CODEWITHSUNDEEP
c++ssesimd
SubliminalBroccoli
SubliminalBroccoli

Reputation: 83

Issue copying small amount of data using SSE intrinsics

I've looked at similar questions here and tried to use similar code, but am going wrong somewhere.. This is just a learning exercise by the way. Here is the code:

struct alignas(16) Data
{
    union
    {
      int field1;
      int field2;
      int field3;
      int field4;
      int field5;
      int field6;
      int field7;
      int field8;
      __m128 v[2];
   };
};

void copy((__m128* __restrict b, const __m128* __restrict a)
{
    *b++ = *a++;
    *b++ = *a++;
}

int main(int, char**)
{
   Data dst={0};
   Data src={0};
   src.field1=1;
   src.field2=500;
   src.field3=200;
   src.field4=393;
   src.field5=29383;
   src.field6=3838;
   src.field7=128484;
   src.field8=111;

   copy(dst.v,src.v);


   std::cout<<" before copy   dst.field1=" << dst.field1 <<" dst.field2=" << dst.field2 << std::endl;
   return 0;
}

It's showing 0 for both field1 and field2 before the copy, but both are 111 after the copy? I'm fairly new to c++ so it appears it's copying the last 32 bits of the struct over 64 bits of the dst struct, but not sure why?

Upvotes: 0

Views: 63

Answers (1)

Alan Birtles
Alan Birtles

Reputation: 36488

Your union is incorrect, it has 1 integer or your array of __m128. field1, field2 etc all point to the same memory. All your assignments apart from your last one to field8 are being overwritten.

You could use this instead:

struct alignas(16) Data
{
    union
    {
      int fields[8];
      __m128 v[2];
   };
};

Upvotes: 4

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