Jakob Stoklund Olesen 6501ea2557 Prefer cheap registers for busy live ranges.
On the x86-64 and thumb2 targets, some registers are more expensive to encode
than others in the same register class.

Add a CostPerUse field to the TableGen register description, and make it
available from TRI->getCostPerUse. This represents the cost of a REX prefix or a
32-bit instruction encoding required by choosing a high register.

Teach the greedy register allocator to prefer cheap registers for busy live
ranges (as indicated by spill weight).

llvm-svn: 129864
2011-04-20 18:19:48 +00:00
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