Re: stabs changes for 64 bit targets

[nick clifton <nickc at redhat dot com>]

Hi David,


> . the new section would be named .stab2 instead of .stab
>
>    [I wasn't sure what to name it; I didn't want to call it .stab64 as I
>     could envision someone someday needing to increase the size of the
>     string table.  I don't really care what it is called provided it is
>     not .stab (backwards compatibility).]

How about: .gnu_stabs ?  (Following the general principle of prefix 
"gnu" to extensions to parts of existing standards).


> Does this sound like a reasonable approach?

Mostly, but...

  1. How would this new feature be documented ?  Ie there ought to be a 
proper document describing the format rather than just a hand waving 
"read the code".

  2. If you are going to extend the .stab binary format then how about 
making a few other improvements too ?  Such as including a header that 
describes the version of the STABS format being used and the length of 
the .stabs entries.  That way if some day you need to have 8-byte string 
indices for example, you can just bump the version number and increase 
the structure size.

An alternative approach, which might occupy less space is to continue to 
generate the .stabs entries as before, except that this time the e_value 
field will be inaccurate.  Then in the .stabs2 (or .gnu_stabs) section 
you just have a run-length encoded list of the high parts of the 
e_values for each entry.  Ie suppose that a .stabs section has 4 entries 
and that the e_value fields ought to be:

  0x0000880012341234
  0x0000880056785678
  0x00008800abcdabcd
  0xffff000000000000

Then the .stabs section would contain e_values of:

  0x12341234
  0x56785678
  0xabcdabcd
  0x00000000

And the ,stabs2 section would contain:

  3: 0x00008800
  1: 0xffff0000

(The assumption here is that the 64-bit addresses needed for the .stabs 
e_value entries are still going to be clumped into 4G pages, so run 
length encoding of the high part of the address will save space).

This scheme also has the advantage that the .stabs section would still 
be present and could be consumed by old tools that were unaware of the 
.stabs2 section, albeit with bogus addresses.

Just a thought...

Cheers
  Nick