  Global Value Numbering(GVN) is a popular method for detecting redundant
computations. A polynomial time algorithm for GVN is presented by Gulwani and
Necula(2006). Here we present two limitations of this GVN algorithm due to
which detection of certain kinds of redundancies can not be done using this
algorithm. The first one is concerning the use of this algorithm in detecting
some instances of the classical global common subexpressions, and the second is
concerning its use in the detection of some redundancies that a local value
numbering algorithm will detect. We suggest improvements that enable the
algorithm to detect these kinds of redundancies as well.
