Re: 'Write attribute vs Write procedure.

[Ole-Hjalmar Kristensen <oleh@vlinux.voxelvision.no>]

Simon Wright <simon@pushface.org> writes:

> Ole-Hjalmar Kristensen <oleh@vlinux.voxelvision.no> writes:
> 
> > tmoran@acm.org writes:
> > 
> > > > Write (This.Link.all,
> > > >    Buffer (1 .. Stream_element_offset (Chunk)));
> > > > Stream_element_array'Write (This.Link,
> > > >    Buffer (1 .. Stream_element_offset (Chunk)));
> > > >...
> > > > the Write procedure is taking no noticeable amount of time/cpu.
> > > >In the contrary, the 'Write attribute takes like 3-4 seconds of 100% CPU
> > >   Perhaps Gnat's 'Write is doing one 'Write(Stream_Element) call for
> > > each element of the Buffer, vs a single call for the whole thing in
> > > the procedure version?  Since you've already got a Stream_Element_Array,
> > > perhaps you could call
> > >   Ada.Streams.Write(Ada.Streams.Root_Stream_Type'class(This.Link),
> > >                     Buffer (1 .. Stream_element_offset (Chunk)));
> > > directly?
> > >   Personally, I think using the default 'write is only reasonable
> > > in the very simplest cases.  If something is big, or complicated,
> > > I'd write my own 'write for it.
> > 
> > Yes, but that removes much of the attraction of using a stream in the
> > first place. It should only be necessary to do your own write if you
> > wanted another representation of it in the external format.  A prime
> > example of rendering a very nice feature of the language pretty much
> > unusable because of a poor implementation.
> 
> ARM 13.13.2(9) says
> 
>    [...] For composite types, the Write or Read attribute for each
>    component is called in canonical order, which is last dimension
>    varying fastest for an array, and positional aggregate order for a
>    record. Bounds are not included in the stream if T is an array
>    type. [...]
> 
> I'm not sure whether an implementation has licence to produce code
> which works "as if" this happened. What about the exceptional cases?
> (where there isn't enough room in the stream for all the elements, for
> example).

Too bad. But I cannot see what stops the compiler to produce code
which works "as if" this happened in the case where every read/write
attribute is compiler generated. After all, the standard does not
specify how the compiler should implement read and write for
elementary types.

-- 
Ole-Hj. Kristensen

******************************************************************************
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