How should i know a clang flag option is default on?

Hi:
I’m interested in how to know a command line option such as flto is default on in clang, what should i do?
Thank you all.
Zhengxianli

You can check the source code Options.td (clang/include/clang/Driver) or Options.inc (the tblgen output), to see the defination of each flag options.
But both ‘enable’ and ‘disable’ flags are in it. I also do not know in which file/function, we can select which one can be default.