Re: "ld -r" on mixed IR/non-IR objects (

[Cary Coutant <ccoutant at google dot com>]

> Here is my proposal. ÂAny comments?

We talked about ld -r a while back during the WHOPR project, and the
two ways that the linker could work: (1) combine all the .o files and
use the plugin to run LTRANS on the IR files, producing a pure,
optimized, object file; and (2) combine the non-IR object files as ld
-r normally would, and combine that result somehow with the IR from
the other files, for later optimization. If I remember correctly,
there was support for both modes of operation. The first mode is
easily handled with the current design (untested as far as I know --
there are probably bugs, and I'm not sure if we get the symbol
visibility correct in those cases).

The second mode corresponds with your proposal here. It's complicated
by the fact that it's difficult to tell, once the objects are
combined, which compiled code came without corresponding IR. For this,
I've got a suggestion that seems a bit simpler than your
".objectonly\004" section, based on an idea for something completely
unrelated[1] that I've been pondering over for a while. Instead of
embedding the non-IR objects into the mixed object file, let's instead
produce an archive file with several members: one that contains the
result of running ld -r on the non-IR objects in the link, and one
member for each of the IR files (alternatively, exactly one member
that contains the result of running ld -r on all of the IR objects).
In order to make the archive such that a subsequent link loads all of
the members unconditionally, I propose to add a special symbol
".FORCE" into the archive symbol table for each member; when the
linker sees that symbol in the archive symbol table, it will load the
corresponding member unconditionally.

>       â Object-only section:
>               Â Section name won't be generated by any tools, something like
>".objectonly\004".
>               Â Contains non-IR object file.
>               Â Input is discarded after link.

Please -- use a special section type, not a magic name.

-cary


[1] My unrelated idea is about "__attribute__ (( used ))" -- when a
symbol is marked as used, it should not only suppress unused warnings
in the compiler, but it should also force the resulting object module
to be linked from an archive library. I've been thinking about a
proposal to mark any object file that contains a used symbol, have ar
recognize that mark and add the ".FORCE" symbol to the archive symbol
table for that object, then have the linker recognize the ".FORCE"
symbol and load the member unconditionally.