sqrt: Fix NaN propagation for IEEE Std 754-2008

The R language has some hacks specifically for mingw-w64 that
were caused by our handling of NaNs in sqrt(x). R uses a
special valued NaN to mean 'Not Available' and expects it to
be retained through various calculations. Our sqrt(x) doesn't
do this, instead it normalises such a NaN (retaining sign).

From:

http://wwwf.imperial.ac.uk/~drmii/M3SC_2016/IEEE_2008_4610935.pdf

"6.2.3 NaN propagation

An operation that propagates a NaN operand to its result and
has a single NaN as an input should produce a NaN with the
payload of the input NaN if representable in the destination
format."

There might even be a slight speed-up from this too.

Thanks to Duncan Murdoch for finding the reference.

1 files changed, 2 insertions, 3 deletions

diff --git a/winsup/cygwin/math/sqrt.def.h b/winsup/cygwin/math/sqrt.def.h
index 1048130d0..2690d1d9f 100644
--- a/winsup/cygwin/math/sqrt.def.h
+++ b/winsup/cygwin/math/sqrt.def.h
@@ -73,9 +73,8 @@ __FLT_ABI (sqrt) (__FLT_TYPE x)
       if (x_class == FP_ZERO)
 	return __FLT_CST (-0.0);
 
-      res = (signbit (x) ? -__FLT_NAN : __FLT_NAN);
-      __FLT_RPT_DOMAIN ("sqrt", x, 0.0, res);
-      return res;
+      __FLT_RPT_DOMAIN ("sqrt", x, 0.0, x);
+      return x;
     }
   else if (x_class == FP_ZERO)
     return __FLT_CST (0.0);