GNU C native vectors: how to broadcast a scalar, like x86's _mm_set1_epi16

Question

How do I write a portable GNU C builtin vectors version of this, which doesn\'t depend on the x86 set1 intrinsic?

typedef uint16_t v8su __attribute__((vector         


        

Answer 1

A generic broadcast solution can be found for GCC and Clang using two observations

1. Clang's OpenCL vector extensions and GCC's vector extensions support scalar - vector operations.
2. x - 0 = x (but x + 0 does not work due to signed zero).

Here is a solution for a vector of four floats.

#if defined (__clang__)
typedef float v4sf __attribute__((ext_vector_type(4)));
#else
typedef float v4sf __attribute__ ((vector_size (16)));
#endif

v4sf broadcast4f(float x) {
  return x - (v4sf){};
}

https://godbolt.org/g/PXr3Xb

The same generic solution can be used for different vectors. Here is an example for a vector of eight unsigned shorts.

#if defined (__clang__)
typedef unsigned short v8su __attribute__((ext_vector_type(8)));
#else
typedef unsigned short v8su __attribute__((vector_size(16)));
#endif

v8su broadcast8us(short x) {
  return x - (v8su){};
}

ICC (17) supports a subset of the GCC vector extensions but does not support either vector + scalar or vector*scalar yet so intrinsics are still necessary for broadcasts. MSVC does not support any vector extensions.