Does the same description exist for clang? They are up to -O6. I only know their pass descriptions http://llvm.org/docs/Passes.html but nothing related to the O level and -g

Are you sure they're up to -O6 ?

Doing this in any autoconf-based program:

    $ ./configure CC="/usr/bin/clang-8" CFLAGS="-Weverything -O6 -flto=full" LDFLAGS="-fuse-ld=/usr/bin/ld.lld-8"

... fails (error: C compiler cannot create executables); same for -O5.

-O4 works, but generates this diagnostic:

    clang: warning: -O4 is equivalent to -O3 [-Wdeprecated]