[m-dev.] for review: change MercuryFile structure

[Peter Ross]

On Tue, Aug 08, 2000 at 03:31:12PM +1000, Fergus Henderson wrote:
> On 28-Jul-2000, Peter Ross <peter.ross at miscrit.be> wrote:
> > For Fergus to review.
> > 
> > I will do the changes required to io.m to handle bidirectional streams
> > as a seperate change, as they are independent of this change.
> > 
> > ===================================================================
> > 
> > Estimated hours taken: 20
> > 
> > Change the MercuryFile structure so that it now contains pointers to
> > functions which operate on the MercuryFile.  This allows us, for
> > example, to create a MercuryFile structure which operates on sockets and
> > use the predicates in io.m to operate on the socket stream.
> 
> OK, so this change is another version of the one previously
> discussed in the thread "Add Mission Critical extensions".
> Unlike the previous version, in this version the changes
> are applied unconditionally, rather than being #ifdef'd.
> 
> The main drawback of that is the possible performance
> implications.  What's the performance impact of this change?
> For example, how much does this change slow down `cat'
> programs written using io__read_char or io__read_line_as_string?
> I don't think we should commit this change until we have
> measured the performance impact.  If the performance impact
> is too high, then we should go with the conditional compilation
> approach.
> 
I ran the following command under WinNT a number of times and took the
best time.  INPUT was about 1Mb in size.

cat < INPUT > OUTPUT

Original version:
real    0m0.062s
user    0m0.015s
sys     0m0.031s

New Version:
real    0m0.171s
user    0m0.140s
sys     0m0.061s

Will we get similar slow downs for a typeclass based approach, or will
type class specialisation remove most of the overhead?

Pete
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