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sqrt: Fix NaN propagation for IEEE Std 754-2008

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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.
This commit is contained in:
Ray Donnelly 2016-04-06 02:59:53 +01:00 committed by Corinna Vinschen
parent 5085ce2106
commit 96fc528397
1 changed files with 2 additions and 3 deletions
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5
winsup/cygwin/math/sqrt.def.h
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@ -73,9 +73,8 @@ __FLT_ABI (sqrt) (__FLT_TYPE x)
if (x_class == FP_ZERO) if (x_class == FP_ZERO)
return __FLT_CST (-0.0); return __FLT_CST (-0.0);
res = (signbit (x) ? -__FLT_NAN : __FLT_NAN); __FLT_RPT_DOMAIN ("sqrt", x, 0.0, x);
__FLT_RPT_DOMAIN ("sqrt", x, 0.0, res); return x;
return res;
} }
else if (x_class == FP_ZERO) else if (x_class == FP_ZERO)
return __FLT_CST (0.0); return __FLT_CST (0.0);