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
gcc to control the optimization level:
No optimization (the default); generates unoptimized code but has the fastest compilation time.
Note that many other compilers do substantial optimization even
if ‘no optimization’ is specified. With gcc, it is very unusual
-O0 for production if execution time is of any concern,
-O0 means (almost) no optimization. This difference
between gcc and other compilers should be kept in mind when
doing performance comparisons.
Moderate optimization; optimizes reasonably well but does not degrade compilation time significantly.
Full optimization; generates highly optimized code and has the slowest compilation time.
Full optimization as in
also uses more aggressive automatic inlining of subprograms within a unit
(Inlining of Subprograms) and attempts to vectorize loops.
Optimize space usage (code and data) of resulting program.
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 the ‘Options That Control Optimization’ section in
Using the GNU Compiler Collection (GCC)
for details about
-O settings and a number of
-f options that
individually enable or disable specific optimizations.
Unlike some other compilation systems,
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
Note regarding the use of
-O3: The use of this optimization level
ought not to be automatically preferred over that of level
since it often results in larger executables which may run more slowly.
See further discussion of this point in Inlining of Subprograms.