+class LiveRegMatrix;
+
+/// A bitmask representing the parts of a register are alive.
+///
+/// Lane masks for sub-register indices are similar to register units for
+/// physical registers. The individual bits in a lane mask can't be assigned
+/// any specific meaning. They can be used to check if two sub-register
+/// indices overlap.
+///
+/// If the target has a register such that:
+///
+/// getSubReg(Reg, A) overlaps getSubReg(Reg, B)
+///
+/// then:
+///
+/// (getSubRegIndexLaneMask(A) & getSubRegIndexLaneMask(B)) != 0
+///
+/// The converse is not necessarily true. If two lane masks have a common
+/// bit, the corresponding sub-registers may not overlap, but it can be
+/// assumed that they usually will.
+typedef unsigned LaneBitmask;