This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Fix erfcf spurious underflows (bug 18217) [committed]
- From: Joseph Myers <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Fri, 15 May 2015 00:16:43 +0000
- Subject: Fix erfcf spurious underflows (bug 18217) [committed]
- Authentication-results: sourceware.org; auth=none
The flt-32 implementation of erfcf produces spurious underflow
exceptions for some arguments close to 0, because of calculations
squaring the argument and then multiplying by small constants. This
patch fixes this by adjusting the threshold for arguments for which
the result is so close to 1 that 1 - x will give the right result from
2**-56 to 2**-26. (If 1 - x * 2/sqrt(pi) were used, the errors would be
on the order of x^3 and a much larger threshold could be used.)
Tested for x86_64 and x86. Committed.
(auto-libm-test-out diffs omitted below.)
2015-05-15 Joseph Myers <joseph@codesourcery.com>
[BZ #18217]
* sysdeps/ieee754/flt-32/s_erff.c (__erfcf): Use 2**-26 not 2**-56
as threshold for returning 1 - x.
* math/auto-libm-test-in: Add more tests of erfc.
* math/auto-libm-test-out: Regenerated.
diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in
index 1deb99a..c4bfe74 100644
--- a/math/auto-libm-test-in
+++ b/math/auto-libm-test-in
@@ -1129,6 +1129,8 @@ erf -0x1.3a0d48p+0
erfc 0.0
erfc -0
+erfc 0x1p-55
+erfc -0x1p-55
erfc 0.125
erfc 0.75
erfc 1.25
diff --git a/sysdeps/ieee754/flt-32/s_erff.c b/sysdeps/ieee754/flt-32/s_erff.c
index d23cff9..2be44cc 100644
--- a/sysdeps/ieee754/flt-32/s_erff.c
+++ b/sysdeps/ieee754/flt-32/s_erff.c
@@ -169,7 +169,7 @@ float __erfcf(float x)
}
if(ix < 0x3f580000) { /* |x|<0.84375 */
- if(ix < 0x23800000) /* |x|<2**-56 */
+ if(ix < 0x32800000) /* |x|<2**-26 */
return one-x;
z = x*x;
r = pp0+z*(pp1+z*(pp2+z*(pp3+z*pp4)));
--
Joseph S. Myers
joseph@codesourcery.com