Re: Update LTO plugin interface

[Cary Coutant <ccoutant at google dot com>]

> This isn't quite what should happen, though. ?If the user does not
> specify -lm on the link line, then we should not add -lm, even if LTO
> somehow introduces a function that is defined in libm. ?Automatically
> adding -lm would introduce a surprising dynamic dependency in some
> cases. ?The user should be explicit about that.

I wouldn't expect the compiler to introduce a call to anything in libm
when -lm isn't already specified on the command line (that would break
even without LTO).

With what I'm suggesting, we'll only resolve libcalls to libraries
that were originally specified on the command line.

> That is, it really doesn't matter which library libcalls come from. ?All
> we need to know is which libraries might satisfy new libcalls.
>
> This is what leads me in the direction of copying certain libraries
> mentioned on the command line to follow the LTO objects, much as
> -pass-through does. ?I think we just need to make that a little bit more
> automatic.

Another way to do this would be to put a marker in the command line
that identifies where those libraries begin, and the linker could just
go back and rescan those libraries if needed, before the final layout
of the endcaps.

-cary