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Fix ldbl-128ibm logbl near powers of 2 (bug 18030). 

The ldbl-128ibm implementation of logbl produces incorrect results
when the high part of the argument is a power of 2 and the low part a
nonzero number with the opposite sign (and so the returned exponent
should be 1 less than that of the high part).  For example, logbl
(0x1.ffffffffffffffp1L) returns 2 but should return 1.  (This is
similar to (fixed) bug 16740 for frexpl, and (fixed) bug 18029 for
ilogbl.)  This patch adds checks for that case.

Tested for powerpc.

	[BZ #18030]
	* sysdeps/ieee754/ldbl-128ibm/s_logbl.c (__logbl): Adjust exponent
	of power of 2 down when low part has opposite sign.
	* math/libm-test.inc (logb_test_data): Add more tests.
This commit is contained in:
Joseph Myers 2015-02-26 15:13:22 +00:00
parent 4a28f4d55a
commit 380bd0fd24
4 changed files with 27 additions and 4 deletions
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7
ChangeLog
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@ -1,3 +1,10 @@
2015-02-26 Joseph Myers <joseph@codesourcery.com>
[BZ #18030]
* sysdeps/ieee754/ldbl-128ibm/s_logbl.c (__logbl): Adjust exponent
of power of 2 down when low part has opposite sign.
* math/libm-test.inc (logb_test_data): Add more tests.
2015-02-26 Andreas Schwab <schwab@suse.de>
[BZ #18032]

2
NEWS
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@ -12,7 +12,7 @@ Version 2.22
4719, 14841, 13064, 14094, 15319, 15467, 15790, 15969, 16560, 16783,
17269, 17523, 17569, 17588, 17792, 17836, 17912, 17916, 17932, 17944,
17949, 17964, 17965, 17967, 17969, 17978, 17987, 17991, 17996, 17998,
17999, 18019, 18020, 18029, 18032.
17999, 18019, 18020, 18029, 18030, 18032.
* Character encoding and ctype tables were updated to Unicode 7.0.0, using
new generator scripts contributed by Pravin Satpute and Mike FABIAN (Red

5
math/libm-test.inc
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@ -7868,6 +7868,11 @@ static const struct test_f_f_data logb_test_data[] =
TEST_f_f (logb, 0x1p-16400L, -16400, NO_INEXACT_EXCEPTION),
TEST_f_f (logb, 0x.00000000001p-16382L, -16426, NO_INEXACT_EXCEPTION),
#endif
#if defined TEST_LDOUBLE && LDBL_MANT_DIG >= 57
TEST_f_f (logb, 0x1.ffffffffffffffp1L, 1, NO_INEXACT_EXCEPTION),
TEST_f_f (logb, -0x1.ffffffffffffffp1L, 1, NO_INEXACT_EXCEPTION),
#endif
};
static void

17
sysdeps/ieee754/ldbl-128ibm/s_logbl.c
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@ -26,11 +26,12 @@
long double
__logbl (long double x)
{
int64_t hx, rhx;
double xhi;
int64_t hx, hxs, rhx;
double xhi, xlo;
xhi = ldbl_high (x);
ldbl_unpack (x, &xhi, &xlo);
EXTRACT_WORDS64 (hx, xhi);
hxs = hx;
hx &= 0x7fffffffffffffffLL; /* high |x| */
if (hx == 0)
return -1.0 / fabs (x);
@ -42,6 +43,16 @@ __logbl (long double x)
though it were normalized. */
rhx -= __builtin_clzll (hx) - 12;
}
else if ((hx & 0x000fffffffffffffLL) == 0)
{
/* If the high part is a power of 2, and the low part is nonzero
with the opposite sign, the low part affects the
exponent. */
int64_t lx;
EXTRACT_WORDS64 (lx, xlo);
if ((hxs ^ lx) < 0 && (lx & 0x7fffffffffffffffLL) != 0)
rhx--;
}
return (long double) (rhx - 1023);
}
#ifndef __logbl