Error : casting user defined data types in c

You don't need to define a custom SIMD vector type (v4si) or mess around with casts and type punning - just use the provided intrinsics in the appropriate *intrin.h header, e.g.

#include <xmmintrin.h> // use SSE intrinsics 

int main(void)
{
    __m128 v;          // __m128 is the standard SSE vector type for 4 x float
    float x, y, z, w;

    v = _mm_set_ps(x, y, z, w);
                       // use intrinsic to set vector contents to x, y, z, w

    // ...

    return 0;
}

You appear to be using GCC vector extensions. The following code shows how to do broadcasts, vector + scalar, vector*scalar, loads and stores using vector extensions. #include

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

void print_v4sf(v4sf a) { for(int i=0; i<4; i++) printf("%f ", a[i]); puts(""); }

int main(void) {
  v4sf a;
  //broadcast a scalar
  a = ((v4sf){} + 1)*3.14159f;  
  print_v4sf(a);

  // vector + scalar
  a += 3.14159f;
  print_v4sf(a);

  // vector*scalar
  a *= 3.14159f;
  print_v4sf(a);

  //load from array
  float data[] = {1, 2, 3, 4};
  a = *(v4sf*)data;
  //a = __builtin_ia32_loadups(data);

  //store to array
  float store[4];
  *(v4sf*)store = a;
  for(int i=0; i<4; i++) printf("%f ", store[i]); puts("");
}

Clang 4.0 and ICC 17 support a subset of the GCC vector extensions. However, neither of them support vector + scalar or vector*scalar operations which GCC supports. A work around for Clang is to use Clang's OpenCL vector extensions. I don't know of a work around for ICC. MSVC does not support any kind of vector extension that I am aware of.

With GCC even though it supports vector + scalar and vector*scalar you cannot do vector = scalar (but you can with Clang's OpenCL vector extensions). Instead you can use this trick.

a = ((v4sf){} + 1)*3.14159f;

I would do as Paul R suggests and use intrinsics which are mostly compatible with the four major C/C++ compilers: GCC, Clang, ICC, and MSVC.

Here is a table of what is supported by each compiler using GCC's vector extensions and Clang's OpenCL vector extensions.

                                gcc  g++  clang  icc   OpenCL
unary operations                
[]                              yes  yes  yes    yes   yes
+, –                            yes  yes  yes    yes   yes
++, --                          yes  yes  no     no    no
~                               yes  yes  yes    yes   yes
!                               no   yes  no     no    yes 

binary vector op vector         
+,–,*,/,%                       yes  yes  yes    yes   yes    
&,|,^                           yes  yes  yes    yes   yes
>>,<<                           yes  yes  yes    yes   yes
==, !=, >, <, >=, <=            yes  yes  yes    yes   yes
&&, ||                          no   yes  no     no    yes

binary vector op scalar         
+,–,*,/,%                       yes  yes  no     no    yes
&,|,^                           yes  yes  no     no    yes
>>,<<                           yes  yes  no     no    yes
==, !=, >, <, >=, <=            yes  yes  no     no    yes                      
&&, ||                          no   yes  no     no    yes

assignment
vector = vector                 yes  yes  yes    yes   yes
vector = scalar                 no   no   no     no    yes                                              

ternary operator
?:                              no   yes  no     no    ?

We see that Clang and ICC do not support GCC's vector operator scalar operations. GCC in C++ mode supports everything but vector = scalar. Clang's OpenCL vector extensions support everything except maybe the ternary operator. Clang's documentation claims it does but I don't get it to work. GCC in C mode additional does not support binary logical operators or the ternary operator.