APL/J the assembler language? 
Author Message
 APL/J the assembler language?

I found this in the computer-architecture newsgroup (comp.arch)
last week; provoked by an elder and better (Hi Lee!) I'm submitting
it for your perusal:

--------- Forwarded NetNews Article --------
Newsgroups: comp.arch
Subject: Re: Branchless conditionals (Was: More on Linpack pivoting)
Date: 22 Jun 91 10:50:28 GMT
Organization: National Radio Astronomy Observatory, C{*filter*}tesville, VA

McNamara) writes:

... Most vector machines, (Ardent Titan, Convex, etc) have a
   vector compare instruction that sets or clears bits in any specified
   register based upon the compare; further, other operations can be
   specified to operate "undermask" of that register.  This facilitates
   vectorization of loops containing conditionals...

In most such machines one can use the bits to mask the "compress" of a
vector of values 1...n by one means or another. The resulting vector
is a list of indicies of the loop for which the condition was
satisfied (or not satisfied). In such machines the list can then be
used as an index vector for load and store (gather, scatter)
operations. This technique is especially advantageous when the truth
ratio (probability of test being satisfied) is low, or when the amount
of work (especially load/store) being done in the loop is large. This
approach is a part of standard bag of tricks for high performance
programming of conditionals on vector boxes. Some compilers, notably
the Fujitsu, produce such code automatically for loops preceded by a
suitable truth ratio compiler directive, based on a execution cost
analysis. It is even practical to use the approach on concurrent
vector machines like the Alliants (yes, a concurrent vector
compress!). Perhaps in this era of supersonic RISC engines nobody
cares anymore about these old vector programming tactics...


National Radio Astronomy Observatory                   +1-804-296-0277
Edgemont Road                                     Fax= +1-804-296-0278
C{*filter*}tesville, {*filter*}ia 22903-2475 USA            78:31.1W, 38:02.2N

Wed, 15 Dec 1993 02:55:01 GMT  
 [ 1 post ] 

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