BUG: Fix underflow error in AVX512 implementation of ufunc exp/f64

[seiko2plus]

closes #18932, related to #18920, #18916.

[seiko2plus]

@seberg, CI(use_wheel (pull_request)) ufunc tests complaining about:

 >               assert_array_equal(res_num.astype("O"), res_obj)
E               AssertionError: 
E               Arrays are not equal
E               
E               Mismatched elements: 1 / 1 (100%)
E               Max absolute difference: 2.7755575615628914e-17
E               Max relative difference: 1.1489924412333298e-16
E                x: array([-0.2415644752704905], dtype=object)
E                y: array([-0.24156447527049046], dtype=object)

MyFloat    = <class 'numpy.core.tests.test_ufunc.TestUfuncGenericLoops.test_unary_PyUFunc_O_O_method_full.<locals>.MyFloat'>
num_arr    = array([0.78539816])
obj_arr    = array([0.7853981633974483], dtype=object)
res_num    = array([-0.24156448])
res_obj    = array([-0.24156447527049046], dtype=object)
self       = <numpy.core.tests.test_ufunc.TestUfuncGenericLoops object at 0x7f897731c950>
ufunc      = <ufunc 'log'>
val        = 0.7853981633974483

why we don't use assert_array_almost_equal in here:

numpy/numpy/core/tests/test_ufunc.py

Lines 177 to 178 in 6ad4650

	res_obj = ufunc(obj_arr)
	assert_array_equal(res_num.astype("O"), res_obj)

[mattip]

Yes, that seems too strong a requirement.

[seiko2plus]

@mattip, Should I replace it with assert_array_almost_equal? I don't think this error is related to this patch.

[seiko2plus]

close/open for another round.

[charris]

LGTM. Go ahead and fix the failing test so everything starts working again.

[seberg]

Oh, you changed the test to almost-equal. I had problems elsewhere and changed it to 0-D, but this is just as well. EDIT: Sorry, that was unclear. That was in a work-in-progress, lets go with this! :)

(I think I may have changed 0-stride vs. contiguous-stride for 0-D, which probably can change whether the SIMD loop is used sometimes?)

[seberg]

Thanks for the quick fix, Sayed!

[seiko2plus]

@seberg,

which probably can change whether the SIMD loop is used sometimes?)

That probably what happened, but avoid using almost_equal is kinda strict especially for CPUs that don't have native support of FMA/Rounding.

[r-devulap]

oh yikes! /me hides in shame.
Thanks @seiko2plus for fixing this.