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libstdc++: Fix std::strong_order to handle NaN on VAX 

I mistakenly believed that VAX floats do not support NaN, but with GCC
__builtin_isnan(__builtin_nan("")) is true. That means my previous
change to <compare> is wrong, because it fails to handle NaN.

When std::numeric_limits<floating-point-type>::is_iec559 is false, as on
VAX, the standard only requires an ordering that is consistent with the
ordering observed by comparison operators. With this change the ordering
is -NaN < numbers < +NaN, and there is no support for different NaN bit
patterns (as I'm not even sure if GCC supports any for VAX).

libstdc++-v3/ChangeLog:

	* libsupc++/compare (_Strong_order::_S_fp_cmp) [__vax__]:
	Handle NaN.
This commit is contained in:
Jonathan Wakely 2022-03-10 14:17:03 +00:00
parent c65d3c7f9d
commit 73f3b8a53e
1 changed files with 10 additions and 2 deletions
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12
libstdc++-v3/libsupc++/compare
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@ -844,8 +844,16 @@ namespace std
_S_fp_cmp(_Tp __x, _Tp __y) noexcept
{
#ifdef __vax__
// VAX format has no NaN, only "excess" for Inf, so totally ordered.
return __builtin_bit_cast(strong_ordering, __x <=> __y);
if (__builtin_isnan(__x) || __builtin_isnan(__y))
{
int __ix = (bool) __builtin_isnan(__x);
int __iy = (bool) __builtin_isnan(__y);
__ix *= __builtin_signbit(__x) ? -1 : 1;
__iy *= __builtin_signbit(__y) ? -1 : 1;
return __ix <=> __iy;
}
else
return __builtin_bit_cast(strong_ordering, __x <=> __y);
#endif
auto __ix = _S_fp_bits(__x);