Mark Dickinson added the comment:
> Do you know a way to get the IEEE 754 rounding behavior without invoking C
> undefined behavior?
One option is to hard-code the actual boundary, which is 2**128 * (1 - 2**-25)
(as opposed to FLT_MAX, which is 2**128 * (1 - 2**-24)): values equal to or
larger than 2**128 * (1 - 2**-25) in absolute value should raise. But that
means assuming IEEE 754 and round-ties-to-even, which isn't an outrageous
assumption but does make the solution feel a bit fragile.
An alternative would be to scale values in the range (FLT_MAX, 2.0 * FLT_MAX]
by 0.5 before doing the conversion, something like this:
if (fabs(x) > FLT_MAX && !Py_IS_INFINITY(x)) {
double half_x = 0.5 * x;
if (half_x > FLT_MAX) {
goto Overflow;
}
float half_y = (float)half_x;
if (half_y > 0.5 * FLT_MAX) {
goto Overflow;
}
}
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<https://bugs.python.org/issue31373>
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