`_mm256_cvtss_f32` is only available in avxintrin.h of gcc-8 or
later. There it is defined as
```
extern __inline float
__attribute__ ((__gnu_inline__, __always_inline__, __artificial__))
_mm256_cvtss_f32 (__m256 __A)
{
return __A[0];
}
```
While explicit `vcurrent[0]` works with gcc-5 and gcc-6,
older gcc-4 fails with the following
error: invalid types 'float __vector__[int]' for array subscript
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