Can you elaborate on the tagging scheme? I'm not sure if the tag is stored in the high bits or low bits of the data pointer. If it's the low bits, how does it interact with the VUE repr, whose pointers are char-aligned (that is, unaligned)?
#define DATA(buf) ((char*)((intptr_t)((buf)->data) << BFT_TYPE_BITS))
Older programmers tend to raise an eyebrow at added computation like the "<<", but here in 2022 we need to remember that we are about to do a memory access and even if it's in L1 cache, that "<<" is basically free by comparison.