[M3devel] Integer overflow

[Rodney M. Bates]

If the type were INTEGER instead of cardinal, (and assuming no checking for overflows),
I think wrapping around to the maximal negative value would be reasonable.  Even for
CARDINAL, it's reasonable for the arithmetic operation itself (which is really being
done in the full INTEGER range).  But then the assignment of the result back to the
CARDINAL variable really must do a range check.  This derives from the assignment
x:= ... in the WITH equivalent, not from the + 1 operation.

Mainly, there is a pervasive principle in the safety of the language that you can
never get a bit pattern stored in a variable that does not map back to an abstract
value of its type.  We really need to preserve this.

Tony Hosking wrote:
> Even under the interpretation for INC that yields:
> 
> VAR s: CARDINAL := LAST(INTEGER);
> BEGIN WITH x = s DO x := VAL(ORD(x) + 1, CARDINAL) END; END;
> 
> the compiler currently does not insert a bounds check, because it reasons that the bound for ORD(x) + 1 is [0+1, LAST(INTEGER)], so the result is always assignable to CARDINAL.
> 
> In reality, the compiler should presumably assume that because ORD(x) + 1 might overflow if ORD(x) = LAST(INTEGER) then the bound for the expression ORD(x)+1 is actually the same as INTEGER: [FIRST(INTEGER),LAST(INTEGER)].  So, because this is larger than the range for CARDINAL it really needs to insert a bounds check on the conversion to CARDINAL.
> 
> On 9 Jan 2010, at 23:33, Tony Hosking wrote:
> 
>> So, in my experiments with range checking on integers, I have been playing with the overflow case:
>>
>> VAR s: CARDINAL := LAST(INTEGER);
>> BEGIN INC(s) END;
>>
>> Should this fail at runtime with a range check error?
>>
>> The language definition says that integer overflow may or may not be checked, depending on the implementation.  (See FloatMode.i3).
>>
>> If it is not checked, is it reasonable that s takes on the value FIRST(INTEGER)?
>>
>> Antony Hosking | Associate Professor | Computer Science | Purdue University
>> 305 N. University Street | West Lafayette | IN 47907 | USA
>> Office +1 765 494 6001 | Mobile +1 765 427 5484
>>
>>
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>>
> 
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