[adrian_b]

This is explained in TFA, where it is mentioned that you can replace inline metadata with a pointer to metadata (or an index), which may be unaligned, if necessary.

However, the pointer to metadata is not really necessary.

The associated metadata could be stored in a table, and the index of the metadata could be computed from the offset of the pointer returned by malloc to the start of the heap (possibly using a hash function).

The ancient versions of the Microsoft C/C++ compilers were using a malloc with inline metadata. I have no idea if they replaced this more recently.

[CodesInChaos]

I consider an inline-pointer a form of inline-metadata. When I mentioned that inline metadata isn't necessary I was referring to the kind of external lookup you describe.