[racket-dev] stypes.h + 3M woes

From: Jon Zeppieri (zeppieri at gmail.com)
Date: Mon May 6 11:58:13 EDT 2013

On Mon, May 6, 2013 at 8:08 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> If you're modifying "stypes.h", though, "public" is not relevant. You
> should be able to add lots of new types to "stypes.h" without reaching
> 512 or otherwise running into GC limits, though you do need to
> registers traversal functions with the GC for the new type tags.

Right, but I was trying to add a new type with the explicit value of
512. That's where I ran into trouble with the fixup_table and

> Maybe you can say more about what you're trying to do overall? I think
> I'm probably giving you incomplete advice here, but I'm not sure which
> of the various implementation details (type tags having GC traversal
> procedures, type-tag names, hashing, versioning) are relevant for your
> task.

I was looking into what it would take to add immediately represented
types, other than fixnums, to racket. The eventual goal is to
represent characters in the (Scheme_Object *) word itself. The code
point would occupy the upper N-1 bytes of the word, and the least
significant byte would contain a type tag. Since fixnums have a 1 in
the least significant bit, and pointers have #b00 in their least
significant bits, the tag would have to end with #b10.

First, I thought about reserving the first 256 type tags for
immediates (which would be sparsely populated, since, again, they need
to end in #b10). Then I realized that the order of groups of tags in
stypes.h is significant. Possibly the order could be reversed, to put
the internal objects last, rather than first. But since I'm trying to
change as little as possible, my next thought was to shift the raw tag
(the least significant byte of the word) 8 bits to the left. So, the
first valid immediate tag would then be 512. (For type-specific
predicates, the shift wouldn't be necessary; CHARP could be defined as
((obj) & 0xff == 2), for example, but the generic SCHEME_TYPE macro
would need to do a bit more work.)

So, I was trying to move scheme_char_type to the end of the list, with
an explicit value of 512. The first step was just to change the type
tag. Then I could try to change the actual representation.

Posted on the dev mailing list.