Lars T. Kyllingstad wrote:
Don wrote:
Jason House wrote:
Don Wrote:
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).
What about function calls?
double z = abs(x/y);
Yeah, it won't catch cases where there are both integer and
floating-point overloads of the same function. abs() and pow() are
the only two I can think of -- and pow() will be covered by ^^.
There's probably a few others.
I think the most subtle cases will be calls to max() and min(). If you do
x = max(1.2, 3/2);
and the 'inexact' flag doesn't survive beyond the function call, there
will be a silent conversion to double inside max() and the function will
return 1.2.
Note that that wouldn't happen if max had a signature like:
max(double a, double b)
or
max(T)(T a, T b)
But it's probably not a very common problem.
-Lars
