Fix ldbl-128 expm1l spurious underflow (bug 16539)
Commit Message
This patch fixes spurious underflows from ldbl-128 expm1l, as reported
in <https://sourceware.org/ml/libc-alpha/2014-06/msg00835.html> and
exposed by the tests added for such a bug in the x86 / x86-64
version. The bug and fix are essentially the same, so no separate bug
is filed in Bugzilla.
Tested for mips64.
2014-06-30 Joseph Myers <joseph@codesourcery.com>
[BZ #16539]
* sysdeps/ieee754/ldbl-128/s_expm1l.c: Include <float.h>.
(__expm1l): Return argument unchanged when small but not
subnormal.
Comments
On 06/30/2014 06:42 PM, Joseph S. Myers wrote:
> This patch fixes spurious underflows from ldbl-128 expm1l, as reported
> in <https://sourceware.org/ml/libc-alpha/2014-06/msg00835.html> and
> exposed by the tests added for such a bug in the x86 / x86-64
> version. The bug and fix are essentially the same, so no separate bug
> is filed in Bugzilla.
>
> Tested for mips64.
Thanks,
Andreas
@@ -54,6 +54,7 @@
#include <errno.h>
+#include <float.h>
#include <math.h>
#include <math_private.h>
@@ -136,6 +137,10 @@ __expm1l (long double x)
if (x < minarg)
return (4.0/big - 1.0L);
+ /* Avoid internal underflow when result does not underflow. */
+ if (fabsl (x) < 0x1p-113L && fabsl (x) >= LDBL_MIN)
+ return x;
+
/* Express x = ln 2 (k + remainder), remainder not exceeding 1/2. */
xx = C1 + C2; /* ln 2. */
px = __floorl (0.5 + x / xx);