svn commit: r215237 - head/lib/msun/src

[Ulrich Spoerlein]

Author: uqs
Date: Sat Nov 13 10:54:10 2010
New Revision: 215237
URL: http://svn.freebsd.org/changeset/base/215237

Log:
  Fix bug in jn(3) and jnf(3) that led to -inf results
  
  Explanation by Steve:
  jn[f](n,x) for certain ranges of x uses downward recursion to compute
  the value of the function.  The recursion sequence that is generated is
  proportional to the actual desired value, so a normalization step is
  taken.  This normalization is j0[f](x) divided by the zeroth sequence
  member.  As Bruce notes, near the zeros of j0[f](x) the computed value
  can have giga-ULP inaccuracy. I found for the 1st zero of j0f(x) only
  the leading decimal digit is correct.  The solution to the issue is
  fairly straight forward.  The zeros of j0(x) and j1(x) never coincide,
  so as j0(x) approaches a zero, the normalization constant switches to
  j1[f](x) divided by the 2nd sequence member.  The expectation is that
  j1[f](x) is a more accurately computed value.
  
  PR:		bin/144306
  Submitted by:	Steven G. Kargl <kargl at troutmask.apl.washington.edu>
  Reviewed by:	bde
  MFC after:	7 days

Modified:
  head/lib/msun/src/e_jn.c
  head/lib/msun/src/e_jnf.c

Modified: head/lib/msun/src/e_jn.c
==============================================================================
--- head/lib/msun/src/e_jn.c	Sat Nov 13 10:38:06 2010	(r215236)
+++ head/lib/msun/src/e_jn.c	Sat Nov 13 10:54:10 2010	(r215237)
@@ -200,7 +200,12 @@ __ieee754_jn(int n, double x)
 			}
 	     	    }
 		}
-	    	b = (t*__ieee754_j0(x)/b);
+		z = __ieee754_j0(x);
+		w = __ieee754_j1(x);
+		if (fabs(z) >= fabs(w))
+		    b = (t*z/b);
+		else
+		    b = (t*w/a);
 	    }
 	}
 	if(sgn==1) return -b; else return b;

Modified: head/lib/msun/src/e_jnf.c
==============================================================================
--- head/lib/msun/src/e_jnf.c	Sat Nov 13 10:38:06 2010	(r215236)
+++ head/lib/msun/src/e_jnf.c	Sat Nov 13 10:54:10 2010	(r215237)
@@ -152,7 +152,12 @@ __ieee754_jnf(int n, float x)
 			}
 	     	    }
 		}
-	    	b = (t*__ieee754_j0f(x)/b);
+		z = __ieee754_j0f(x);
+		w = __ieee754_j1f(x);
+		if (fabsf(z) >= fabsf(w))
+		    b = (t*z/b);
+		else
+		    b = (t*w/a);
 	    }
 	}
 	if(sgn==1) return -b; else return b;