Re: [patch,libgfortran] PR48298 DTIO implementation for Internal Units

[Jerry DeLisle <jvdelisle at charter dot net>]

On 09/14/2016 12:47 AM, Manfred Schwarb wrote:
> Hi Jerry,
>
> Am 14.09.2016 um 07:56 schrieb Jerry DeLisle:
> > Hi All,
> >
> > The attached patch implements the necessary changes for DTIO to/from internal units.
> >
> > Prior to this patch, internal unit character strings and related data were kept within the dtp structure with a pseudo unit number assigned. Since child I/O procedures need this information passed to them through the unit number, it is necessary to move this information into the gfc_unit structure.
> >
> > This also implies that the internal unit needs a legitimate and unique unit number. This is accomplished using the existing newunit mechanisms to obtain a unit number and then allocating the unit structure on the existing treap mechanism used for all units.  The child I/O procedures can then find the units for use.
>
> How does this affect performance?
> I mean, reading and writing to/from internal units is a common thing and is
> certainly performance critical in some cases. It is already slow as is,
> so it would be sad to slow down this mechanism even further.
> Does acquiring a new unit and allocating the structure impair performance?
> Or does your stack approach mitigate this completely?
>
> Thanks,
> Manfred

I did think about this issue a little. Honestly have not had time to test the 
actual performance.

There is a trade-off between the time to do the stack check and treap search vs 
the allocation of the unit structure.  Currently I do nothing to try to 
recognize if the same string is being used from one I/O statement to the next, 
so there is that possibility in the future...

It has been my hope that this stack will avoid allocations and speed things up a 
little.  We now have something to test.

Regards,

Jerry