What does the C standard mean by "converted to its semantic type" for the floating-point macros?

Question

I'll quote from N1570, but the C11 standard has similar wording:

The fpclassify macro classifies its argument value as NaN, infinite, normal, subnormal, zero, or into another implementation-defined category. First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then classification is based on the type of the argument.

(my emphasis)

And a footnote:

Since an expression can be evaluated with more range and precision than its type has, it is important to know the type that classification is based on. For example, a normal long double value might become subnormal when converted to double, and zero when converted to float.

What does it mean for the argument to be "converted to its semantic type". There is no definition of "semantic type" anywhere evident.

My understanding is that that any excess precision is removed, as if storing the expression's value to a variable of float, double or long double, resulting in a value of the precision the programmer expected. In which case, using fpclassify() and friends on an lvalue would result in no conversion necessary for a non-optimising compiler. Am I correct, or are these functions much less useful than advertised to be?

(This question arises from comments to a Code Review answer)

Answer 1

The semantic type is simply the type of the expression as described elsewhere in the C standard, disregarding the fact that the value is permitted to be represented with excess precision and range. Equivalently, the semantic type is the type of the expression if clause 5.2.4.2.2 paragraph 9 (which says that floating-point values may be evaluated with excess range and precision) were not in the standard.

Converting an argument to its semantic type means discarding the excess precision and range (by rounding the value to the semantic type using whatever rounding rule is in effect for the operation).

Regarding your hypothesis that applying fpclassify to an lvalue does not require any conversion (because the value stored in an object designated by an lvalue must have already been converted to its semantic type when it was assigned), I am not sure that holds formally. Certainly when the object’s value is updated by assignment, 5.2.4.2.2 9 requires that excess range and precision be removed. But consider alternate ways of modifying the value, such as the postfix increment operator. Does that count as an assignment? Its specification in 6.5.2.4 2 says to see the discussion of compound assignment for information on its conversions and effects. That is a bit vague. One would have to consider all possible ways of modifying an object and evaluate what the C standard says about them.