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

[James E Wilson <wilson at specifixinc dot 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.

> 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.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com