvm-dev wrote:
> Dear Community,
>
> To improve current interprocedural register allocation (IPRA) , we 
> have planned to set callee saved registers to none for local 
> functions, currently I am doing it in following way:
>
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {

As an aside, you might want to analyze how many functions have both 
local linkage and are not address taken.  I recall that many functions 
returned false for hasAddressTaken() because some direct calls casted 
the function to a different function type before calling it.  Such 
functions are still not address taken, but the simple hasAddressTaken() 
method can't determine it.

If you see that happening, you can simply scan through a function's 
def-use chains and see if any "indirect calls" are really direct calls 
that cast the function pointer.  I believe SAFECode has some code 
somewhere that does this if you need it.


>    DEBUG(dbgs() << "Function has LocalLinkage \n");
> F->setCallingConv(CallingConv::GHC);
>  }
>
> but we think threre should be clean and properway to do this perhaps like:
>
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>   }
>
> So I would like to know any better suggestions and if it is better to 
> add a new CC for this purpose then what aspects should be considered 
> while defining a new CC. Actually in this case the new CC does not 
> really required to define how parameters should be passed or any 
> special rule for return value etc , it just required to set callee 
> saved registers to be none. So what are the minimal things required to 
> define such a CC?
>
> Other alternative that I have thought was to add new attribute for 
> function and use it like following in 
> TargetFrameLowering::determineCalleeSaves()
>
>  // In Naked functions we aren't going to save any registers.
>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>     return;
>
> Any suggestions / thoughts are welcomed !

My humble opinion is that you should avoid hacks as they will likely 
break as LLVM changes.  If the GHC calling convention or the naked 
function attribute guarantee that you will always get the behavior that 
you want on all architectures, then go ahead and use them; just make 
sure to add a clear and conspicuous comment explaining why are you using 
them as it is not obvious.

If the GHC calling convention or the naked attribute does not guarantee 
to give you what you need, I'd add an attribute or a calling convention.

That said, I'm not familiar enough with the code generator or these 
attributes/calling conventions to tell you what you should do. You'll 
need input from others more familiar with them.

My two cents,

John Criswell

>
> Sincerely,
> Vivek
>
>
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-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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