A question about subsumption in register allocators

A have a question about the use of register subsumtion in practical
graph coloring register allocators.


Before the actual coloring, subsumption is normally done, to remove
unnecessary copy operations, and to make the graph to be coloured
smaller.


No the question is: What do register allocators out there do if there
is more than one way of doing the subsumption, and they are mutually
incompatible?


{ define a }
if ... then
x := a
...
else
y := a
...
fi
{ use x, y }


Assuming there is no conflict between x and a, or y and a, but there
is a conflict between x and y, then we have a choice in merging either x
and a, or y and a.
So how do register allocators out there make the choice?


-- Thomas Johnsson
--

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