123 lines
6.5 KiB
Markdown
123 lines
6.5 KiB
Markdown
---
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layout: news
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author: Torsten
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---
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I just read mri 2.4 "unifies" Fixnum and Integer. This, it turns out, is something quite
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different from what i though, mostly about which class names are returned.
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And that it is ok to have two implementations for the same class, Integer.
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But even it wasn't what i thought, it did spark an idea, and i hope a solution to a problem
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that i have seen lurking ahead. Strangely the solution maybe even more radical than the
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cross function jumps it replaces.
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## A problem lurking ahead
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As i have been thinking more about what happens when a type changes, i noticed something:
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An object may change it's type in one method (A), but may be used in a method (B), far up the call
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stack. How does B know to treat the object different. Specifically, the calls B makes
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on the object are determined by the type before the change. So they will be wrong after the change,
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and so B needs to know about the type change.
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Such a type change was supposed to be handled by a cross method jump, thus fixing the problem
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in A. But the propagation to B is cumbersome, there can be just so many of them.
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Anything that i though of is quite a bit too involved. And this is before even thinking about closures.
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## A step back
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Looking at this from a little higher vantage there are maybe one too many things i have been trying
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to avoid.
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The first one was the bit-tagging. The ruby (and smalltalk) way of tagging an integer
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with a marker bit. Thus loosing a bit and gaining a gazillion type checks. In mri c land
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an object is a VALUE, and a VALUE is either a tagged integer or a pointer to an object struct.
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So on **every** operation the bit has to be checked. Both of these i've been trying to avoid.
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So that lead to a system with no explicit information in the lowest level representation and
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thus a large dance to have that information in an external type system and keeping that type
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information up to date.
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Off course the elephant in the room here is that i have also be trying to avoid making integers and
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floats objects. Ie keeping their c, or machine representation, just like anyone else before me.
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Too wasteful to even think otherwise.
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## And a step forward
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The inspiration that came by reading about the unification of integers was exactly that:
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**to unify integers** . Unifying with objects, ie **making integers objects**
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I have been struggling with the dichotomy between integer and objects for a long time. There always
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seemed something so fundamentally wrong there. Ok, maybe if the actual hardware would do the tagging
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and that continuous checking, then maybe. But otherwise: one is a direct, the other an indirect
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value. It just seemed wrong.
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Making Integers (and floats etc) first class citizens, objects with a type, resolves the chasm
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very nicely. Off course it does so at a price, but i think it will be worth it.
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## The price of Unification
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Initially i wanted to make all objects the size of a cache line or multiples thereof. This is
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something i'll have to let go of: Integer objects should naturally be 2 words, namely the type
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and the actual value.
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So this is doubling the amount of ram used to represent integers. But maybe worse, it makes them
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subject to garbage collection. Both can probably be alleviated by having the first 256 pinned, ie
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a fixed array, but still.
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Also using a dedicated memory manager for them and keeping a pool of unused as a linked list
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should make it quick. And off course the main hope lies in the fact that your average program
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nowadays (especially oo) does not really use integers all that much.
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## OO to the rescue
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Off course this is not the first time my thought have strayed that way. There are two reasons why
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they quickly scuttled back home to known territory before. The first was the automatic optimization
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reflex: why use 2 words for something that can be done in one, and all that gc on top.
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But the second was probably even more important: If we then have the value inside the object
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(as a sort of instance variable or array element), then when return it then we have the "naked"
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integer wreaking havoc in our system, as the code expects objects everywhere.
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And if we don't return it, then how do operations happen, since machines only operate on values.
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The thing that i had not considered is that that line of thinking is mixing up the levels
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of abstraction. It assumes a lower level than one needs: What is needed is that the system
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knows about integer objects (in a similar way that the other ways assumes knowledge of integer
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values.)
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Concretely the "machine", or compiler, needs to be able to perform the basic Integer operations,
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on the Integer objects. This is really not so different from it knowing how to perform the
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operations on two values. It just involves getting the actual values from the object and
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putting them back.
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OO helps in another way that never occurred to me. **Data hiding:** we never actually pass out
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the value. The value is private to the object and not accessible from the outside. In fact it not
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even accessible from the inside to the object itself. Admittedly this means more functionality in
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the compiler, but since that is a solved problem (see builtin), it's ok.
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## Unified method caching
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So having gained this unification, we can now determine the type of an object very very easily.
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The type will *always* be the first word of the memory that the object occupies. We don't have
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immediate values anymore, so always is always.
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This is *very* handy, since we have given up being god and thus knowing everything at any time.
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In concrete terms this means that in a method, we can *not* know what type an object is.
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In fact it's worse, we can't even say what type it is, even if we have checked it, but after we
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have passed it as an argument to another method.
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Luckily programs are not random, and it quite rare for an object to change type, and so a given
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object will usually have one of a very small set of types. This can be used to do method caching.
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Instead of looking up the method statically and calling it unconditionally at run-time, we will
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need some kind of lookup at run-time.
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The lookup tables can be objects that the method carries. A small table (3 entries) with pairs of
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type vs jump address. A little assembler to go through the list and jump, or in case of a miss
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jump to some handler that does a real lookup in the type.
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In a distant future a smaller version may be created. For the case where the type has been
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checked already during the method, a further check may be inlined completely into the code and
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only revert to the table in case of a miss. But that's down the road a bit.
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Next question: How does this work with Parfait. Or the interpreter??
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