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[PATCH] Fix spurious undeflow for ldbl-128ibm erfl
- From: Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>
- To: "GNU C. Library" <libc-alpha at sourceware dot org>
- Date: Thu, 31 May 2012 17:04:17 -0300
- Subject: [PATCH] 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.
---
2012-05-31 Adhemerval Zanella <azanella@linux.vnet.ibm.com>
* sysdeps/ieee754/ldbl-128ibm/s_erfl.c: Fix spurious underflow for
values higher than 25.6283.
diff --git a/sysdeps/ieee754/ldbl-128ibm/s_erfl.c b/sysdeps/ieee754/ldbl-128ibm/s_erfl.c
index f91a00f..3572e93 100644
--- 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 of 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;
--
1.6.0.2
--
Adhemerval Zanella Netto
Software Engineer
Linux Technology Center Brazil
Toolchain / GLIBC on Power Architecture
azanella@linux.vnet.ibm.com / azanella@br.ibm.com
+55 61 8642-9890