How to negate (change sign) of the floating point elements in a __m128 type variable?
c, simd, sse, vectorization, x86
Solution
In practice your compiler should do a good job of generating the constant vector for 0.0. It will probably just use `_mm_xor_ps`, and if your code is in a loop it should hoist the constant generation out of the loop anyway. So, bottom line, use your original idea of:
v = _mm_sub_ps(_mm_set1_ps(0.0), v);
or another common trick, which is:
v = _mm_xor_ps(v, _mm_set1_ps(-0.0));
which just flips the sign bits instead of doing a subtraction (not quite as safe as the first method, since it doesn't do the right thing with NaNs, but may be more efficient in some cases).
Problem
Is there any single instruction or function that can invert the sign of every float inside a __m128? i.e. `a = r0:r1:r2:r3 ===> a = -r0:-r1:-r2:-r3`? I know this can be done by `_mm_sub_ps(_mm_set1_ps(0.0),a)`, but isn't it potentially slow since `_mm_set1_ps(0.0)` is a multi-instruction function?