[RFC] gcc lto&binutils: Load different gcc's bfd-plugin automatically

[JunMa <JunMa@linux.alibaba.com>]

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).

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