[cfe-dev] Passing structs byval

[Chris Lattner]

On Apr 28, 2008, at 2:11 AM, Matthijs Kooijman wrote:
> I'm looking at the argpromotion optimizer pass in LLVM, but it  
> doesn't seem to
> reliably work right now in combination with clang (didn't try with  
> llvm-gcc).

Ok.  Most optimization stuff has only been looked at with llvm-gcc.

> When passing a struct to a function, clang currently generates a  
> call to
> memcpy and passes the pointer to the (temp copy of the) struct to the
> function. ArgPromotion can succesfully promote this struct argument  
> (i.e.,
> replace it by multiple scalar arguments) but only if the called  
> function does
> not modify the struct.

I don't think there is any specific reason for this, it is just what  
was easiest to do at the time.  We should follow llvm-gcc's lead and  
generate byval.

> Any suggestions on how and where to implement this byval passing of  
> structs?
> I'm still digging around in the source code, following call paths to  
> see what
> actually happens right now :-)

CodeGen/CGExprAgg.cpp and friends is the most relevant.  Thanks for  
working on this!

-Chris