The alpha is a 64 bit architecture. Therefore, the use of pointers is rather expensive. In alto all objects are accessed by a 32 bit index relative to a base address rather that directly by a 64 bit address, ie. the objects are stored in an array. The drawbacks is are that (1) one has to know in advance how many objects of a certain type will be used during program execution and that (2) accessing objects is a little cumbersome.
We assume rather high upper bounds on the number of objects to remedy the first problem. They can also be changed using comand line options. Since the OS uses demand paging that is allocated memory does not really consume RAM unless it is used there is no big disadvantage. We address the second problem by providing macros to access object members.
Having all objects of a certain type in an array increases locality and allows us to iterate over them without an additional data structure.
The elements of an object are accessed indirectly using macros this allows us to change the underlying datastructures with having to change the entire program, eg. one of the plans is to stripe the individual components of a datastructure. In fact if one needs additional elements of an object for one particular optimization we suggest to allocate a separate array just for these new elements and define the necessary macros. An example can be found in "eval.c".
Since alto is rather space efficient we can affort the luxury of never deleting an object in order to reuse its space. Instead we just allocate a new object. This eliminates bugs to dangling pointers and simplifies iterating over lists which are simultaneously modified. New objects will always have higher numbers than old ones!
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