+ /// VarInfo - This represents the regions where a virtual register is live in
+ /// the program. We represent this with three difference pieces of
+ /// information: the instruction that uniquely defines the value, the set of
+ /// blocks the instruction is live into and live out of, and the set of
+ /// non-phi instructions that are the last users of the value.
+ ///
+ /// In the common case where a value is defined and killed in the same block,
+ /// DefInst is the defining inst, there is one killing instruction, and
+ /// AliveBlocks is empty.
+ ///
+ /// Otherwise, the value is live out of the block. If the value is live
+ /// across any blocks, these blocks are listed in AliveBlocks. Blocks where
+ /// the liveness range ends are not included in AliveBlocks, instead being
+ /// captured by the Kills set. In these blocks, the value is live into the
+ /// block (unless the value is defined and killed in the same block) and lives
+ /// until the specified instruction. Note that there cannot ever be a value
+ /// whose Kills set contains two instructions from the same basic block.
+ ///
+ /// PHI nodes complicate things a bit. If a PHI node is the last user of a
+ /// value in one of its predecessor blocks, it is not listed in the kills set,
+ /// but does include the predecessor block in the AliveBlocks set (unless that
+ /// block also defines the value). This leads to the (perfectly sensical)
+ /// situation where a value is defined in a block, and the last use is a phi
+ /// node in the successor. In this case, DefInst will be the defining
+ /// instruction, AliveBlocks is empty (the value is not live across any
+ /// blocks) and Kills is empty (phi nodes are not included). This is sensical
+ /// because the value must be live to the end of the block, but is not live in
+ /// any successor blocks.