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Fix tanf spurious underflows (bug 18221) [committed]
- From: Joseph Myers <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Fri, 15 May 2015 17:48:37 +0000
- Subject: Fix tanf spurious underflows (bug 18221) [committed]
- Authentication-results: sourceware.org; auth=none
The flt-32 implementation of tanf produces spurious underflow
exceptions for some small arguments, through computing values on the
order of x^5. This patch fixes this by adjusting the threshold for
returning x (or, as applicable, +/- 1/x) to 2**-13 (the next term in
the power series being x^3/3).
Tested for x86_64 and x86. Committed.
(auto-libm-test-out diffs omitted below.)
2015-05-15 Joseph Myers <joseph@codesourcery.com>
[BZ #18221]
* sysdeps/ieee754/flt-32/k_tanf.c (__kernel_tanf): Use 2**-13 not
2**-28 as threshold for returning x or +/- 1/x.
* math/auto-libm-test-in: Add more tests of tan.
* math/auto-libm-test-out: Regenerated.
diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in
index 2a88403..e483569 100644
--- a/math/auto-libm-test-in
+++ b/math/auto-libm-test-in
@@ -2418,6 +2418,8 @@ tan pi/4
tan 0.75
tan 0x1p65
tan -0x1p65
+tan 0x1p-27
+tan -0x1p-27
tan 0xc.9p-4
tan 0xc.908p-4
tan 0xc.90cp-4
diff --git a/sysdeps/ieee754/flt-32/k_tanf.c b/sysdeps/ieee754/flt-32/k_tanf.c
index d918826..a67f36e 100644
--- a/sysdeps/ieee754/flt-32/k_tanf.c
+++ b/sysdeps/ieee754/flt-32/k_tanf.c
@@ -45,7 +45,7 @@ float __kernel_tanf(float x, float y, int iy)
int32_t ix,hx;
GET_FLOAT_WORD(hx,x);
ix = hx&0x7fffffff; /* high word of |x| */
- if(ix<0x31800000) /* x < 2**-28 */
+ if(ix<0x39000000) /* x < 2**-13 */
{if((int)x==0) { /* generate inexact */
if((ix|(iy+1))==0) return one/fabsf(x);
else return (iy==1)? x: -one/x;
--
Joseph S. Myers
joseph@codesourcery.com