[MLton] card and cross maps

[Matthew Fluet]

> Same idea.  BTW, the code (forwardIntergenerationPointers) 
> doesn't actually need to step back through the cross map to find the 
> nonempty entry, since, as it scans the old generation from beginning to 
> end, it maintains as an invariant (objectStart) the last object prior to 
> the current card.  Actually, given that, I'm not quite sure why 
> skipObjects is needed, since I don't see how the object at objectStart 
> could point earlier than the card being scanned.  If it did, wouldn't 
> the crossMap have caused us to have moved objectStart forward?  Hmmm. 
> If you can figure out a situation where minorBytesSkipped will be 
> nonzero I'd like to see it.

Yeah, I was curious about the skipObjects as well.  It seemed like it 
wasn't assuming the invariant maintained by the crossMap.

>> My question: why does the crossMap offset count 32-bit words, rather
>> than 8-bit bytes?
>
> I don't think it's anything deeper than the fact that all objects are
> 32-bit aligned, so why waste the extra 2 bits.

Fair enough.

> As you mention, it does allow a larger card size (2^9) to be used.

But, certainly not an arbitrary card size.

> I remember testing various card sizes, up to about 2^12 IIRC, but that
> must have been when the crossMap had its old meaning as a bit-map.

Right, for some reason the -card-size-log2 runtime option sticks in my 
mind, despite the fact that it wasn't ever around for very long.

> So, if you want to change crossMap offset to use bytes instead of
> words, I have no objection.  I'm curious to know why it might help,
> though, or if it's more for general cleanliness and consistency.

More for genearl cleanliness and consistency.  I'm really trying to count 
everything in 8-bit bytes.  I'm very suspicious of scaling by WORD_SIZE, 
because sometimes that is assuming the word size is the native pointer 
size, sometimes it is assuming that word size is a header size, and 
sometimes (like this case) it is assuming that word size is just 32-bits.

I'll add changing the meaning of crossMap to my TODO list.