On Sun, 9 Jul 2017 09:59:44 -0400 (EDT) Nicolas Pitre <nicolas.pi...@linaro.org> wrote:
> On Sun, 9 Jul 2017, Masahiro Yamada wrote: > > > Hi. > > > > 2017-07-09 18:05 GMT+09:00 Ingo Molnar <mi...@kernel.org>: > > > > > > * Nicholas Piggin <npig...@gmail.com> wrote: > > > > > >> FYI, easiest way to check if you forgot to KEEP a linker table is > > >> to look at `readelf -S vmlinux` differences, and to see what is > > >> being trimmed, look at nm differences or use --print-gc-sections > > >> LD option to see what symbols you're trimming. Linker tables, > > >> boot entry, and exception entry tends to require anchoring. > > > > > > Could you please add a debug build target to display all discarded > > > symbols/sections? Something like: > > > > > > make lto-check > > > > > > ... or so? Some kind of option like this could be a good idea. It could apply to any kind of link-time optimization we do. I'll think about it. > > > > > > Thanks, > > > > > > Ingo > > > > > > Actually, LTO activity existed some years ago > > (but not pulled in). > > > > http://www.spinics.net/lists/linux-kbuild/msg09242.html > > > > > > IIUC, this patch enables "dead code elimination", > > (or "garbage collection"?), > > but I think it is different from what is called LTO. > > Yes, it is different. With gc-sections the linker simply drops code > sections that have no references to them. Yes, I shouldn't have confused the terms. gc-sections is a trivial form of LTO, but not "LTO". > This is therefore fast and low > complexity. LTO postpones the compiler's code optimization passes at > the point where everything is linked together and can do things like > constant propagation across multiple files, etc. LTO is therefore more > efficient at removing unused code but compilation time is much longer > due to the added complexity and inherent difficulty to parallelize the > operation across multiple CPUS. > > I think we should aim for gc-sections to be used by default and have LTO > as a possible option only. I agree after it starts getting implemented and debugged by small system users, we could make it default in the interest of sharing testing and reducing combinations. Thanks, Nick
