Detecting inadvertent use of integer division

[lws]

On 2009-12-14 03:16:27 -0800, Don <nospam at nospam.com> said:

> Walter Bright wrote:
>> Don wrote:
>>> Consider this notorious piece of code:
>>> 
>>> assert(x>1);
>>> double y = 1 / x;
>>> 
>>> This calculates y as the reciprocal of x, if x is a floating-point 
>>> number. But if x is an integer, an integer division is performed 
>>> instead of a floating-point one, and y will be 0.
>>> 
>>> It's a very common newbie trap, but I find it still catches me 
>>> occasionally, especially when dividing two variables or compile-time 
>>> constants.
>>> 
>>> In the opPow thread there were a couple of mentions of inadvertent 
>>> integer division, and how Python is removing this error by making / 
>>> always mean floating-point division, and introducing a new operator for 
>>> integer division.
>>> 
>>> We could largely eliminate this type of bug without doing anything so 
>>> drastic. Most of the problem just comes from C's cavalier attitude to 
>>> implicit casting. All we'd need to do is tighten the implicit 
>>> conversion rules for int->float, in the same way that the int->uint 
>>> rules have been tightened:
>>> 
>>> "If an integer expression has an inexact result (ie, involves an 
>>> inexact integer divison), that expression cannot be implicitly cast to 
>>> a floating-point type."
>> 
>> But the compiler cannot reliably tell if it will produce an inexact result.
>> 
>> 
>>> (This means that double y = int_val / 1;  is OK, and also:
>>>  double z = 90/3; would be OK. An alternative rule would be:
>>> "If an integer expression involves integer divison, that expression 
>>> cannot be implicitly cast to a floating-point type").
>> 
>> This is kinda complicated if one has, say:
>> 
>>     double z = x/y + 3;
> 
> Integer expressions remain inexact until there's a cast.
> 
> (It's very simple to implement, you just use the integer range code, 
> adding an 'inexact' flag. Division sets the flag, casts clear the flag, 
> everything else just propagates it if a unary operation, or ORs the two 
> flags if a binary operation).

Seems like when the product of integer devision is cast to a double is 
when it should produce a warning.

-SC