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From: Gautier write-only <gautier_niouzes@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Performance of the Streams 'Read and 'Write
Date: Thu, 29 Oct 2009 16:29:47 -0700 (PDT)
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Date: 2009-10-29T16:29:47-07:00
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Hello.
I got used to think that I/O was the last spot where our preferred
language was condemned to slowness.
Now consider this. Variant 1 of a buffered I/O:

  type Buffer is array(Natural range <>) of Unsigned_8;

  procedure Read( b: out Buffer ) is
  begin
    Buffer'Read(Stream(f_in), b);
  exception
    when Ada.Streams.Stream_IO.End_Error =>
      null;
      -- Nothing bad, just some garbage in the buffer
      -- after end of compressed code
  end Read;

  procedure Write( b: in Buffer ) is
  begin
    Buffer'Write(Stream(f_out), b);
  end Write;

Bad luck, it is as slow as doing I/O's with single bytes and
Sequential_IO! But if it is slow by receiving/sending a whole buffer,
how to make it faster ? Now someone (in a slightly different context)
came with this (call it variant 2):

  procedure Read( b: out Buffer ) is
    use Ada.Streams;
    First     : constant Stream_Element_Offset:= Stream_Element_Offset
(b'First);
    Last      :          Stream_Element_Offset:= Stream_Element_Offset
(b'Last);
    SE_Buffer : Stream_Element_Array (First..Last);
  begin
    Read(Stream(f_in).all, SE_Buffer, Last);
    for i in First..Last loop
      b(Natural(i)):= Unsigned_8(SE_Buffer(i));
    end loop;
  end Read;

  procedure Write( b: in Buffer ) is
    use Ada.Streams;
    First     : constant Stream_Element_Offset:= Stream_Element_Offset
(b'First);
    Last      : constant Stream_Element_Offset:= Stream_Element_Offset
(b'Last);
    SE_Buffer : Stream_Element_Array (First..Last);
  begin
    for i in SE_Buffer'Range loop
      SE_Buffer(i):= Stream_Element(b(Natural(i)));
    end loop;
    Write(Stream(f_out).all, SE_Buffer);
  end Write;

Naively, you would say it is even slower: you do even more by copying
a buffer into another one, right ?
Indeed, not at all, it is *lots* faster (on GNAT and ObjectAda)!
To give an idea, the variant 1 applied to a bzip2 decompressor makes
it 4x slower than the C version, and variant 2 makes it only 7%
slower! With only I/O (like copying a file) you would get an even much
larger difference.

Now, it raises some questions:
Is there maybe a reason in the RM why the 'Read and 'Write have to be
that slow ?
Or are these two compilers lazy when compiling these attributes ?
Should I bug Adacore about that, then ?
Do some other compilers do it better ?
_________________________________________________________
Gautier's Ada programming -- http://sf.net/users/gdemont/
NB: For a direct answer, e-mail address on the Web site!