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From: Andoni Arregi <andoni.arregui@gtd-gmbh.de>
To: newlib@sourceware.org
Subject: [PATCH 1/2] Improve lgammaf range for very small cases
Date: Wed, 09 Feb 2022 17:28:07 +0100	[thread overview]
Message-ID: <99cdb95d3263da03844fa0343c22d9e4ab87375e.camel@gtd-gmbh.de> (raw)
In-Reply-To: <mw35kxbyxz.fsf@tomate.loria.fr>

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.
(Courtesy 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


  reply	other threads:[~2022-02-09 16:32 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-05  6:38 regression in Newlib 4.2.0 (lgammaf) Paul Zimmermann
2022-02-09 16:28 ` Andoni Arregi [this message]
2022-02-10 14:58   ` [PATCH 1/2] Improve lgammaf range for very small cases Corinna Vinschen
2022-02-10 16:07     ` [PATCH 0/2] Fix lgammaf and lgamma for small ranges and in [2, 3[ Andoni Arregi
2022-02-10 16:10     ` [PATCH 1/2] Improve lgammaf range for very small cases Andoni Arregi
2022-02-10 16:21       ` Paul Zimmermann
2022-02-10 16:12     ` [PATCH 2/2] Add a missing default case in lgamma Andoni Arregi
2022-02-10 17:29       ` Paul Zimmermann
2022-02-11 11:14         ` Andoni Arregi
2022-02-10 16:16     ` [PATCH 1/2] Improve lgammaf range for very small cases Andoni Arregi
2022-02-11 11:19     ` Andoni Arregi
2022-02-09 16:30 ` [PATCH 2/2] Add a missing default case in lgamma Andoni Arregi
2022-02-09 16:49 ` regression in Newlib 4.2.0 (lgammaf) Andoni Arregi
2022-02-11 11:16 ` [PATCH] Improve lgammaf range for very small cases Andoni Arregi
2022-02-14 13:46   ` Corinna Vinschen

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