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

[Mehdi Amini via llvm-dev]

> On Jun 20, 2016, at 11:12 AM, John Criswell via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 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.

Looks like hasAddressTaken could be updated to handle these simple case maybe?

> 
> 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.  

In-tree code with appropriate tests is unlikely to break. 
But we should avoid hacks anyway because they make LLVM harder to change in general.


— 
Mehdi


> f 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 <http://www.cs.rochester.edu/u/criswell>_______________________________________________
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