On Tue, Mar 5, 2019 at 10:41 AM JunMa <ju...@linux.alibaba.com> wrote: > > Hi All > > We are now optimizing some projects with lto enabled, however, > there are some issues. > First, lto_plugin.so needs to be passed to ar/nm/ranlib. > For example, build static library with lto: > > gcc -flto -O2 a.c -c -o a.o > gcc -flto -O2 b.c -c -o b.o > ar rcs --plugin=/path/to/lto_plugin.so libx.a a.o b.o > > This is a little bit anoying. Also, it is not easy and convincible to use > gcc-ar/gcc-nm/gcc-ranlib on those projects. Luckily, binutils offers a > default plugin searching path(/lib/bfd-plugins), it can load plugin > automatically from that directory. > > However, this brings up the second issue: binutils doesn't support multiple > version plugins of gccs which have the same plugin name:"lto_plugin.so" > int the same directory. Since these projects require different versions of > gccs, multiple gccs co-exist in our build system, we cannot put lto_plugin > in /lib/bfd-plugins. Although plugins may be compatible with each other, > we do want to decouple it among different versions of GCC. > > I also have seen some discussions in > https://bugzilla.redhat.com/show_bug.cgi?id=1467409 > where I don't seeany clear solutions. > > I thought about this and had some ideas. I want ask for some feedback here. > The idea is to use versioned plugin searching paths > (/lib/bfd-plugins/$cc/$target/$version) > The $cc and $version information can be found in .comment section > of elf file, as for file which does not have comment section, just keep > original searching path(/lib/bfd-plugins).
Well, why not go a step further and add a bfd-plugin note that suggests the plugin to be used if it is installed? That could contain for example lto_plugin_gcc8.so (to be installed in /lib/bfd-plugins/). Alternatively a full path could be specified (though the files wouldn't then necessarily work when moving between different compiler installs). The install location could be modified to a location BFD searches just for this note/comment but not for other auto-loading tries. Or, since mostly archive related stuff is the issue, we could bundle the plugin as a special archieve member... (ok, that creates a chicken and egg issue at archieve creation time). Richard. > Here are few steps: > 1) ar/nm/ranlib keep same behavior when '--plugin' is passed. > 2) If --plugin is missed, find whether there are at least one .comment > section in object file/archive file if not goto 5). > 3) for elf object file, get $cc and $version information. for elf > archive file, > iterate comment section of all object files in archive, make sure $cc and > $version are same. if not, goto 5). > 4) Get $target from target_alias variable in configure. > 5) Find plugin from /lib/bfd-plugins/$cc/$target/$version or > /lib/bfd-plugins directory. > > Looking forward to your replies! > > Regards > Jun
