[Omp] A question about OpenMP 2.5

[Haab, Grant]

I think it is unreasonable for the OpenMP specification to enforce an
atomicity constraint on the entire application.  What if some called
modules weren't compiled with the OpenMP compiler?  How do we insert
atomic instructions for all accesses in those? What about
system/language libraries?  Where does this stop?  

If you need enforced atomicity for data access smaller than the target
machine word size, then either code it with conditional atomic
directives/pragmas (for that machine) or complain to your hardware
manufacturer.  

- Grant

-----Original Message-----
From: omp-bounces at openmp.org [mailto:omp-bounces at openmp.org] On Behalf
Of Bronis R. de Supinski
Sent: Wednesday, March 21, 2007 10:33 AM
To: Jakub Jelinek
Cc: omp at openmp.org
Subject: Re: [Omp] A question about OpenMP 2.5


Jakub:

Re:
> On Wed, Mar 21, 2007 at 05:00:29AM -0700, Bronis R. de Supinski wrote:
> > Note that it is not clear where to define the hardware
> > aromicity level, which is why the specification has tried
> > to avoid doing so. I could easily argue that the right
> > level of write atomicity for a DSM implementation is at
> > the page granularity. While I don't think anyone would
> > accept that, it is very unclear where we stop. If Brad's
> > example used a char array, does it work? I would hope so...
>
> Not everywhere.  E.g. older Alpha CPUs (without BWX extension, like
> ev4, ev45 or ev5) couldn't write 8 and 16-bit quantities atomically,
only
> 32-bit and above, so for 8 and 16-bit accesses it would need to read
> old 32-bit word, update the byte in question in it and store it again.

Yes, this does not surprise me. My point was a position
on how we need to modify the spec. I think Brad's question
demonstrates that we will need to specify SOME minimum
level of atomicity. I was staking a position of byte level
with char; if the hardware doesn't support it then the
OpenMP implementation would need to include sufficient
synchronization to guarantee it. I understand this might
entail undue hardship and I could be convinced that four
bytes is the right level (more than that seems unreasonable).

Bronis
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