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Subject: Re: Why is assembly more effecient than C?

Author: Robert Hyatt

Date: 06:13:29 09/28/98

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On September 28, 1998 at 03:09:32, Danniel Corbit wrote:

>On September 27, 1998 at 18:15:36, Robert Hyatt wrote:
>[snip]
>>Depends on how far you take it.  Cray Blitz... pure fortran (using the
>>very best fortran compiler on the planet, too) is 5x *slower* than the
>>hand-coded assembly language version.  Different approach in the asm
>>version, keeping things in registers longer, doing things that are
>>architecture specific, etc...
>>
>>But it can make a big difference.
>Do they have a KAP preprocessor for the CRAY?  If so, was that used?  I have
>seen it make a 2 or 3x improvement in some VAX multi-cpu code.
they were the first customer of Ken Kennedy's group.  But KAP does *nothing*
for a recursive tree search, because it doesn't recognize how to "unroll"
the recursion to get parallel results.  I tried this on Cray Blitz once
when Dennis Moen (Cray compiler/library guru) asked me to, and on a 4
processor XMP,cray blitz ran 1.01 times faster.  It found ways to parallelize
the move input/output code because there are loops there.  But it was totally
lost in the alpha/beta search which is where *all* the work goes on.



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