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Date: Tue, 01 Sep 2009 16:36:17 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Ada Shootout program for K-Nucleotide (patches)
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Date: 2009-09-01T16:36:18+02:00
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Ole-Hjalmar Kristensen schrieb:
>>>>>> "GB" == Georg Bauhaus <rm.dash-bauhaus@futureapps.de> writes:
> 
> I think the problem is not the explicit copy which happens when we hit
> the end of the buffer (easily verified, just increase the buffer size
> 10 times, the copying will happen 1/10 of the time, but execution time
> stays the same), but the implicit copy which seems to happen in the statement
> return In_Buf(Start..I-1); I do not know if this copy is unavoidable,
> or if the compiler could have optimized it away.

WRT return In_Buf(Start..I-1), there seems to be a way to have the
bounds shift towards 1 .. Result'Length without copying:

with Ada.Text_IO;
procedure Bnds is
   function S return String is
   begin
      return String'(10 .. 20 => '*');
   end S;

   X : constant String := S;
begin
   Ada.Text_IO.Put_Line(Positive'Image(X'First));
   declare
      subtype XString is String(1 .. X'Length);
      Z : XString;
      pragma Import (Ada, Z);
      for Z'Address use X'Address;
   begin
      Ada.Text_IO.Put_Line(Positive'Image(Z'First));
   end;
end Bnds;

Is this "phantom sliding" accidental or can it be relied upon?


> The idea of using a storage pool is an interesting one, since it could
> allow you to avoid copying when you want to return a string from a
> function. But how would you overlay the string on top of the read
> buffer? An Ada string is not a C string, you would need to put the
> descriptor somewhere as well.

I'll reread "Accessing Memory as a String",
http://adapower.com/adapower1/lang/accessmem.html

Maybe this leads somewhere regarding this question.