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Re: Mixing 32-bit and 64-bit DWARF2/3 sections


   Date: Mon, 08 Nov 2004 12:54:30 -0800
   From: James E Wilson <wilson@specifixinc.com>

   Mark Kettenis wrote:
   > This argues that what's currently done in gas and GCC for most 64-bit
   > MIPS targets is wrong, since these use the 64-bit DWARF format
   > unconditionally for the n64 ABI.

   SGI Irix6 created the N64 ABI, and they decreed that one must use a 
   (non-standard) 64-bit DWARF format with it.  We are just following the 
   ABI here.  If we change it, then we lose compatibility with Irix6, and 
   the published ABI.

Yes I left out the non-standard SGI 64-bit DWARF format.  Indeed it
makes sense to maintain compatibility with the vendor tools if
possible.  So for the IRIX n64 ABI the GNU toolchain should generate
the non-standard 64-bit DWARF.

The situation is different though for other 64-bit MIPS systems.  The
n64 ABI for Linux and OpenBSD is already incompatible with IRIX
(bfd_elf64_bigmips_vec vs. bfd_elf64_tradbigmips_vec).  On these
systems, gas currently generates the new DWARF3 64-bit format.  I
merely want to argue that it probably would have been better if the
default had been 32-bit for Linux.

   > P.S. It seems that MIPS is the only 64-bit target for which this is a
   >      problem, since all other 64-bit targets (AMD64, UltraSPARC) still
   >      use the 32-bit DWARF format.

   This is because SGI decided that DWARF2 was broken, and tried to fix it 
   by adding 64-bit extensions.  These extensions are only needed if you 
   have more than 4GB of DWARF debug info, and hence is unlikely, but SGI 
   claims that they did hit this limit.  No one else has apparently ever 
   hit the limit.

   Unfortunately, the way that SGI extended DWARF2 caused problems, as it 
   resulted in ambiguous DWARF info, so the DWARF3 committee defined a 
   different way that is self descriptive.  But since this happened after 
   Irix6 had already been released, and SGI could not break backwards 
   compatibility with Irix6, N64 still uses the non-standard 64-bit DWARF 
   format that SGI invented.  And gcc uses this non-standard 64-bit DWARF 
   format for all mips64 targets, for consistency, and to conform with the ABI.

If this is true, there's a real problem in 64-bit MIPS Linux now,
since gas clearly uses the non-standard 64-bit format only on IRIX
according to this fragment from gas/config/tc-mips.c:

enum dwarf2_format
mips_dwarf2_format (void)
{
  if (mips_abi == N64_ABI)
    {
#ifdef TE_IRIX
      return dwarf2_format_64bit_irix;
#else
      return dwarf2_format_64bit;
#endif
    }
  else
    return dwarf2_format_32bit;
}

On the bright side, if all non-IRIX 64-bit MIPS targets are currently
broken, this gives us the opportunity to fix things the "right" way by
using standard 32-bit DWARF2 by default.

Mark


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