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C–– Efficiency
5-38
repetition), but for loops are implemented with much greater overhead (one
conditional jump and three unconditional jumps per repetition.) For this
reason, it is best to replace for loops with while loops. (This was not done in
the example projects for the sake of readability and to provide an example of
a C–– for loop.) If the number of repetitions is both fixed and small, the code
will execute faster if the loop is unwrapped. Switch statements and if-else
blocks have similar overhead. Switch statements are slightly more efficient
because the values being compared are only looked up once, while an if-else
block looks up the values for each comparison. Switch statements do not use
a table lookup; they use a fall through structure like an if-else block. Because
of the fall through structure of switch and if-else blocks, items occurring first
are executed with less overhead than items occurring last. If it is known that
certain cases will occur more frequently than others, the code will execute
fastest if the most frequently occurring cases are put before the less frequently
occurring ones.
Space for global variables is allocated at compile time. Space for local
variables is allocated on the stack at run time. This means that the compiler
will not generate a warning if local variables exceed the available RAM. The
compiler will generate an error message if the global variables exceed the
available RAM. Caution must be used to avoid overflowing the stack by
allocating too many local variables. During a call, parameters, return address,
local variables, and the frame pointer are stored on the stack using a stack
frame. The stack frame structure allows recursive calls, but the elegant
solution provided by a recursive program is often offset by inefficiency. Using
recursive calls is not recommended with the C–– compiler.
Dividing the program into too many functions can be inefficient also. It may be
stylish to separate portions of the program into functions based on what they
are designed to do, but unless the functions will be used in multiple places in
the program, it is better not to make a function call. There is a tradeoff between
ROM usage and RAM usage depending on the number of times a function will
be needed. Using a function call requires more RAM and instruction overhead.
Not using a function call can require more ROM depending on the size of the
function and the number if times it is used.