> Do you happen to have some hard numbers for this improvement?

I can tell you that it removes the need for metadata headers from simple allocators like those based solely on free lists for various size classes. It will reduce memory usage by up to 50%, and that comes with a large performance benefit.

 For general purpose allocators where this data may already be around, it's still faster. In jemalloc and TCMalloc, an optional size parameter will save at least one cache miss. Taking advantage of the *guarantee* would require a complete redesign.

> When an object is deallocated, we compute its page number and look it up in the central array to find the corresponding span object. The span tells us whether or not the object is small, and its size-class if it is small.