Re: Cray-2 Fast Memory

Patrick Delano <delano@cs.berkeley.edu> writes:
> Apparently the Cray-2 had a fast memory that unlike cache memory was
> explicitly managed by the compiler. Can anyone tell me what software
> techniques were used, or point me to some references?


Basically, no software techniques were used. The compiler does very
little to take advantage of the local memory. As far as I am aware, the
only ways in which it is used are the following:


    o Temporary storage for register spillage.


    o As a means of extracting scalar values from vector registers
        (which can be done directly on the X-MP and Y-MP).


    o When the programmer, by directive, explicitly indicates that a
        variable is to be placed in local rather than common memory.


I believe that the primary reason that more sophisticated techniques
were not used by the compiler is that less than 40 Cray-2 machines were
manufactured and sold, compared to hundreds of X-MP and Y-MP type
machines.


j-grout@uiuc.edu writes:
> This paper talks a lot about techniques to minimize the use of
> temporary storage to one or several lengths of a vector register (which
> obviously fits well with the availability of local memory).


I don't understand this. Minimizing temporary storage is _less_
important with a fast local memory, because register spillage is less
expensive than if it had to go to common memory.


I agree that minimizing spillage is an important issue, but I don't see
what it has to do with local memory, or the lack of it.


                                                                                David desJardins
--

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