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Fix ldbl-128 roundl for exponents in [31, 47] (bug 18346).

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The implementation of roundl for ldbl-128 involves undefined behavior
for arguments with exponents from 31 to 47 inclusive, from the shift:

      u_int64_t i = -1ULL >> (j0 - 48);

For example, on mips64, this means roundl (0xffffffffffff.8p0L)
wrongly returns its argument, which is not an integer.  A condition
checking for exponents < 31 should actually be checking for exponents
< 48, and this patch makes it do so.  (That condition is for whether
the bit representing 0.5 is in the high 64-bit half of the
floating-point number.  The value 31 might have arisen from an
incorrect conversion of the ldbl-96 version to handle ldbl-128.)

This was originally reported as a GCC libquadmath bug
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65757>.

Tested for mips64; also tested for x86_64 and x86 to make sure the new
tests pass there.

	[BZ #18346]
	* sysdeps/ieee754/ldbl-128/s_roundl.c (__roundl): Handle all
	exponents less than 48 as cases where high part of mantissa needs
	examining to determine whether argument is integral.
	* math/libm-test.inc (round_test_data): Add more tests.
This commit is contained in:
Joseph Myers
2015-04-28 17:27:02 +00:00
parent fb4041ae53
commit 7d0b257541
4 changed files with 21 additions and 2 deletions
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11
math/libm-test.inc
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@ -9012,6 +9012,17 @@ static const struct test_f_f_data round_test_data[] =
TEST_f_f (round, 2097152.5, 2097153),
TEST_f_f (round, -2097152.5, -2097153),
#ifndef TEST_FLOAT
TEST_f_f (round, 0xffffffffffff.0p0L, 0xffffffffffff.0p0L),
TEST_f_f (round, 0xffffffffffff.4p0L, 0xffffffffffff.0p0L),
TEST_f_f (round, 0xffffffffffff.8p0L, 0x1000000000000.0p0L),
TEST_f_f (round, 0xffffffffffff.cp0L, 0x1000000000000.0p0L),
TEST_f_f (round, -0xffffffffffff.0p0L, -0xffffffffffff.0p0L),
TEST_f_f (round, -0xffffffffffff.4p0L, -0xffffffffffff.0p0L),
TEST_f_f (round, -0xffffffffffff.8p0L, -0x1000000000000.0p0L),
TEST_f_f (round, -0xffffffffffff.cp0L, -0x1000000000000.0p0L),
#endif
#ifdef TEST_LDOUBLE
/* The result can only be represented in long double. */
TEST_f_f (round, 4503599627370495.5L, 4503599627370496.0L),