Why compile-time floating point calculations might not have the same results as run-time calculations?

Question

In constexpr: Introduction, the speaker mentioned "Compile-time floating point calculations might not have the same results as runtime calculations":

And the reason is related to "cross-compiling".

Honestly, I can't get the idea clearly. IMHO, different platforms may also have different implementation of integers.

Why does it only affect floating points? Or I miss something?

Answer 1

Why does it only affect floating points?

For the standard doesn't impose restrictions on floating-point operation accuracy.

As per expr.const, emphasis mine:

[ Note: Since this document imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution. [ Example:

bool f() {
    char array[1 + int(1 + 0.2 - 0.1 - 0.1)];  // Must be evaluated during translation
    int size = 1 + int(1 + 0.2 - 0.1 - 0.1);   // May be evaluated at runtime
    return sizeof(array) == size;
}

It is unspecified whether the value of f() will be true or false. — end example ]
— end note ]