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From: Jacob Sparre Andersen <sparre@nbi.dk>
Newsgroups: comp.lang.ada
Subject: Re: Copying string slices before calling subroutines?
Date: Sat, 05 May 2007 09:33:26 +0200
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Date: 2007-05-05T09:33:26+02:00
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Simon Wright <simon.j.wright@mac.com> writes:
> Jeffrey Creem <jeff@thecreems.com> writes:

>>  function Index
>>      (Source  : String;
>>       Pattern : String;
>>       Going   : Direction := Forward;
>>       Mapping : Maps.Character_Mapping := Maps.Identity) return Natural
>>    is
>>       Cur_Index     : Natural;
>>       Mapped_Source : String (Source'Range);
>
> There's the problem that I had (in my case Source'Length was >
> 2_000_000!) calling Index from one of my tasks.
>
> My reworking begins
[...]
> which calls Ada.Strings.Maps.Value rather more often than I suppose
> it could. But since the normal case is for the mapping to be the
> identity mapping the most useful optimisation would be to check for
> Identity and just compare directly.

Wouldn't the proper solution be to separate out Mapping =
Maps.Identity and Mapping /= Maps.Identity?  Even though we can do
cool things with default values for arguments, it isn't always
efficient.

   function Index
     (Source  : String;
      Pattern : String;
      Going   : Direction := Forward) return Natural;

   function Index
     (Source  : String;
      Pattern : String;
      Going   : Direction := Forward;
      Mapping : Maps.Character_Mapping) return Natural;

I can't see that there can be any cases, where existing code will work
differently with these two specifications instead of the existing
single specification.

We should also remember that for a compiler writer team, Mapping =
Maps.Identity may not be the normal case.

> Someone else was asking whether GNAT copies on the stack before the
> call -- I see no evidence of that.

Now that the implementation of Index has been pointed out to me, I
agree.  I could just see that it happened at some point during the
call to Index.

Greetings,

Jacob
-- 
�For there are only two reasons why war is made against a
 republic: The one, to become lord over her: the other, the
 fear of being occupied by her.�       -- Nicolo Machiavelli