[PATCH] middle-end: Fix PR middle-end/85811: Introduce tree_expr_maybe_nan_p et al.

Joseph Myers joseph@codesourcery.com
Mon Aug 17 22:31:08 GMT 2020


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
joseph@codesourcery.com


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