Re: MIPS stack tracing

[David Anderson <davea@quasar.engr.sgi.com>]

Daniel Jacobowitz <drow@mvista.com>
> 
> .mdebug is the ECOFF/Third-Eye debugging info format; binutils recently
> switched to generating stabs-in-ELF like other targets instead.
> 
> To my complete surprise, we apparently get PDR information out of the
> .mdebug section.  This is somewhat bizarre, as it is also present in a
> .pdr section independent of data format.  We need to read in this
> information.  I'll investigate next week (if no one beats me to it :).

The original  third-eye was (as you say) added to COFF
by MIPS (SGI was separate then, mid 1980's). 
No real sections.  the PDR was inside the third-eye data.

When MIPS/SGI moved to Elf we made the third-eye
data be a section named  .mdebug  but with
bit-for-bit identical contents to the coff-object
third-eye. There was no .pdr section.

The third-eye data structures were not ideal, in that
they had absolute (not section-relative) file offsets,
and those, of course, were always wrong for objects-inside-archives
as such objects-inside-archives were simply plunked in by ar(1)
and extracted by ld with no update of the mdebug data.

Third-eye was designed this way precisely because
various object formats of the time  third-eye was
designed did not have real sections
and adding extra sections was a major hassle (compatibility-wise
and more). So section-relative offsets were an idea that
just did not work back then (and I guess Peter Rowell,
the designer (he was Third-eye Software), 
just did not think of making the mdebug-internal
offsets be mdebug-relative (or decided not to, I don't know...)).

Andrew Cagney <ac131313@cygnus.com>
>It would go back to SGI (Hmm, didn't sgi switch to dwarf2?) which was 
>using mdebug info.  A number of embedded MIPS toolchains would have been 
>mdebug for compatability (I know this as I filed gdb/150, gdb/152 and 
>gdb/149) and I have a sinking feeling that they haven't yet ``just gone 
>away''.

Ok. More than you want to know (sorry).
SGI has 3 ABIs, all ELF (since the mid 1990's).
An executable can be built in any ABI.

	o32.  Uses mdebug (third-eye). Follows
	 	the MIPS Processor Supplement.
		Intended for MIPS-1 (and now MIPS-2
		cpu chips only.
		This will never change, compiler in stasis :-)

	n32   Uses dwarf-2. For MIPS-3 and MIPS-4 cpu chips,
		takes advantage of 64bit integer registers
		and additional floating point registers.
		pointers are 32 bits.
		Not yet using dwarf-3.
		ABI defined in N32 Handbook (http://techpubs.sgi.com)

	64   Uses dwarf-2. For MIPS-3 and MIPS-4 cpu chips,
		takes advantage of 64bit integer registers
		and additional floating point registers.
		pointers are 64 bits.
		Not yet using dwarf-3.


David Anderson davea@sgi.com