I have a __m256
value that holds random bits.
I would like to to "interpret" it, to obtain another __m256
that holds float
values in a uniform [0.0f, 1.0f]
range.
Planning to do it using:
__m256 randomBits = /* generated random bits, uniformly distribution */;
__m256 invFloatRange = _mm256_set1_ps( numeric_limits<float>::min() ); //min is a smallest increment of float precision
__m256 float01 = _mm256_mul(randomBits, invFloatRange);
//float01 is now ready to be used
Question 1:
However, will this cause a problem in very rare cases where randomBits
has all bits as 1 and is therefore NAN?
What can I do to protect myself from this?
I want the float01
to always be a usable number
Question 2:
Will the [0 to 1] range remain uniform after I obtain it using the above approach? I know float has varying precision at different magnitudes
Reinterpreting an int32_t as float, one can
auto const one = _mm256_set1_epi32(0x7f800000);
a = _mm256_and_si256(a, _mm256_set1_epi32(0x007fffff));
a = _mm256_or_si256(a, one);
return _mm256_sub_ps(_mm256_castsi256_ps(a), _mm256_castsi256_ps(one));
The and/or sequence will reuse the 23 LSBs of the input sequence to produce a uniform distribution of values between 1.0f <= a < 2.0f. And then the bias of 1.0f is removed.
As @Soonts has pointed out, floats can be created uniformly in [0, 1] range:
https://stackoverflow.com/a/54873925/9007125
I ended up using the answer below:
https://stackoverflow.com/a/54893167/9007125
//converts __m256i values into __m256 values, that contains floats in [0,1] range.
//https://stackoverflow.com/a/54893167/9007125
inline void int_rand_int_toFloat01( const __m256i* m256i_vals,
__m256* m256f_vals){ //<-- stores here.
const static __m256 c = _mm256_set1_ps(0x1.0p-24f); // or (1.0f / (uint32_t(1) << 24));
__m256i* rnd = ((__m256i*)m256i_vals);
__m256* output = ((__m256*)m256f_vals);
// remember that '_mm256_cvtepi32_ps' will convert 32-bit ints into a 32-bit floats
__m256 converted = _mm256_cvtepi32_ps(_mm256_srli_epi32(*rnd, 8));
*output = _mm256_mul_ps( converted, c);
}
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