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What's the meaning of the -O4 optimization flag in gcc (3.2.3)? What's the difference to O3? When would you use one vs. the other?
The man pages only mention O, O0-3, Os, no word of the mysterious O4. Thanks!
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Turning on optimization flags 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. The compiler performs optimization based on the knowledge it has of the program.
GCC has a range of optimization levels, plus individual options to enable or disable particular optimizations. The overall compiler optimization level is controlled by the command line option -On, where n is the required optimization level, as follows: -O0 . (default).
gcc -c compiles source files without linking.
The gcc option -O enables different levels of optimization. Use -O0 to disable them and use -S to output assembly. -O3 is the highest level of optimization.
There is no -O4 in 3.2.3. Everything above -O3 results in -O3 being chosen.
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