Every thread-local allocation buffer (TLAB) contains a reserved area to accommodate allocation prefetches (i.e., to make sure that instructions performing allocation prefetch do not cause faults). The reserved area is at the end of the TLAB.

Currently, all the reserved area is created in all builds, even though only the C2 compiler generates allocation prefetches. As a result, in builds without C2, some heap space is wasted at the end of each TLAB. Creating the reserved area also causes problems for internal testing of the AllocatePrefetchLines/AllocateInstancePrefetchLines command-line flags.