Re: fwd from Jim Demmel: More on repeatability
On 2011-08-17 12:49:29 +0100, N.M. Maclaren wrote:
> On Aug 17 2011, Vincent Lefevre wrote:
> >IMHO, this should be done only by specifying the full behavior, possibly
> >depending on well-identified platform-dependent parameters. Otherwise it
> >may be tricky to define "repeatable mode" and "same machine" (e.g. what
> >if the machine is upgraded[*], some environment variables are changed,
> >and so on?)
>
> Once you do that for any non-trivial operation, you are forbidding any
> improvements in the future. State of the art today may be one result,
> but someone may develop an improvement.
With forbidden contraction and in tightest mode, there would be
no possible improvements.
> The most extreme example I know of is the specification of the shift
> operators in most programming languages, which is an artifact of the
> restrictions of discrete-logic hardware (remember it?), and is STILL
> around!
But without specification, the behavior can depend on the compiler
version, the optimization level and so on.
--
Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)