+// This file implements the LiveVariable analysis pass. For each machine
+// instruction in the function, this pass calculates the set of registers that
+// are immediately dead after the instruction (i.e., the instruction calculates
+// the value, but it is never used) and the set of registers that are used by
+// the instruction, but are never used after the instruction (i.e., they are
+// killed).
+//
+// This class computes live variables using are sparse implementation based on
+// the machine code SSA form. This class computes live variable information for
+// each virtual and _register allocatable_ physical register in a function. It
+// uses the dominance properties of SSA form to efficiently compute live
+// variables for virtual registers, and assumes that physical registers are only
+// live within a single basic block (allowing it to do a single local analysis
+// to resolve physical register lifetimes in each basic block). If a physical
+// register is not register allocatable, it is not tracked. This is useful for
+// things like the stack pointer and condition codes.
+//