Incremental LTO linking part 2: lto-plugin support

[H.J. Lu]

On Tue, May 8, 2018 at 8:14 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
> Hi,
> with lto, incremental linking can be meaninfuly done in three ways:
>  1) read LTO file and produce non-LTO .o file
>     this is current behaviour of gcc -r or ld -r with plugin
>  2) read LTO files and merge section for later LTO
>     this is current behaviour of ld -r w/o plugin
>  3) read LTO files into the compiler, link them and produce
>     incrementaly linked LTO object.
>
> 3 makes most sense and I am maing it new default for gcc -r. For testing purposes
> and perhaps in order to have tool to turn LTO object into real object, we want
> to have 1) available as well.  GCC currently have -flinker-output option that
> decides between modes that is decided by linker plugin and can be overwritten
> by user (I have forgot to document this).
>
> I am targeting for -flinker-output=rel to be incremental linking into LTO
> and adding -flinker-output=nolto-rel for 1).
>
> The main limitation of 2 and 3 is that you can not link LTO and non-LTO
> object files theger.  For 2 HJ's binutils patchset has support and I think
> it can be extended to handle 3 as well. But with default binutils we want
> to warn users.  This patch implements the warning (and prevents linker plugin
> to add redundat linker-ouptut options.


My users/hjl/lto-mixed/master branch is quite flexible.  I can extend
it if needed.

-- 
H.J.