[Open64-devel] Question on Aliases & register promotion

[Peng Z. <pen...@cs...>]

Hi,
     BB1
    /   \
  BB2   BB3
    \   /
     BB4

Let's consider the above dimond shape in a CFG. Assume BB1->BB2->BB4
is a very hot path and BB3 is rarely touched. And in BB3, there is a 
function call my_call(&V). V is used in all the four BBs and its address
is passed to my_call() in BB3. This inter-procedural alias relationship 
makes it unsafe to keep V in 
registers. ORC is smart in that it tries to put V in register and then 
spill it to memory when possible. However, if without frequency 
information, ORC might spill V in the hot paths. For example, I found that 
ORC keeps spilling V at the end of BB1, causing lot more instructions and 
performance penalities. It would be much better to spill
V  in BB3 (just before the function call my_call()).
Can somebody tell me how ORC makes the decision on where to spill a 
varialbe because of aliases? And where is it implemented in ORC?
Why doesnot ORC spill V as close to the call site (therefore in BB3) as
possible? Is there anyother factors considered or the compiler is at a
total different perspective with me?

Thanks.