Bug 14241 is a regression in pow where pow (0.0, -Inf) returns the
wrong result on architectures using the dbl-64/e_pow.c
implementation. The problem is in the use of ABS(x) (inside the case
where x is a zero); the ABS macro in mydefs.h is
#define ABS(x) (((x)>0)?(x):-(x))
which gives -0.0 when x is 0.0. (This also affects the similar code
in the case for finite negative powers with absolute value up to 1e20
and not odd integers, but this doesn't show as testsuite failures
because in those cases the correct return value ends up coming from
__kernel_standard.)
I propose this patch fixing this by using 0.0 instead of ABS(x) in the
relevant places (not changing the definition of the ABS macro, given
that I don't know the reasons behind the present choice and given all
the other uses of that macro). Tested x86 and x86_64. Note that the
use of DIVIDE_BY_ZERO_EXCEPTION_OK is in accordance with C11 where
this exception is permitted but not required in this case (a
correction to C99 following N1515; ideally the exception would not be
present for these cases of infinite exponent, but now is not the time
to ensure and test consistency in this regard across the various pow
implementations).
2012-06-14 Joseph Myers <joseph@codesourcery.com>
[BZ #14241]
* sysdeps/ieee754/dbl-64/e_pow.c (__ieee754_pow): Use 0.0 instead
of ABS(x) in calculating zero to negative powers other than odd
integers.
* math/libm-test.inc (pow_test): Add more tests.
diff --git a/math/libm-test.inc b/math/libm-test.inc
index d32dde3..8e4d02e 100644
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -6290,6 +6290,8 @@ pow_test (void)
TEST_ff_f (pow, minus_zero, 11.1L, 0);
TEST_ff_f (pow, 0, plus_infty, 0);
TEST_ff_f (pow, minus_zero, plus_infty, 0);
+ TEST_ff_f (pow, 0, minus_infty, plus_infty, DIVIDE_BY_ZERO_EXCEPTION_OK);
+ TEST_ff_f (pow, minus_zero, minus_infty, plus_infty, DIVIDE_BY_ZERO_EXCEPTION_OK);
#ifndef TEST_INLINE
/* pow (x, +inf) == +inf for |x| > 1. */
diff --git a/sysdeps/ieee754/dbl-64/e_pow.c b/sysdeps/ieee754/dbl-64/e_pow.c
index 6c41af9..3fd5e65 100644
--- a/sysdeps/ieee754/dbl-64/e_pow.c
+++ b/sysdeps/ieee754/dbl-64/e_pow.c
@@ -112,12 +112,12 @@ __ieee754_pow(double x, double y) {
if (((v.i[HIGH_HALF] & 0x7fffffff) == 0x7ff00000 && v.i[LOW_HALF] != 0)
|| (v.i[HIGH_HALF] & 0x7fffffff) > 0x7ff00000)
return y;
- if (ABS(y) > 1.0e20) return (y>0)?0:1.0/ABS(x);
+ if (ABS(y) > 1.0e20) return (y>0)?0:1.0/0.0;
k = checkint(y);
if (k == -1)
return y < 0 ? 1.0/x : x;
else
- return y < 0 ? 1.0/ABS(x) : 0.0; /* return 0 */
+ return y < 0 ? 1.0/0.0 : 0.0; /* return 0 */
}
qx = u.i[HIGH_HALF]&0x7fffffff; /* no sign */