Additional options to selectively pass to LTO backend (under -- features=thin_lto) when compiling certain backend objects. This option can be passed multiple times. Syntax: regex_filter@option_1,option_2,..., option_n. Where regex_filter stands for a list of include and exclude regular expression patterns. option_1 to option_n stand for arbitrary command line options. If an option contains a comma it has to be quoted with a backslash. Options can contain @. Only the first @ is used to split the string. Example: --per_file_ltobackendopt=//foo/.*\.o,-//foo/bar\.o@-O0 adds the -O0 command line option to the LTO backend command line of all o files in //foo/ except bar.o.