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NNTP-Posting-Date: Sat, 14 May 2011 10:09:03 -0500
Date: Sat, 14 May 2011 08:09:02 -0700
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: Java puzzler
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On 5/14/2011 2:18 AM, Tom Anderson wrote:
> On Thu, 12 May 2011, Patricia Shanahan wrote:
>
>> On 5/12/2011 5:40 PM, markspace wrote:
>>> On 5/12/2011 4:05 PM, Patricia Shanahan wrote:
...
>> The original proposal was to make all arithmetic detect overflow, and
>> that would remove the existing ability to do unsigned arithmetic by
>> ignoring overflow.
>
> The semantics of which, i've realised, is an utter can of worms. Does this:
>
> byte b = 100;
> byte c = (b + b) - b;
>
> Throw an exception? A interpreter which evaluates
> expression-by-expression would blow up when it came to the b + b. An
> optimising compiler can pretty easily optimise that expression out of
> existence altogether, but in doing so, it eliminates any opportunity to
> detect an overflow. If you require the exception, you close the door to
> a lot of optimisations.
...

These issues already exist in Java, and there are established rules for
dealing with them. Optimizers are required to make it look as though
expressions were evaluated exactly the way the JLS defines, regardless
of whether that is how the expression was actually evaluated.

Consider:

double d = Double.MAX_VALUE;
double e = (d + d) - d;

The correct result is Double.POSITIVE_INFINITY, not Double.MAX_VALUE.

In effect, adding integer overflow exceptions would extend the
non-associativity and non-commutativity properties of floating point
arithmetic to integers.

Patricia