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http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-O-775 documents the -f* flags enabled by -O*. And the -f* options document their respective -O* options which enables them.
However, for -Og this is not documented. The information can be obtained using:
gcc -Q --help=optimizers -Og|grep enabled
It might be useful to have this information in the man page for -Og as well.
But it might be misleading (similar to -O0 vs -O2) - enabling for example
-ftree-pre won't enable PRE for -Og as it has a completely different
pass pipeline which is not based on -O. Those enumerations are only
relevant for the 'numbered' optimization levels (excluding -O0).
So we miss to filter options in --help optimizers that can be enabled at all.