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In g++ 4.6 (or later), what extra optimisations does -Ofast enable other than -ffast-math?
The man page says this option "also enables optimizations that are not valid for all standard compliant programs". Where can I find more information about whether this might affect my program or not?
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The compiler optimizes to reduce the size of the binary instead of execution speed. If you do not specify an optimization option, gcc attempts to reduce the compilation time and to make debugging always yield the result expected from reading the source code.
We say that code optimization is writing or rewriting code so a program uses the least possible memory or disk space, minimizes its CPU time or network bandwidth, or makes the best use of additional cores. In practice, we sometimes default to another definition: Writing less code.
GCC has since added -Og to bring the total to 8. From the man page: -O0 (do no optimization, the default if no optimization level is specified) -O1 (optimize minimally)
Here's a command for checking what options are enabled with -Ofast:
$ g++ -c -Q -Ofast --help=optimizers | grep enabled
Since I only have g++ 4.4 that doesn't support -Ofast, I can't show you the output.
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The -Ofast options might silently enable the gcc C++ extensions. You should check your sources to see if you make any use of them. In addition, the compiler might turn off some obscure and rarely encountered syntax checking for digraphs and trigraphs (this only improves compiler performance, not the speed of the compiled code).
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