.eh_frame section on alpha

[Simon Burge]

Hi Ian,

Ian Lance Taylor wrote:

> Simon Burge <simonb@wasabisystems.com> writes:
> 
> > http://sources.redhat.com/ml/binutils/2003-05/msg00863.html added the
> > generation of a .eh_frame section to all objects that otherwise didn't
> > have them.
> > 
> > On NetBSD, we have some tight code size constraints for our first stage
> > boot loader - it needs to fit in 7.5k of space at the start of the disk.
> > With binutils 2.14, our boot loader was 6880 bytes.  With binutils 2.15,
> > the boot loader is 7848.  The only difference in size between the two is
> > the .eh_frame section:
> > 
> >   3 .eh_frame     000003c4  0000000020001990  0000000020001990  00001a40  2**3
> >                   CONTENTS, ALLOC, LOAD, READONLY, DATA
> > 
> > What exactly is this section used for?  A bit of googling seems to
> > indicate it's for exception handling, which isn't a C issue.
> > 
> > More importantly, how can I disable the generation of this section?  It
> > seems that only alpha generates an .eh_frame section unconditionally.
> 
> Hi Simon.  I haven't tested it, but there should be two ways to
> disable to automatic creation of the .eh_frame section: 1) do not emit
> the directives which create one--.ent, .end, .frame, etc.; 2) create
> your own .eh_frame section and put nothing in it--for example, the
> assembler doesn't generate its own unwind information for compiled
> code, because the compiler creates a .eh_frame section anyhow.

This worked, but was difficult to make sure all objects got to
include a dummy .eh_frame section.

For now, James Chacon has come up with a change to our make rules for
.{c,S,s}->.o builds that just calls "objcopy -R .eh_frame".  This at
least doesn't need any code changes and is quite a local change.

Simply removing the .eh_frame section from the final linked object
didn't work, since we use "objcopy --output-target=binary" to make a
blob to put on disk and just removing the .eh_frame section at this
stage left a hole in the file (ie, text started at the same address
and data finished at the same address but the area where the .eh_frame
section was in the middle was now just zero'ed out).

> However, I also think it would be a good idea to have an assembler
> option to not create the unwind information.

Strongly agreed :-)  I can come up with a patch, but I'm not sure what a
good option name would be.  --no-auto-eh-frame seems a little unwieldy.

Simon.
--
Simon Burge                            <simonb@wasabisystems.com>
NetBSD Support and Service:         http://www.wasabisystems.com/