Re: Convert cross reference table to resolution file for LTO

[Richard Guenther <richard dot guenther at gmail dot com>]

On Thu, Jul 1, 2010 at 5:02 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
> Hi,
> I did some experiments to convert cross-reference table
> to resolution files. Patches are attached and still crude.
>
> The initial idea is to have as little as possible change
> in GNU LD. It turns out that cross reference table doesn't
> always print out definition at the first line. So
> some change is needed there. In addition, offset within
> archive file is also useful. Nonetheless, such changes
> may break existing cref users. Maybe it is better to dump
> to a different file.
>
> Collect2 is modified to convert the enhanced cross-reference
> table to a full resolution file and used it in subsequent
> linking steps. Maybe I can move this part into LD. I am just
> not sure if it is easy to have similar functions in BFD
> as those in libelf. And it will be a much bigger change on
> GNU LD then.
>
> I have tried with small examples, and our mixed C/assembly
> applications. It works well with -flto/-fwhole-program.
>
> Any opinion on this approach?

Nice.  I didn't know of --cref.  Btw, the resolution file ideally
should come from the driver like for the -fuse-linker-plugin case.
See

#define LINK_COMMAND_SPEC "\
...
    -plugin-opt=-fresolution=%u.res \

you'd add that for -flto/-fwhopr as well.

What exactly is the ELF handling in collect2 for?  Is it only for
archive file offset parsing?  It's compile of course would need to
be conditionalized on host ELF support availability.

It would also be a nice thing to get rid of the special symbol
marking LTO files but instead look if some of the common
sections are there (likewise for other supported object formats).
So probably best move that LTO file detection and handling
code into a separate file and share it with the LTO backend code.

Does --cref work with .comm symbols properly (listing the biggest
one first)?

Richard.

> Thanks,
> Bingfeng
>