Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the subprogram and get exactly the results you would expect from the source code.
Turning on optimization makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
The default is optimization off. This results in the fastest compile times, but GNAT makes absolutely no attempt to optimize, and the generated programs are considerably larger and slower than when optimization is enabled. You can use the -O switch (the permitted forms are -O0, -O1 -O2, -O3, and -Os) to gcc to control the optimization level:
Note that many other compilers do fairly extensive optimization
even if “no optimization” is specified. With gcc, it is
very unusual to use -O0 for production if
execution time is of any concern, since -O0
really does mean no optimization at all. This difference between
gcc and other compilers should be kept in mind when doing
Higher optimization levels perform more global transformations on the program and apply more expensive analysis algorithms in order to generate faster and more compact code. The price in compilation time, and the resulting improvement in execution time, both depend on the particular application and the hardware environment. You should experiment to find the best level for your application.
Since the precise set of optimizations done at each level will vary from release to release (and sometime from target to target), it is best to think of the optimization settings in general terms. See Options That Control Optimization, for details about the -O settings and a number of -f options that individually enable or disable specific optimizations.
Unlike some other compilation systems, gcc has been tested extensively at all optimization levels. There are some bugs which appear only with optimization turned on, but there have also been bugs which show up only in unoptimized code. Selecting a lower level of optimization does not improve the reliability of the code generator, which in practice is highly reliable at all optimization levels.
Note regarding the use of -O3: The use of this optimization level is generally discouraged with GNAT, since it often results in larger executables which run more slowly. See further discussion of this point in Inlining of Subprograms.