Performance of the Streams 'Read and 'Write
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From: Gautier write-only on 1 Nov 2009 16:38 Jeffrey R. Carter: > If you're > really paranoid, you can add a test that Unsigned_8'Size = Stream_Element'Size, > which you seem to be assuming. Yet a bit more paranoid: checking the size of arrays! workaround_possible: Boolean; procedure Check_workaround is test_a: constant Byte_Buffer(1..10):= (others => 0); test_b: constant Ada.Streams.Stream_Element_Array(1..10):= (others => 0); begin workaround_possible:= test_a'Size = test_b'Size; end Check_workaround; It's the code I've put into Zip-Ada - big success! :-) _________________________________________________________ Gautier's Ada programming -- http://sf.net/users/gdemont/ NB: For a direct answer, e-mail address on the Web site!
From: Randy Brukardt on 2 Nov 2009 16:32 "Jeffrey R. Carter" <spam.jrcarter.not(a)spam.acm.org> wrote in message news:hcfdpo$p62$1(a)news.tornevall.net... > Gautier write-only wrote: >> On 30 Okt., 01:39, "Jeffrey R. Carter" >> <spam.jrcarter....(a)spam.acm.org> wrote: >> >>> And if you overlay the Stream_Element_Array onto the Buffer, thus >>> eliminating >>> the copying and the stream attribute operations? >> >> With an Unchecked_conversion ? > > No, that still does a copy. It doesn't have to, there is a permission to avoid copying in 13.9(12). So it depends on what the compiler is able to do optimization-wise. Randy.
From: Randy Brukardt on 2 Nov 2009 16:37 "Gautier write-only" <gautier_niouzes(a)hotmail.com> wrote in message news:715d78f4-9b01-4598-88c1-6fc202983bae(a)p35g2000yqh.googlegroups.com... > On 30 Okt., 04:36, Georg Bauhaus <rm-host.bauh...(a)maps.futureapps.de>: > >> I finally thought that the above procedures are faster than 'Read >> or 'Write because the latter are defined in terms of stream elements: >> When there is a composite object like b : Buffer and you >> 'Write it, then for each component of b the corresponding 'Write >> is called. This then writes stream elements, probably >> calling Stream_IO.Write or some such in the end. >> So Write from above appears closer to writing bulk loads >> of stream elements than a bulk load of 'Writes can be. > > Sure, it is the safe way: write records field by field, arrays element > by element (that recursively). The compiler avoids problems with non- > packed data. Nothing against that. The general case is well done, > fine. But the compiler could have a look a the type left to the > attribute and in such a case (an array of Unsigned_8, or a String) > say: "Gee! that type Buffer is coincidentally the same as > Stream_Element_Array, then I take the shortcut to generate the code to > write the whole buffer and, this time, not the code to write it > element by element". IMHO, Ada compilers should do that. (There's specifically a permission to do this optimization in Ada 2005: 13.13.2(56/2).) That's an intergral part of the stream attribute implementation on Janus/Ada. (Disclaimer: the entire stream attribute implementation on Janus/Ada doesn't work right, quite probably because it is too complicated. So perhaps there is a reason that other Ada compilers don't do that. :-) Note, however, that it is pretty rare that you could actually do that (only about 15% of the composite types I've seen in Janus/Ada would qualify). So I'm not surprised that implementers have left that capability out in favor of things that happen more often. Randy.
From: Gautier write-only on 2 Nov 2009 17:16 On 2 nov, 22:37, "Randy Brukardt" <ra...(a)rrsoftware.com> wrote: > IMHO, Ada compilers should do that. (There's specifically a permission to do > this optimization in Ada 2005: 13.13.2(56/2).) Excellent news! > That's an intergral part of > the stream attribute implementation on Janus/Ada. (Disclaimer: the entire > stream attribute implementation on Janus/Ada doesn't work right, quite > probably because it is too complicated. So perhaps there is a reason that > other Ada compilers don't do that. :-) Note, however, that it is pretty rare > that you could actually do that (only about 15% of the composite types I've > seen in Janus/Ada would qualify). Sure - but imagine that these 15% might transport 95% of the information. It could happen, couldn't it ? And if type T qualifies, a record type R with fields of types T,U,V (U and V not qualifying) will be also transmitted faster, an array of R will also go faster, and so on... > So I'm not surprised that implementers > have left that capability out in favor of things that happen more often. I am not surprised either... Gautier
From: Gautier write-only on 17 Nov 2009 07:45
Hello, Following the discussion about stream attributes for arrays of bytes (see Performance of the Streams 'Read and 'Write), I have an idea on how to improve GNAT on that point. Basically, GNAT has a shortcut for the type {Wide_}String. The idea would consist in applying the same when the base type is an array of bytes (any array-of-mod-2**8 type which can be safely detyped to a String). More concretely, it would consist in adding at the end of Find_Stream_Subprogram (exp_attr.adb) something like: if Is_Array_Type(Typ) and then Is_Bit_Packed_Array (Typ) and then Component_Size (Typ) = 8 and then then Comp_Typ:= Component_Type (Typ); if Is_Modular_Integer_Type(Comp_Typ) and then [modulus ??] (Comp_Typ) = 2**8 then [ go ahead with some unchecked conversion to String ] return [the right thing]; end if; end if; Now, I have no experience with building GNAT. Is there an easy way on Windows ? Or could someone give a try on a "working" GCC tree ? TIA _________________________________________________________ Gautier's Ada programming -- http://sf.net/users/gdemont/ NB: For a direct answer, e-mail address on the Web site! |