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Optimizing for Small Code, No Optimization

Optimizing for Small Code

Selecting the second button causes the compiler to minimize the size of the code it produces. You can easily guess the trade-off between the speed and size optimizations. If selecting fast code produces speed at the expense of a larger EXE, selecting small code may generate a more compact file at the expense of performance. As you have already seen, a shorter list of instructions doesn’t necessarily correlate with greater performance.

No Optimization

If you select the third button, the compiler still generates native code, but it will no longer be optimized. Before you decide that Microsoft was asleep at the switch when this option was released, think about what it takes to develop an optimizing compiler. It isn’t easy to determine precisely how to handle every conceivable combination of factors governing the use of an optimization. Detecting when it is safe to move an instruction out of a loop or when a particular register value ought to be retained is no mean feat, and it is possible that the optimizer may make a mistake.

In other words, the opportunity to optimize one’s code is also another opportunity to introduce a bug. Generally speaking, it is certainly possible to introduce a bug via an optimization switch. The initial release of Visual C++ 5.0 had some problems with its speed optimizations, for example. (The problem was quickly identified and remedied with a service pack.)

Optimized or not, native code still ought to execute faster than P-Code. If you find that your program doesn’t run properly when you compile it with optimization features activated, you should try compiling with no optimization. If your program still misbehaves, it is probably not the fault of the compiler—this bug belongs to you! If the program behaves properly after being compiled with no optimization, however, it is just possible that you have found a bug in the VB6 optimization routines.


  

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