On Thu, Apr 28, 2011 at 5:12 PM, Alexander Belopolsky <alexander.belopolsky@gmail.com> wrote:
Furthermore, IEEE 754 specifies exactly what I propose:
""" IEEE 754 assigns values to all relational expressions involving NaN. In the syntax of C , the predicate x != y is True but all others, x < y , x <= y , x == y , x >= y and x > y, are False whenever x or y or both are NaN, and then all but x != y and x == y are INVALID operations too and must so signal. """ -- Lecture Notes on the Status of IEEE Standard 754 for Binary Floating-Point Arithmetic by Prof. W. Kahan http://www.cs.berkeley.edu/~wkahan/ieee754status/ieee754.ps
Note that this text refers to the obsolete IEEE 754-1985, not the current version of the standard. IEEE 754 isn't really much help here: the current version of the standard specifies (in section 5.11: Details of comparison predicates) *twenty-two* distinct comparison predicates. That includes, for example: 'compareSignalingGreater' which is a greater-than comparison that signals an invalid operation exception on a comparison involving NaNs. But it also includes: 'compareQuietGreater' which returns False for comparisons involving NaNs. And IEEE 754 has nothing to say about how the specified operations should be mapped to language constructs---that's out of scope for the specification. (It does happen to list plain '>' as one of the names for 'compareSignalingGreater', but I don't think it's realistic to try to read anything into that.) I'm -0 on the proposal: I don't think there's enough of a real problem here to justify the change. Mark