var_args for rs6000 backend

[Yao qi]

I am working on variable arguments on rs6000 backend  and I have browsed 
gcc/confi        g/rs6000/rs6000.c for several times,  I found there are 
some functions relavtive to this issue, they are  setup_incoming_varargs, 
rs6000_build_builtin_va_list ,rs6000_va_start  ,rs6000_gimplify_va_arg .

I could not know what they do just by source code.   Could anybody tell me 
the relationship among these routines?  I think it  is important for me to 
understand the mechnism of GCC backend.  Any comments or advice are highly 
appreciate.

I do not know what is the precondation if I want to do it?  May I need to  
know the architecture of PowerPC and ABI for it?

Any one could clarify it for me.  Thanks in advance!


Best Regards
----------------
Yao Qi
Bejing Institute of Technology

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