[llvm-commits] [llvm-gcc] static chain changes for PR1146

[Devang Patel]

Applied.

-
Devang

On May 7, 2007, at 1:15 AM, Duncan Sands wrote:

> Hi Reid,
>
>> I'm working on PR1146. It remove parameter attributes from function
>> types and places them on functions can call/invoke instructions.
>> Consequently, some changes are needed in llvm-gcc. One of those  
>> changes
>> I'm unsure about. In llvm-types.cpp, in the ConvertArgListToFnType
>> function, we have this pice of code:
>>
>>  if (static_chain) {
>>    // Pass the static chain in a register.
>>    ParamAttrsVector Attrs;
>>    ParamAttrsWithIndex PAWI; PAWI.index = 1; PAWI.attrs =
>> ParamAttr::InReg;
>>    Attrs.push_back(PAWI);
>>    PAL = ParamAttrsList::get(Attrs);
>>  }
>>
>> Obviously this needs to be deleted as a ParamAttrsList is not  
>> needed to
>> instantiate a FunctionType object.  The question is, where does this
>> need to be moved to. The static chain has something to do with nested
>> functions, right? So, do we need to move this "inreg" attribute to  
>> the
>> place where the call is actually made?  Where is that?
>
> I think the inreg attribute should not be applied to the static chain
> parameter at all.  The attached patch removes it.  This also fixes a
> bug in which a nested function returning a struct was getting the  
> inreg
> attribute on the struct return parameter, and the sret attribute on  
> the
> static chain parameter!
>
> I originally marked the static chain inreg because gcc passes it in a
> register (ecx on x86).  Inreg is not needed for correctness or even  
> for
> compatibility (nested functions are always internal, so cannot be  
> called
> from code compiled by another compiler).  And I doubt it is even  
> needed
> for efficiency - after all, nested functions end up having the fastcc
> calling convention because they are internal, which means all the
> parameters will be passed in registers anyway!  Marking it inreg is
> just trying to second-guess the code generators, and I don't see why
> the static chain should be favored more than any other parameter.  A
> final comment: gcc is pretty much obliged to pass the static chain in
> a register, because it is not in the list of function parameters  
> (unlike
> in LLVM), so it has to pass it specially.  So the fact that gcc  
> passes it
> in a register is not necessarily a sign that using a register is a  
> good
> idea performance wise.
>
> Ciao,
>
> Duncan.
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