Gwendal Rou&#233; - 1 year ago 92
C Question

# How to test for lossless double / integer conversion?

I have one double, and one int64_t. I want to know if they hold exactly the same value, and if converting one type into the other does not lose any information.

My current implementation is the following:

int int64EqualsDouble(int64_t i, double d) {
return (d >= INT64_MIN)
&& (d < INT64_MAX)
&& (round(d) == d)
&& (i == (int64_t)d);
}

My question is: is this implementation correct? And if not, what would be a correct answer? To be correct, it must leave no false positive, and no false negative.

Some sample inputs:

• int64EqualsDouble(0, 0.0) should return 1

• int64EqualsDouble(1, 1.0) should return 1

• int64EqualsDouble(0x3FFFFFFFFFFFFFFF, (double)0x3FFFFFFFFFFFFFFF) should return 0, because 2^62 - 1 can be exactly represented with int64_t, but not with double.

• int64EqualsDouble(0x4000000000000000, (double)0x4000000000000000) should return 1, because 2^62 can be exactly represented in both int64_t and double.

• int64EqualsDouble(INT64_MAX, (double)INT64_MAX) should return 0, because INT64_MAX can not be exactly represented as a double

• int64EqualsDouble(..., 1.0e100) should return 0, because 1.0e100 can not be exactly represented as an int64_t.

Yes, your solution works correctly because it was designed to do so, because int64_t is represented in two's complement by definition (C99 7.18.1.1:1), on platforms that use something resembling binary IEEE 754 double-precision for the double type. It is basically the same as this one.

Under these conditions:

• d < INT64_MAX is correct because it is equivalent to d < (double) INT64_MAX and in the conversion to double, the number INT64_MAX, equal to 0x7fffffffffffffff, rounds up. Thus you want d to be strictly less than the resulting double to avoid triggering UB when executing (int64_t)d.

• On the other hand, INT64_MIN, being -0x8000000000000000, is exactly representable, meaning that a double that is equal to (double)INT64_MIN can be equal to some int64_t and should not be excluded (and such a double can be converted to int64_t without triggering undefined behavior)

It goes without saying that since we have specifically used the assumptions about 2's complement for integers and binary floating-point, the correctness of the code is not guaranteed by this reasoning on platforms that differ. Take a platform with binary 64-bit floating-point and a 64-bit 1's complement integer type T. On that platform T_MIN is -0x7fffffffffffffff. The conversion to double of that number rounds down, resulting in -0x1.0p63. On that platform, using your program as it is written, using -0x1.0p63 for d makes the first three conditions true, resulting in undefined behavior in (T)d, because overflow in the conversion from integer to floating-point is undefined behavior.

If you have access to full IEEE 754 features, there is a shorter solution:

#include <fenv.h>
…
#pragma STDC FENV_ACCESS ON
feclearexcept(FE_INEXACT), f == i && !fetestexcept(FE_INEXACT)

This solution takes advantage of the conversion from integer to floating-point setting the INEXACT flag iff the conversion is inexact (that is, if i is not representable exactly as a double).

The INEXACT flag remains unset and f is equal to (double)i if and only if f and i represent the same mathematical value in their respective types.

This approach requires the compiler to have been warned that the code accesses the FPU's state, normally with #pragma STDC FENV_ACCESS on but that's typically not supported and you have to use a compilation flag instead.

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