Fix spurious undeflow for ldbl-128ibm erfl

For values higher than 25.6283 erflc underflow, so adjust erfl
to return a constant value based argument sign.

diff --git a/ChangeLog b/ChangeLog
index 925e986..dc61b0d 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2012-06-05  Adhemerval Zanella  <azanella@linux.vnet.ibm.com>
+
+       * sysdeps/ieee754/ldbl-128ibm/s_erfl.c: Fix spurious underflow for
+       values higher than 25.6283.
+
 2012-06-04  Adhemerval Zanella  <azanella@linux.vnet.ibm.com>
 
        * sysdeps/ieee754/ldbl-128ibm/e_fmodl.c (__ieee754_fmodl): Fix

diff --git a/sysdeps/ieee754/ldbl-128ibm/s_erfl.c b/sysdeps/ieee754/ldbl-128ibm/s_erfl.c
index f91a00f..6a4475e 100644 (file)
--- a/sysdeps/ieee754/ldbl-128ibm/s_erfl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_erfl.c
@@ -72,6 +72,8 @@
  *              z=1/x^2
  *         The interval is partitioned into several segments
  *         of width 1/8 in 1/x.
+ *     erf(x) = 1.0 - erfc(x) if x < 25.6283 else
+ *     erf(x) = sign(x)*(1.0 - tiny)
  *
  *      Note1:
  *        To compute exp(-x*x-0.5625+R/S), let s be a single
@@ -85,6 +87,9 @@
  *             erfc(x) ~ ---------- * ( 1 + Poly(1/x^2) )
  *                       x*sqrt(pi)
  *
+ *      Note3:
+ *        For x higher than 25.6283, erf(x) underflows.
+ *
  *      5. For inf > x >= 107
  *     erf(x)  = sign(x) *(1 - tiny)  (raise inexact)
  *     erfc(x) = tiny*tiny (raise underflow) if x > 0
@@ -770,9 +775,19 @@ __erfl (long double x)
 
   if (ix >= 0x3ff00000) /* |x| >= 1.0 */
     {
-      y = __erfcl (x);
-      return (one - y);
-      /*    return (one - __erfcl (x)); */
+      if (ix >= 0x4039A0DE)
+       {
+       /* __erfcl (x) underflows if x > 25.6283 */
+         if (sign)
+           return one-tiny;
+         else
+           return tiny-one;
+       }
+      else
+       {
+         y = __erfcl (x);
+         return (one - y);
+       }
     }
   u.parts32.w0 = ix;
   a = u.value;