---------------- live on entry/exit info ------------------------------
This structure is used to represent live range information on SSA based
trees. A partition map must be provided, and based on the active partitions,
live-on-entry information and live-on-exit information can be calculated.
As well, partitions are marked as to whether they are global (live
outside the basic block they are defined in).
The live-on-entry information is per block. It provide a bitmap for
each block which has a bit set for each partition that is live on entry to
that block.
The live-on-exit information is per block. It provides a bitmap for each
block indicating which partitions are live on exit from the block.
For the purposes of this implementation, we treat the elements of a PHI
as follows:
Uses in a PHI are considered LIVE-ON-EXIT to the block from which they
originate. They are *NOT* considered live on entry to the block
containing the PHI node.
The Def of a PHI node is *not* considered live on entry to the block.
It is considered to be "define early" in the block. Picture it as each
block having a stmt (or block-preheader) before the first real stmt in
the block which defines all the variables that are defined by PHIs.
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