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Optimizing for Fast Code

After you have selected Compile to Native Code, select the first option button underneath it to generate the fastest code possible (the Optimize for Fast Code option). Even if the compiler decides that it needs to produce more machine instruction code to handle certain portions of your application, thereby resulting in a bigger EXE file, the end result ought to be faster than the smaller alternatives.

You may wonder how one set of instructions can be faster than another, if each accomplishes the same end result. Well, you can get to your next-door neighbor’s house by walking about 25,000 miles around the earth or by walking a few steps the other direction and knocking on the door. You get the same result either way. A compiler that can optimize for speed just knows how to take those shorter routes.

The VB programmer can perform some kinds of optimizations. VB doesn’t short-circuit expressions like C or C++, for example. That is, in a conditional expression such as the following:

If iConditionOne < 1 and iConditionTwo < 10 then
‘ do something
End If

VB evaluates both parts of the conditional expression every time. Even if the value of iConditionOne were 5, so that the overall expression must evaluate to False, VB would still evaluate the value of iConditionTwo. If a C or C++ compiler evaluated this conditional, it would know that the overall expression must evaluate to False as soon as it evaluated the first expression. This is called short-circuiting.

If a programmer knows that VB doesn’t short-circuit logical expressions, it is simple to develop the more efficient habit of coding like this:

If iConditionOne < 1 then
If iConditionTwo < 10 then
‘ do something
End If
End If

It takes two extra lines of code, but the second fragment executes more quickly than the first when the first condition is false. In this case, knowing how the language behaves makes it possible for you to write smarter code.

Optimization for performance generally occurs in two ways: globally and at the register level. If a compiler employs global optimization methods, it tries to change the order in which your program’s instructions are executed. This can save time if an action is being repeated unnecessarily, as in a loop such as this:

Do
iBadlyPlacedVariable = 1
‘ more processing occurs here, but
‘doesn’t change the value of the variable
Loop

In this case, the variable is assigned a value of 1 every time this loop repeats. If the loop iterates several thousand times, that’s several thousand unnecessary assignments. Clearly, the assignment should have been done outside the loop, but the programmer made a mistake. If it uses global methods, an optimizing compiler can correct this mistake.

Register-level optimization tries to save time by putting data where it can be reached most quickly. Generally speaking, the data your computer needs can be found in one of just three places. In order of increasing speed, these are as follows:

  • Physical storage

  • Random access memory

  • A CPU register

When possible, register optimization tries to put data into a register for quick access.

It takes a relatively long time to find data on a physical storage device such as a hard disk. Even fast hard disks have average seek times measuring in the millisecond range, which is an awfully long time compared to the nanoseconds used to measure RAM chips. Given a choice, it is always better to search RAM than to search a hard disk.

(That’s why disk-caching programs are useful: They store recently accessed data from the hard drive in memory for faster access.)

Because they are part of the CPU itself, registers are even faster than RAM. If data or an instruction is in RAM, the CPU has to wait for it to be copied into a register to do anything with it. If it is already in a register, the CPU obviously doesn’t have to wait on the RAM access operation. Register optimization occurs when a compiler can reduce the amount of register manipulation necessary to give the CPU what it needs to run the program.

You will see how well the basic speed optimizations work shortly; but first, this section looks at the other basic optimization choices. The explanations of the remaining basic optimizations will be brief;
they’re quite simple.


  

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