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Fix ldbl-128ibm logbl near powers of 2 (bug 18030) [committed]
- From: Joseph Myers <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Thu, 26 Feb 2015 15:16:11 +0000
- Subject: Fix ldbl-128ibm logbl near powers of 2 (bug 18030) [committed]
- Authentication-results: sourceware.org; auth=none
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. Committed.
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.
diff --git a/math/libm-test.inc b/math/libm-test.inc
index 6045764..7acd29b 100644
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -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
diff --git a/sysdeps/ieee754/ldbl-128ibm/s_logbl.c b/sysdeps/ieee754/ldbl-128ibm/s_logbl.c
index dbd3478..22e5fc2 100644
--- a/sysdeps/ieee754/ldbl-128ibm/s_logbl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_logbl.c
@@ -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
--
Joseph S. Myers
joseph@codesourcery.com