On Sat, 15 Aug 2020, Segher Boessenkool wrote: > Hi! > > On Sat, Aug 15, 2020 at 12:10:42PM +0100, Roger Sayle wrote: > > I'll quote Joseph Myers (many thanks) who describes things clearly as: > > > (a) When both arguments are NaNs, the return value should be a qNaN, > > > but sometimes it is an sNaN if at least one argument is an sNaN. > > Where is this defined? I can't find it in C11, in 18661, and of course > it isn't what GCC does (it requires -fsignaling to even acknowledge the > existence of signaling NaNs :-) )
The semantics of fmax and fmin are those of the maxNum and minNum operations in IEEE 754-2008 (that were removed in IEEE 754-2019); see the table of IEEE operation bindings that 18661-1 adds to Annex F. minNum(x, y) is the canonicalized number x if x < y, y if y < x, the canonicalized number if one operand is a number and the other a quiet NaN. Otherwise it is either x or y, canonicalized (this means results might differ among implementations). When either x or y is a signalingNaN, then the result is according to 6.2. maxNum(x, y) is the canonicalized number y if x < y, x if y < x, the canonicalized number if one operand is a number and the other a quiet NaN. Otherwise it is either x or y, canonicalized (this means results might differ among implementations). When either x or y is a signalingNaN, then the result is according to 6.2. where the relevant wording from 6.2 is Under default exception handling, any operation signaling an invalid operation exception and for which a floating-point result is to be delivered shall deliver a quiet NaN. Signaling NaNs shall be reserved operands that, under default exception handling, signal the invalid operation exception (see 7.2) for every general-computational and signaling-computational operation except for the conversions described in 5.12. For non-default treatment, see 8. (and maxNum and minNum are in 5.3 "Homogeneous general-computational operations"). -- Joseph S. Myers jos...@codesourcery.com
