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Fix ldbl-128 powl sign of result in overflow / underflow cases (bug 17097).

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This patch fixes bug 17097, ldbl-128 powl producing overflowing /
underflowing results with positive sign when the result should have
been negative.  This was shown up by the tests in non-default rounding
modes added by my patch for bug 16315, but isn't actually limited to
non-default rounding modes: rather, when rounding to nearest the
wrappers produced a result with the correct sign and so always hid the
bug unless -lieee was used to disable the wrappers.  The problem is
that in the cases where Y is large enough that the result overflows or
underflows for X not very close to 1, but not large enough to overflow
or underflow for all X != +/- 1 (in the latter case Y is always an
even integer), a positive overflowing / underflowing result is always
returned, rather than one with the correct sign.  This patch moves the
relevant part of computation of the sign earlier and returns a result
of the correct sign.

Tested for mips64.

	[BZ #17097]
	* sysdeps/ieee754/ldbl-128/e_powl.c (__ieee754_powl): Return
	result with correct sign in case of exponents that produce
	overflow except for X very close to 1.
This commit is contained in:
Joseph Myers 2014-06-29 11:49:08 +00:00
parent dd0ba01812
commit edea402804
3 changed files with 21 additions and 14 deletions
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7
ChangeLog
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@ -1,3 +1,10 @@
2014-06-29 Joseph Myers <joseph@codesourcery.com>
[BZ #17097]
* sysdeps/ieee754/ldbl-128/e_powl.c (__ieee754_powl): Return
result with correct sign in case of exponents that produce
overflow except for X very close to 1.
2014-06-28 Paul Eggert <eggert@cs.ucla.edu>
mktime: merge #if/#ifdef usage from glibc

2
NEWS
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@ -21,7 +21,7 @@ Version 2.20
16882, 16885, 16888, 16890, 16912, 16915, 16916, 16917, 16918, 16922,
16927, 16928, 16932, 16943, 16958, 16965, 16966, 16967, 16977, 16978,
16984, 16990, 16996, 17009, 17022, 17031, 17042, 17048, 17050, 17058,
17061, 17062, 17069, 17075, 17079, 17084, 17086, 17092.
17061, 17062, 17069, 17075, 17079, 17084, 17086, 17092, 17097.
* Optimized strchr implementation for AArch64. Contributed by ARM Ltd.

26
sysdeps/ieee754/ldbl-128/e_powl.c
View File

@ -148,7 +148,7 @@ long double
__ieee754_powl (long double x, long double y)
{
long double z, ax, z_h, z_l, p_h, p_l;
long double y1, t1, t2, r, s, t, u, v, w;
long double y1, t1, t2, r, s, sgn, t, u, v, w;
long double s2, s_h, s_l, t_h, t_l, ay;
int32_t i, j, k, yisint, n;
u_int32_t ix, iy;
@ -262,6 +262,11 @@ __ieee754_powl (long double x, long double y)
if (((((u_int32_t) hx >> 31) - 1) | yisint) == 0)
return (x - x) / (x - x);
/* sgn (sign of result -ve**odd) = -1 else = 1 */
sgn = one;
if (((((u_int32_t) hx >> 31) - 1) | (yisint - 1)) == 0)
sgn = -one; /* (-ve)**(odd int) */
/* |y| is huge.
2^-16495 = 1/2 of smallest representable value.
If (1 - 1/131072)^y underflows, y > 1.4986e9 */
@ -277,9 +282,9 @@ __ieee754_powl (long double x, long double y)
}
/* over/underflow if x is not close to one */
if (ix < 0x3ffeffff)
return (hy < 0) ? huge * huge : tiny * tiny;
return (hy < 0) ? sgn * huge * huge : sgn * tiny * tiny;
if (ix > 0x3fff0000)
return (hy > 0) ? huge * huge : tiny * tiny;
return (hy > 0) ? sgn * huge * huge : sgn * tiny * tiny;
}
ay = y > 0 ? y : -y;
@ -366,11 +371,6 @@ __ieee754_powl (long double x, long double y)
t1 = o.value;
t2 = z_l - (((t1 - t) - dp_h[k]) - z_h);
/* s (sign of result -ve**odd) = -1 else = 1 */
s = one;
if (((((u_int32_t) hx >> 31) - 1) | (yisint - 1)) == 0)
s = -one; /* (-ve)**(odd int) */
/* split up y into y1+y2 and compute (y1+y2)*(t1+t2) */
y1 = y;
o.value = y1;
@ -386,11 +386,11 @@ __ieee754_powl (long double x, long double y)
{
/* if z > 16384 */
if (((j - 0x400d0000) | o.parts32.w1 | o.parts32.w2 | o.parts32.w3) != 0)
return s * huge * huge; /* overflow */
return sgn * huge * huge; /* overflow */
else
{
if (p_l + ovt > z - p_h)
return s * huge * huge; /* overflow */
return sgn * huge * huge; /* overflow */
}
}
else if ((j & 0x7fffffff) >= 0x400d01b9) /* z <= -16495 */
@ -398,11 +398,11 @@ __ieee754_powl (long double x, long double y)
/* z < -16495 */
if (((j - 0xc00d01bc) | o.parts32.w1 | o.parts32.w2 | o.parts32.w3)
!= 0)
return s * tiny * tiny; /* underflow */
return sgn * tiny * tiny; /* underflow */
else
{
if (p_l <= z - p_h)
return s * tiny * tiny; /* underflow */
return sgn * tiny * tiny; /* underflow */
}
}
/* compute 2**(p_h+p_l) */
@ -441,6 +441,6 @@ __ieee754_powl (long double x, long double y)
o.parts32.w0 = j;
z = o.value;
}
return s * z;
return sgn * z;
}
strong_alias (__ieee754_powl, __powl_finite)