implict SIMD (SSE/AVX) broadcasts with GCC

Question

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?

This appears to work fine in Clang using the OpenCL vector extentions typedef float float4 __attribute__((ext_vector_type(8)));. However, Clang does not allow the broadcasts with a register using the GCC vector extensions so I'm not sure it is entirely compatible with GCC. — Z boson
@MarcGlisse, I tested your solution in GCC Explorer and I can confirm that it gets converted to vbroadcastss. If you want to write up an answer I'll accept it. I understand the ambiguity of double vs. float. I guess it's even worse for integers since they can be 8-bits, 16-bits, 32-bits, or 64-bits. — Z boson

Marc Glisse Marc Glisse · Accepted Answer · 2014-02-12T15:43:25

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).

implict SIMD (SSE/AVX) broadcasts with GCC

2 Answers