Re: A proposal for the next motion
> Date: Sun, 17 May 2009 22:34:39 -0400
> From: Nate Hayes <nh@xxxxxxxxxxxxxxxxx>
> Subject: Re: A proposal for the next motion
> To: STDS-1788@xxxxxxxxxxxxxxxxx
>
> Vincent Lefevre wrote:
> > Dear Nate,
> >
> > On 2009-05-13 20:47:53 -0400, Nate Hayes wrote:
> >> Dear Vincent,
> >>
> >>> Vincent Lefevre wrote:
> >>> On 2009-05-13 18:23:30 -0400, Nate Hayes wrote:
> >>>> Global flags are technology from the 70's and don't work well in
> >>>> modern multi-core, multi-threaded computing environments.
> >>>
> >>> . . .
>
> My understanding is IEEE 754-2008 attempts to stop speaking of global
> flags and instead speak of "attributes". For example, a conforming
> application could provide four op-codes for addition, one each with a
> different hard-wired rounding mode. This is not unlike the examples in
> Ulrich's letters to the IEEE.
>
> It is a more modern concept, i.e., "attribute" (I think I also heard Michel
> refer to this idea as "local flag"). The main idea is there is nothing that
> requires it to be global, like the rounding-mode control flags on current
> x87 FPUs, for example (which can flush the pipeline and give processor
> stalls when you change them).
>
> I don't actually have a copy of IEEE 754-2008 yet, so I'll let Dan or Michel
> correct me if I misrepresent this idea.
>
> . . .
>
> Nate
Nate,
Vincent has it correct with attributes as modes. In a sense,
Michel does as well but attributes as flags is a bit misleading.
"Attributes" (nee modes) are the subject of Clause 4: Attributes
and Rounding. And we tried to make it clear that they can have
a local scope. This covers what used to be called rounding
modes as well as exeption modes.
But "flags" are the subject of Clause 7: Default Exception
Handling & Clause 8: Alternate Exception Handling Attributes.
In Clause 7 the use of 'flags' is much the same as in 754-1985
but as have tried to make the distinction between 'exception'
(the thing that caused the trouble) & 'flag' (the place it was
recorded).
In Clause 8 the word 'attributes' is used in an attempt to apply
scope to exceptional results.
But Clause 8 is, unfortunately, optional. So we can't count on
it to be there even if we were limiting ourselves to a 754-2008
environment (which it seems we are not doing).
Still, we can require more of an implementation than the minimum
of 754-2008. So if you think its important we could specify
that they implement the option of Clause 8. But if we couldn't
get restriction to 754-2008 to pass I doubt we could get that to
pass. IMHO, of course.
BTW, get your copy of 754-2008. You may find that some of your
problems with modals can be attacked by things in there. My
reply to Vincent's previous post should give a hint.
Take care,
Dan