RE: [MI400] CPU/MI Timings

["Kaare Plesner, SOSY A/S" <kaaple@xxxxxxxxxxxxx>]

I will have to use what is claimed to be IBM's favorite answer (often for
good reasons):
"It depends"!

Generally speaking nothing can beat the very basic and simple computer
functions like add, subtract, load, store, etc. - so therefore the simple
answer for e.g. a binary or a floating point field is that subtracting the
field from itself is as fast as anything else.

You must however remember that a RISC machine does not have a set of packed
format instructions available, so for packed fields the rules of the game
may be different. I would assume that the fastest would be to move a
character redefinition of a packed zero contents field to the character
redefinition of the packed field you want to reset.

Using AND operation to set all bits except the last half byte of the packed
field to zero is probably equally fast.

If you want to reset several fields next to each other, then redefine all of
them as one character field (data structure), and have another data
structure containing the wanted empty values, and make one character move.

However, I have to ask: Why bother? If you use PEX *PROFILE to analyze your
program doing such things then you may not be able to detect any difference
at all as long as we are speaking about just a few fields.

If you are coding in a high level language you should use ILE's optimizing
function - the optimizer is really advanced now, IBM came a long way here -
so the optimizer would probably find a good way of doing it. You cannot list
any generated MI for and ILE program, but you can of course list the
generated assembler - but this gets a little more complex to look at.

FOR OPM you can list the MI code by compiling with GENOPT(*LIST). Looking at
the generated MI will give you a good feeling for what is faster.

In any case you might want to run a PEX (Performance Explorer) *PROFILE
analysis on one of your programs if you are concerned with performance. That
would give you an indication of which instructions are consuming your CPU
time. You will see that moving zeroes to fields is so fast that they may not
even show up in the statistics produced be PEX PROFILE.

Kaare Plesner

============================================
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Kaare Plesner        I     SOSY A/S
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-----Original Message-----
From: mi400-bounces@midrange.com [mailto:mi400-bounces@midrange.com]On
Behalf Of Paul_Miller@TDWH.Co.Uk
Sent: Friday, January 10, 2003 11:45 AM
To: mi400@midrange.com
Subject: [MI400] CPU/MI Timings


Newbie again...!

Does anyone know which compiled RPG code is quicker...

So for example is it better to Z-ADD 0 Field or Clear Field or Move 0 Field
or maybe use an Initialise on a data structure, etc.

I am thinking of the cpu time each line will take when compiled. or would
the MI look the same for both...

Paul Miller
TD Waterhouse Bank N.V.
________________________________________
* Exchange Court, Duncombe Street, Leeds, LS1 4AX
+ Paul.Miller@TDWH.Co.Uk
+ Paul@Millennium-Computers.Uk.Com

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