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Re: Undefined evaluation order
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Date: -- (:)
From: David McClain <dmcclain@a...>
Subject: Re: Undefined evaluation order
... but the same would be true the other way too... (0.0 * a * b).  I am a
numeric programmer and these things are unavoidable no matter how you choose
to order the evaluations.

If (a * b) raises a NaN then what would be the value of 0.0 times that? The
IEEE spec would say the result would have to continue to be a NaN.

I normally perform all arithmetic with exception processing supressed or
deferred. The only time an exception is useful to me is if there is some
remedial action that could be taken. I want my NaN's and INF's to appear in
my answers. In particular, if something should go awry at one point out of
millions I don't want that one point to hose my entire calculation. (Note
that I do not look kindly at the Fortran way of aborting an entire program
for one bad point...) In signal and image processing, especially in a
real-time environment, you just drop the bad points on the floor and
continue running.

- DM

-----Original Message-----
From: Dave Berry <>
To: Greg Morrisett <>;
Date: Thursday, October 12, 2000 4:53 AM
Subject: RE: Undefined evaluation order

>May I toss in a possible complication?   I'm thinking of numeric code, and
>the possibilities of optimisation.  To take a simple example, (a * b * 0.0)
>should always be zero.  Except that (a * b) could raise an exception or
>return a NaN.  I imagine there exist more complex numeric optimisations
>a compiler may wish to perform.
>So my question is whether numeric operations might be hampered by requiring
>a defined evaluation order, even in the case that changing the order has a
>visible (and desired!) effect.  I'm not a numeric programmer, and I know
>there are some numeric programmers on the list, so perhaps they would care
>to comment.
>Perhaps an alternative would be to specify the evaluation order, but allow
>the compiler to modify the evaluation order to reduce the possibilities of
>NaN results or numeric exceptions.  It wouldn't be as elegant as a
>rule, but might be more practical.
>-----Original Message-----
>From: Greg Morrisett []
>Sent: Wednesday, October 11, 2000 1:23 PM
>To: 'Hendrik Tews'
>Subject: RE: Undefined evaluation order
>> I would like to vote for leaving the evaluation order
>> unspecified (implicitly repeating all suitable arguments from
>> previous postings). The specification should only regulate the
>> necessary things not more.
>I don't see why.  As far as I can tell, the only reason
>to not specify the order is for performance.  I've never
>seen a systematic study that significant performance
>gains are achievable across a range of applications.
>Most compilers only do very local re-orderings, and
>these can typically be achieved with local effects
>analysis (at least for languages like ML that are
>relatively effect free.)
>We've heard promises of expression-level parallelism
>since the dawn of Fortran and Lisp.  But for 40 years,
>they speedups have yet to be realized because the granularity
>is always too small to do the necessary synchronization
>for multi-processors, and the granularity is too large
>for instruction-level parallelism (i.e., other hazards
>manifest.)  If you truly believe that magic compilers
>will someday come along and parallelize things, then
>why are you worried that these compilers will be stopped
>by a specified evaluation order?
>IMHO, there are compelling reasons to at least specify
>an evaluation order, if not to standardize on left-to-
>right.  In spite of the fact that programmer's *should*
>realize that expressions could be evaluated in any order,
>they tend to assume the order that the current compiler
>uses.  Then when someone else ports the code, or the
>compiler changes, things break.
>As I mentioned earlier, when teaching, it's nice for
>a language to be simple and uniform.  Explaining to
>a student why:
> let x = input() in
> let y = input() in
> (x,y)
>is not equivalent to:
> (input(), input())
>is one more thing that confuses them -- especially when
>we emphasize that the whole point of anonymous functions
>is to avoid naming things that need not be named!
>A standard trick for Scheme coders is, as someone suggested,
>to randomize the order of evaluation in the hopes of
>tripping across such bugs.  Ugh.  Maybe the type-checker
>should just randomly type-check a few expressions too :-)
>If you're going to have an unspecified order of evaluation,
>then I think you realistically need an effects analysis
>in order to warn the programmer that what they are writing
>is dependent upon the order.  Unfortunately, either the
>analysis would need to be global (to get rid of all the
>false positives) or else you'd have to augment function
>types with effects information, add in polymorphic effects,
>etc.  In other words, you're buying into a whole ball of wax.
>Neither option seems all that wonderful.