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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: Mneson announcement and help request
Date: Thu, 03 Jun 2004 11:02:49 +0200
Organization: AdaCL
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Date: 2004-06-03T11:02:49+02:00
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Jeffrey Carter wrote:

> Martin Dowie wrote:
> 
>> Standard idiom:                         MAA Idiom:
>> 
>> while not End_Of_File (Current_Input)   begin
>> loop                                       loop
>>    Get_Immediate (C);                         Get_Immediate (C);
>> end loop;                                  end loop;
>>                                         exception
>>                                            when End_Error =>
>>                                               null;  -- End of file
>>                                         end;
> 
> The standard idiom should be
> 
> loop
>     exit when End_Of_File (Current_Input);
> 
>     Get_Immediate (C);
> end loop;
> 
> which eliminates the calls to "not". In addition, it uses positive
> logic, rather than the negative logic required by while, which is easier
> to read and understand.

I once tried a "loop optimization" with MS-C 6.00. Tried various options and
looked at the assembler output. Guess what: "do while", "while" and "for"
all redered the very same result in assembler.

Starting with:

loop
  Instructions_1;

   exit when Condition;

   Instructions_2;
end loop;

or alternative:

Instructions_1;

while not Condition loop
   Instructions_2;
   Instructions_1;
end loop;

I got something like this:

goto center;

loop
   Instructions_2;
 
<<center>>

   Instructions_1;

while not Condition;

Disclaimers:

1) I know that Ada does not allow a goto into a loop;
2) I know that Ada does not have a while at the and of a loop. 

I never ever tried to optimize a loop again. I just write it down so I can
read it nicely.

On a side note: At the same time I also made a "if case" optimization. There
where several small groups with large holes in between. Again no matter if
I used "if" or "case" the asembler result was the same: A binary tree
search for the holes and jump tables for the groups.

And that was MSC-6 more then 10 year ago.

With Regards

Martin
-- 
mailto://krischik@users.sourceforge.net
http://www.ada.krischik.com