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| author | Szabolcs Nagy <szabolcs.nagy@arm.com> | 2018-12-10 11:08:36 +0000 |
|---|---|---|
| committer | Szabolcs Nagy <szabolcs.nagy@arm.com> | 2018-12-11 10:01:43 +0000 |
| commit | 505b5b292293a5d6bd4046a6bc7f8c2381a33da4 (patch) | |
| tree | feeacf415d6fdbb306f2911deede485a6d92fda7 /sysdeps | |
| parent | 304c61a24f909168c16793ccf7c686237e53d003 (diff) | |
| download | glibc-505b5b292293a5d6bd4046a6bc7f8c2381a33da4.tar.gz | |
Fix powf overflow handling in non-nearest rounding mode [BZ #23961]
The threshold value at which powf overflows depends on the rounding mode
and the current check did not take this into account. So when the result
was rounded away from zero it could become infinity without setting
errno to ERANGE.
Example: pow(0x1.7ac7cp+5, 23) is 0x1.fffffep+127 + 0.1633ulp
If the result goes above 0x1.fffffep+127 + 0.5ulp then errno is set,
which is fine in nearest rounding mode, but
powf(0x1.7ac7cp+5, 23) is inf in upward rounding mode
powf(-0x1.7ac7cp+5, 23) is -inf in downward rounding mode
and the previous implementation did not set errno in these cases.
The fix tries to avoid affecting the common code path or calling a
function that may introduce a stack frame, so float arithmetics is used
to check the rounding mode and the threshold is selected accordingly.
[BZ #23961]
* math/auto-libm-test-in: Add new test case.
* math/auto-libm-test-out-pow: Regenerated.
* sysdeps/ieee754/flt-32/e_powf.c (__powf): Fix overflow check.
Diffstat (limited to 'sysdeps')
| -rw-r--r-- | sysdeps/ieee754/flt-32/e_powf.c | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/sysdeps/ieee754/flt-32/e_powf.c b/sysdeps/ieee754/flt-32/e_powf.c index d0c912188f..965f1fc2e7 100644 --- a/sysdeps/ieee754/flt-32/e_powf.c +++ b/sysdeps/ieee754/flt-32/e_powf.c @@ -17,6 +17,8 @@ <http://www.gnu.org/licenses/>. */ #include <math.h> +#include <math-barriers.h> +#include <math-narrow-eval.h> #include <stdint.h> #include <shlib-compat.h> #include <libm-alias-float.h> @@ -207,7 +209,17 @@ __powf (float x, float y) { /* |y*log(x)| >= 126. */ if (ylogx > 0x1.fffffffd1d571p+6 * POWF_SCALE) + /* |x^y| > 0x1.ffffffp127. */ return __math_oflowf (sign_bias); + if (WANT_ROUNDING && WANT_ERRNO + && ylogx > 0x1.fffffffa3aae2p+6 * POWF_SCALE) + /* |x^y| > 0x1.fffffep127, check if we round away from 0. */ + if ((!sign_bias + && math_narrow_eval (1.0f + math_opt_barrier (0x1p-25f)) != 1.0f) + || (sign_bias + && math_narrow_eval (-1.0f - math_opt_barrier (0x1p-25f)) + != -1.0f)) + return __math_oflowf (sign_bias); if (ylogx <= -150.0 * POWF_SCALE) return __math_uflowf (sign_bias); #if WANT_ERRNO_UFLOW |