Visual C + + optimization Overview

Summary: Demonstrates several features in the many code optimization features provided by the Visual C + + 2003 compiler.

Microsoft Visual C + + Toolkit 2003 contains optimized C + + compilers. Most switches are fairly concise and are already present in multiple versions of the Visual C + + product, but there are still two switches that are newer and can significantly improve speed without rewriting the code. They are/GL (Whole program Optimization) and/G7 (which can produce code optimized for Pentium 4 or AMD Athlon). There is also an option/ARCH:SSE2, which produces code optimized for SSE2 registers and instructions.

The sample code passed three Tests:

1. Call functions that act as inline candidate functions.

2. Perform a large number of floating-point multiplication and addition operations.

3. Perform a large number of integer multiplication and addition operations.

Full program Optimization

The sample code defines two very similar functions: Add () and Displayadd (). Displayadd () is displayed to the screen, so it is unlikely to be inline:

void DisplayAdd(int a, int b)
{
　 cout << a << " + " << b << " = " << a + b << endl;
　 cout << "Return address from " << __FUNCTION__
　　　　<< " " << _ReturnAddress() << endl;
}

_returnaddress is an internal function that reports where control will be returned. You can use it to identify inline functions.

ADD () is declared in Gl-g7.cpp and also declares a global variable that is set by it:

void* inlineReturnAddress; // set in Add()
int Add(int a, int b); // implementation in module.cpp

Implementation is in Module.cpp:

int Add(int a, int b)
{
　 inlineReturnAddress = _ReturnAddress();
　 return a+b;
}

To compile the program and not use Whole Prgram optimization, use the following command line:

CL/O2/ML/EHSC Gl-g7.cpp Module.cpp

To run test 1, use the following command:

Gl-g7 1

You should see output similar to the following: The numeric address will be different.

1 + 2 = 3
Return address from DisplayAdd 00401D0A
1 + 2 = 3
Return address from Add 00401D13
Return address from Test1 00402125

The return address of Add () is different from the return address of Test1 (): Add () is not inline.

Now, recompile with/GL:

cl /O2 /ML /EHsc /GL GL-G7.cpp module.cpp

Run Test 1 again and you should see the output shown below:

1 + 2 = 3
Return address from DisplayAdd 00401242
1 + 2 = 3
Return address from Add 0040179F
Return address from Test1 0040179F

Now, add () and Test1 () return the same address: add () is inline within Test1 (), even if its code comes from another file.