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Comment by gbin | Hacker News Reader

They explain it in the article, it is to differenciate between an int directly or a pointer. Pointers are aligned so they cannot have their LSB at 1.

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Also less well known (even amongst OCaml programmers) is that the "10" bottom bits combination is used to indicate a result containing an exception. So all pointers must be aligned to at least 4 bytes, of course not a problem since 16 bit arches are not supported.

One can re-align when accessing; this is how SBCL e.g. handles tagging. e.g. the first slot would be accessed by MOV RAX, [RBX - 1].

Lisps have had this for decades. The realization that one could do this was one of the things that killed lisp machines, since it enabled lisps to be implemented efficiently on stock microprocessors, which were much cheaper due to economies of scale.

In SBCL, this also allows cons cells to have their own offset, so car and cdr are implemented with one instruction and are safe (that is, throw an exception if applied to something they don't work on) even in code compiled at (safety 0).

Now just waiting for parallel GC in SBCL. ;) Well, also waiting for some other things, I will admit.

> so car and cdr are implemented with one instruction and are safe (that is, throw an exception if applied to something they don't work on) even in code compiled at (safety 0).

It is? By default x86 will happily do unaligned accesses, and I'm pretty sure SBCL doesn't change that. Not sure about other instruction sets.

SBCL also uses double-word alignment and 4-bit tags, so you could CDR (expecting lowtag #b0111) a structure object (lowtag #b1111).

Um, you're right. I have no idea why I thought that was the case.

In your defense, I recall Self did use unaligned traps to deal with dispatching on heap object/immediate, when immediates didn't usually appear at a call site.

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