how to understand which compiler flags and what value are set in 11.0? i’ve read somewhere in docs that a lot of flags are set “by default” and can compile (almost) everything without set any flag on command line for difference of previous versions.
the question could be read also like “with what flags is compiled 11.0 kernel?” 
Do you mean gcc options or kernel configuration? If you want the kernel configuration, look in /boot/config-blahblah. If you want the gcc options used, then look at the build files in the kernel source package I suppose.
i mean gcc (compiler) flags - $CFLAGS
Look in the spec file, and probably also the makefiles inside the kernel SRPM.
Just curious, why? If it’s Gentoo you want, you know where that can be had.
because, in 10.3 for example, i was not able to compile any bigger project without specifying flags like
-fomit-frame-pointer
-funroll-all-loops
now it seems they are on by default. but the real reason is i have to built into the kernel some modules i need and would like to keep my kernel as close as possible to default