[PATCH 1/2] Improve lgammaf range for very small cases

Andoni Arregi andoni.arregui@gtd-gmbh.de
Thu Feb 10 16:10:35 GMT 2022


The original cut for small arguments at |x|<2**-70 (copied from the
double version) produces that when computing nadj we get a subnormal
number for t*x and thus, the division of pi/subnormal will be INF and
the logarithm of it too, which is wrong as a result for lgammaf in this
range.
The proposed new limit seems to be safe and has been tested to
produce accurate results.
(Courstesy of Andreas Jung, ESA)
---
 newlib/libm/math/erf_lgamma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/newlib/libm/math/erf_lgamma.c b/newlib/libm/math/erf_lgamma.c
index f88f63092..84d02159b 100644
--- a/newlib/libm/math/erf_lgamma.c
+++ b/newlib/libm/math/erf_lgamma.c
@@ -168,7 +168,7 @@ static float zero=  0.0000000000e+00;
 	       *signgamp = -1;
 	   return one/(x-x);
 	}
-	if(ix<0x1c800000) {	/* |x|<2**-70, return -log(|x|) */
+	if(ix<0x30800000) {	/* |x|<2**-30, return -log(|x|) */
 	   if(hx<0) {
 	       *signgamp = -1;
 	       return -__ieee754_logf(-x);
-- 
2.35.1




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