Narrowing conversion on a (int64_t * static_cast<float>(double))

Question

I'm fixing some lint errors in a codebase I am working with, and I see the following line

// note offset is int64_t, scale is int64_t, x is double
int64_t y = offset + lrintf(scale * static_cast<float>(x))

the linter complains and says narrowing conversion from 'int64_t' (aka 'long') to 'float'.

It seems the scale * static_cast<float>(x) is what's causing issues, and I'm wondering what is the best way to handle this error? should I cast scale to a double, and then call lrintd instead? Or is there a better approach for this case?

Answer 1

You're right. The static_cast discards some bits from the mantissa and exponent. This also limits the number of bits in the mantissa of float(scale).

Note that a typical IEEE754 double also has insufficient bits in the mantissa to fully express every int64_t value. That's logical: both are 8 byte types. But float is only 4 bytes in IEEE754.

Answer 2

Seems more logical to use

int64_t y = offset + llrint(scale * x);

scale * x will be double and should be rounded to long long (AKA int64_t).

Note anyway that this might modify the program behavior in some cases (truncating the double to float or not could result in different y).