RE: Convert cross reference table to resolution file for LTO

["Bingfeng Mei" <bmei at broadcom dot com>]

> -----Original Message-----
> From: Richard Guenther [mailto:richard.guenther@gmail.com]
> Sent: 01 July 2010 16:13
> To: Bingfeng Mei
> Cc: gcc@gcc.gnu.org; Jan Hubicka
> Subject: Re: Convert cross reference table to resolution file for LTO
> 
> 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.
I will look at it. 

> 
> 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.
Main purpose is to locate the "index" information for each symbol.
I will conditionalize Elf part for sure. 

> 
> 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.
> 
Actually, this part is mainly copied from lto-plugin implementation.

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

Yes, after my patch. The original --cref just prints out all comm
symbols. The first one is not necessarily used one. 

> 
> Richard.
> 
> > Thanks,
> > Bingfeng
> >