Re: [9fans] NUMA

[tlaronde@polynum.com]

On Sun, Jul 17, 2011 at 01:44:11AM -0700, Bakul Shah wrote:
> On Sun, 17 Jul 2011 09:38:47 +0200 tlaronde@polynum.com  wrote:
> >
> > Furthermore, I don't know for others, but I prefer correctness over
> > speed. I mean, if a program is proved to be correct (and very few are),
> > complex acrobatics from the compiler, namely in the "optimization" area,
> > able to wreak havoc all the code assumptions, is something I don't buy.
>
> C's design has compromises in favor of speed to correctness
> (mainly by underspecifying, by leaving more things upto the
> implementor). So if you really prefer correctness over speed,
> you should be using Scheme, ML or Haskell etc but not C!

Yes and no. IMHO one of the greatest strengths of C is that the language
is small (standard lib is apart) and its description short and to the
point. K&R and, at least, ANSI C were short and if there are subtleties
(promotions... signed/unsigned etc.), knowing what is guaranteed
and what is not can be achieved. (And C is a general purpose
language, but not an all purposes language [standard C]: calculus
is not its realm, since even integer overflow is unspecified.)

My woe is more that an optimization can say "this may improve speed (or
may not, even slow down processing...)": OK. But an optimization
that can break a program, that is an optimization whose correctness
is not guaranteed, is something I can't understand why it is even
proposed (since I fail to see why someone would be happy to have
more rapidly an incorrect result, that can even not be said to be close
to the correct one for some epsilon...).

>[...]
> Also note that the ISA implementations these days are quite
> complex (perhaps even more than your typical program).  We
> don't see this complexty because it is all hidden behind a
> relatively simple ISA.  But remember the FOOF bug? Usually the
> vendor has a long errata list (typically only available on a
> need to know basis and only under NDA!). And usually they
> don't formally prove the implementation right; they just run
> zillions of test vectors! I bet you would be scandalized if
> you knew what they do :-)

Scandalized, perhaps... surprised: not! Because stating explicitely
the domain of definition of operations on not integer, not scaled
and certainly not "reals" is not trivial at all precisely by lack
of uniformity, specially on a higher level when you consider not
one manipulation, but a sequence... And the the decreasing size of
the hardware components will lead to impredictability by design!

>[...]
> I seriously think you will be happier with Scheme!

That's on my TODO list. Since a long time now... ;)
--
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
                      http://www.kergis.com/
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