C simd _m128 fabs

Question

How to make fabs() for __m128 vector ?

Does I have to use sign bits to multiply the original vector by 1.0f/-1.0f ?

Didn't find any instruction set to do it.

I don't want __m256 or 512. I'm searching for __m128 data type

Answer 1

A single AND-NOT using a prepared constant to turn off the sign bits suffices:

#include <stdio.h>

#include <xmmintrin.h>


int main(void)
{
    static const __m128 MinusZero = { -0.f, -0.f, -0.f, -0.f };

    __m128 x = { +1, -2, +3, -4 };

    __m128 y = _mm_andnot_ps(MinusZero, x);

    printf("%g %g %g %g\n", y[0], y[1], y[2], y[3]);
}

Output:

1 2 3 4

Answer 2

I have find a persone proposing this, in a related post, but if you have better, please propose it. I'm new to simd.

__m128 _m128_fabs(__m128 x)
{
    __m128 minus_zero = _mm_set1_ps(-0.0);  // epi32(1U<<31)
    __m128 signbits = _mm_and_ps(x, minus_zero);
    __m128 flipped = _mm_xor_ps(x, signbits);

    // reuse the zero constant we already have, maybe saving an instruction
    __m128 nonzero = _mm_cmpneq_ps(x, minus_zero);
    return _mm_and_ps(flipped, nonzero);
}

Answer 3

One way using SSE4.1 instructions:

__m128 _mm_fabs_ps(__m128 vec) {
  return _mm_blendv_ps(vec, _mm_mul_ps(vec, _mm_set_ps1(-1.0)),
                       _mm_cmplt_ps(vec, _mm_set_ps1(0.0)));
}

Not sure how it compares performance-wise to the other answer.

Basic idea is to pick either the positive number or the negative number multiplied by -1.0.