From 380bd0fd2418f8988217de950f8b8ff18af0cb2b Mon Sep 17 00:00:00 2001 From: Joseph Myers Date: Thu, 26 Feb 2015 15:13:22 +0000 Subject: 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. --- sysdeps/ieee754/ldbl-128ibm/s_logbl.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) (limited to 'sysdeps/ieee754') 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 -- cgit v1.1