http://clang.llvm.org/docs/InternalsManual.html#the-qualtype-class
the QualType class is designed to be an efficient value class which contains a pointer to the unqualified type together with the qualifiers (const, volatile etc.). The class is implemented as a llvm::PointerIntPair
/// PointerIntPair - This class implements a pair of a pointer and small /// integer. It is designed to represent this in the space required by one /// pointer by bitmangling the integer into the low part of the pointer. This /// can only be done for small integers: typically up to 3 bits, but it depends /// on the number of bits available according to PointerLikeTypeTraits for the /// type. /// /// Note that PointerIntPair always puts the IntVal part in the highest bits /// possible. For example, PointerIntPair<void*, 1, bool> will put the bit for /// the bool into bit #2, not bit #0, which allows the low two bits to be used /// for something else. For example, this allows: /// PointerIntPair<PointerIntPair<void*, 1, bool>, 1, bool> /// ... and the two bools will land in different bits.
原文：http://www.cnblogs.com/qsort/p/4034889.html