How much effort do you have to put in to get gains from using SSE?
- by John
Case One
Say you have a little class:
class Point3D
{
private:
float x,y,z;
public:
operator+=()
...etc
};
Point3D &Point3D::operator+=(Point3D &other)
{
this->x += other.x;
this->y += other.y;
this->z += other.z;
}
A naive use of SSE would simply replace these function bodies with using a few intrinsics. But would we expect this to make much difference? MMX used to involve costly state cahnges IIRC, does SSE or are they just like other instructions? And even if there's no direct "use SSE" overhead, would moving the values into SSE registers and back out again really make it any faster?
Case Two
Instead, you're working with a less OO-based code base. Rather than an array/vector of Point3D objects, you simply have a big array of floats:
float coordinateData[NUM_POINTS*3];
void add(int i,int j) //yes it's unsafe, no overlap check... example only
{
for (int x=0;x<3;++x)
{
coordinateData[i*3+x] += coordinateData[j*3+x];
}
}
What about use of SSE here? Any better?
In conclusion
Is trying to optimise single vector operations using SSE actually worthwhile, or is it really only valuable when doing bulk operations?
