C++ for Scientists - Technische Universität Dresden

5.3. EXPRESSION TEMPLATES 151
}
std::cout ≪ ”x = ” ≪ x ≪ std::endl;
std::cout ≪ ”y = ” ≪ y ≪ std::endl;
std::cout ≪ ”z = ” ≪ z ≪ std::endl;
w= x + y + z;
std::cout ≪ ”w= x + y + z = ” ≪ w ≪ std::endl;
return 0;
If this works properly, what is wrong with it? From the software engineering prospective:
nothing. From the performance prospective: a lot.
How is the statement executed:
1. Create temporary variable sum for the addition of x and y;
2. Perform a loop reading x and y, adding it element-wise, and writing the result to sum;
3. Copy sum to a temporary variable, say t xy, in the return statement;
4. Delete sum;
5. Create temporary variable sum for the addition of t xy and z;
6. Perform a loop reading t xy and z, adding it element-wise, and writing the result to sum;
7. Copy sum to a temporary variable, say t xyz, in the return statement;
8. Delete sum;
9. Delete t xy;
10. Perform a loop reading t xyz and writing to w;
11. Delete t xyz;
This is admittedly the worst-case scenario. But it was the code that old compilers generated.
Modern compilers perform more optimizations by static code analysis and can avoid copying
the return value into the temporaries t xy and t xyz. Instead of creating the temporaries t xy
and t xyz, they become aliases for the respective sum temporaries.
The optimized version performs:
1. Create temporary variable sum (for distinction sum xy) for the addition of x and y;
2. Perform a loop reading x and y, adding it element-wise, and writing the result to sum;
3. Create temporary variable sum (for distinction sum xyz) for the addition of sum xy and z;
4. Perform a loop reading sum xy and z, adding it, and writing the result to sum xyz;
5. Delete sum xy;
6. Perform a loop reading sum xyz and writing to w;
7. Delete sum xyz;
How much operations did we perform? Say our vectors have lenght n then we have in total:
• 2n additions;