August 29th, 2006, 05:05 AM
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Command Line Warrior
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Quote from "http://www.netadmintools.com/html/1gcc.man.html"
Quote: Options That Control Optimization
These options control various sorts of optimizations:
-O
-O1
Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
Without -O, 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 function and get exactly the results you would expect from the source code.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
-O2
Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optional optimizations except for loop unrolling, function inlining, and register renaming. It also turns on the -fforce-mem option on all machines and frame pointer elimination on machines where doing so does not interfere with debugging.
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
-O3
Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions and -frename-registers options.
-O0
Do not optimize.
-Os
Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. |
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