implict SIMD (SSE/AVX) broadcasts with GCC

问题:

I have manged to convert most of my SIMD code to us the vector extensions of GCC. However, I have not found a good solution for doing a broadcast as follows

__m256 areg0 = _mm256_broadcast_ss(&a[i]);

I want to do

__m256 argeg0 = a[i];

If you see my answer at Mutiplying vector by constant using SSE I managed to get broadcasts working with another SIMD register. The following works:

__m256 x,y;
y = x + 3.14159f; // broadcast x + 3.14159
y = 3.14159f*x;  // broadcast 3.14159*x

but this won't work:

 __m256 x;
 x = 3.14159f;  //should broadcast 3.14159 but does not work

How can I do this with GCC?

回答1:

I think there is currently no direct way and you have to work around it using the syntax you already noticed:

__m256 zero={};
__m256 x=zero+3.14159f;

It may change in the future if we can agree on a good syntax, see PR 55726.

Note that if you want to create a vector { s, s, ... s } with a non-constant float s, the technique above only works with integers, or with floats and -fno-signed-zeros. You can tweak it to __m256 x=s-zero; and it will work unless you use -frounding-math. A last version, suggested by Z boson, is __m256 x=(zero+1.f)*s; which should work in most cases (except possibly with a compiler paranoid about sNaN).

回答2:

It turns out that with a precise floating point model (e.g. with -O3) that GCC cannot simplify x+0 to x due to signed zero. So x = zero+3.14159f produces inefficient code. However GCC can simplify 1.0*x to just x therefore the efficient solution in this case is.

__m256 x = ((__m256){} + 1)*3.14159f;

https://godbolt.org/g/5QAQkC

See this answer for more details.

---

A simpler solution is just x = 3.14159f - (__m256){} because x - 0 = x irrespective of signed zero.