Re: linker debug info editing

[Daniel Jacobowitz <drow@false.org>]

On Fri, Mar 10, 2006 at 11:19:21PM +1030, Alan Modra wrote:
> /* This file implements removal of DIEs for discarded link-once sections.
>    Such DIEs are detected by parsing debug sections to find DIE boundaries,
>    then examining relocations for each DIE.  Any DIE containing a field
>    with a relocation against a symbol in a discarded section is marked for
>    removal.  A removed DIE in .debug_info results in all of its children
>    and associated data in .debug_loc and .debug_ranges being removed too.
>    A removed CIE in .debug_frame results in all FDEs using that CIE
>    being removed, but no attempt is made to remove usused CIEs (as can
>    happen when all of a CIE's FDEs are removed).  Likewise, other unused
>    shared info, ie. .debug_abbrev and .debug_str entries, are not removed.
> 
>    We assume that
>    a) debug section relocations are sorted by r_offset,
>    b) .debug_info location lists (references to .debug_loc) occur
>       in increasing order of offset into .debug_loc,
>    c) .debug_info ranges (references to .debug_ranges) occur
>       in increasing order of offset into .debug_ranges,
>    d) entries in .debug_loc and .debug_ranges are not shared,
>    e) FDEs always occur at higher offsets than their associated CIEs
>       in .debug_frame.
> 
>    ld -r using perverse linker scripts can break the first three
>    assumptions.  */

Jim mentioned references to DIEs.  I'm also concerned by deleting just
the DIEs containing discarded relocations and their children; that's
not necessarily a logical place to cut the DIE tree.  In general, this
will work for you, because the relocations you're interested in are
those on DW_AT_high_pc and DW_AT_low_pc; but if the function uses
.debug_ranges instead I guess you won't find it.  And there's probably
some other cases where this is a problem, e.g. if you are cutting
member functions or static variables out of a class type.  Removing
the children can affect the interpretation of the parent.

All these complications are a shame; I think it would be useful if the
linker could edit DWARF data.  But it may be a bit complicated.

-- 
Daniel Jacobowitz
CodeSourcery