Re: How many vector registers are useful?

In article 93-01-195 jlg@cochiti.lanl.gov (J. Giles) writes:
>Vectors and scalars are not really all that different. Unless you have
>overlap between the vectors they really behave just like large scalars.


Good point. If you program in assembler on a Cray, you also use registers
to pre-fetch memory operands before you need 'em. In this regard, re-use
makes things even better, but even if you only do the equivalent of i++ in
C, you can win big if you don't run out of registers.... ie, load i n
clocks before it is needed, then s1 s1+1, then store it and don't use s1
for n clocks. the result is a three clock i++, as fast as it can be done
on a Cray. (n varies by cpu type.... and by memory type. can at least
span the interval 14 clocks to 24 clocks and even up to 50 clocks on some
Cray-2 processors I have used.)


Same holds true for vectors. You don't necessarily have to have high
reuse to win, just enough registers to keep data flowing (a) from memory
(b) to memory and (c) between functional units....
--
!Robert Hyatt Computer and Information Sciences !
!hyatt@cis.uab.edu University of Alabama at Birmingham !
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