SpiderMonkey uses one of the rather elaborate, carefully-tuned modern allocators (tcmalloc or jemalloc, I forget which) that's designed around clustering alloca

SpiderMonkey uses one of the rather elaborate, carefully-tuned modern allocators (tcmalloc or jemalloc, I forget which) that's designed around clustering allocations into 'size categories', and carving allocations out of those categories. As a result, a given allocation will end up with overhead based on the size bucket it's put into, and certain buckets may perform better. In this environment it's very useful for each type to be fixed-size (within reason) and be consistently sized. You'll end up with all your JSStrings allocated out of the same size bucket and other things get simpler and faster. For example, with fixed-size JSStrings allocating out of a fixed-size allocation bucket, perhaps thread-local, you can allocate a string by atomically incrementing a pointer. Speedy!