bit sets in IntSet vs. Integer

[Henning Thielemann]

In my set-cover package I make extensive use of bit vectors. I tested both 
IntSet and Integer for this purpose. In GHC-7.4.2 Integer was visibly 
faster than IntSet, but in GHC-7.8.4 and later, IntSet clearly wins over 
Integer. I guess this can be attributed to IntSet using BitMaps at the 
leaves since containers-0.5. However, on a 64-bit machine a BitMap is a 
Word64. I wondered whether we could get further speedup by using a vector 
type. E.g. AVX processors could perform bit manipulations on 256 bit 
words. Do the bit operations on Integer make use of vector instructions?