https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71618
--- Comment #5 from Marc Glisse <glisse at gcc dot gnu.org> ---
We already have, just above the description of -O in the manual:
" Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on the
command line. Otherwise they are disabled, even if individual
optimization flags are specified.
Depending on the target and how GCC was configured, a slightly
different set of optimizations may be enabled at each -O level than
those listed here. You can invoke GCC with -Q --help=optimizers to
find out the exact set of optimizations that are enabled at each level."
Though I guess a bit of redundancy in the place you suggest may indeed help...
