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Fix powf overflow handling in non-nearest rounding mode 

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.
This commit is contained in:
Szabolcs Nagy 2018-12-10 14:40:01 +00:00 committed by Corinna Vinschen
parent 55db4a8e3a
commit df6915f029
1 changed files with 10 additions and 0 deletions
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10
newlib/libm/common/sf_pow.c
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@ -220,7 +220,17 @@ 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
&& eval_as_float (1.0f + opt_barrier_float (0x1p-25f)) != 1.0f)
|| (sign_bias
&& eval_as_float (-1.0f - opt_barrier_float (0x1p-25f))
!= -1.0f))
return __math_oflowf (sign_bias);
if (ylogx <= -150.0 * POWF_SCALE)
return __math_uflowf (sign_bias);
#if WANT_ERRNO_UFLOW