"ld -A" on PMAXen -- help wanted

[Thomas Maslen]

[ Posted to comp.unix.ultrix and comp.sys.mips, followup-to
  comp.sys.mips; hope that's appropriate.  My apologies if this has
  been around the net previously, but I haven't seen it in the traffic
  since 15-Aug. ]

I'm trying to port a Prolog implementation (SICSTUS 0.6, including the
foreign function interface) which wants to use incremental loading
("ld -A") to do clever things at runtime;  I'm not having much fun.
Has anyone Out There successfully used the -A flag? 

If it matters, I'm working on one of the server boxes in the same
family as the DECStation 3100, running Ultrix 3.1 (at least, I think
that's what the login gibberish is trying to say).  It is my
impression, however, that my problems stem from MIPS-derived software
rather than from anything weird that DEC did -- please tell me if I'm
wrong.

The main thing that makes life difficult is GP-addressed variables
(small data and small bss).  I understand why incremental loading
demands that all the newly loaded modules be compiled with -G 0, but
it's still a pain to deal with.  For modules under my control, I can
simply grit my teeth and add -G 0 to the Makefile, but when those
modules refer to something in a library (e.g. libc.a) which isn't
already linked into the program, and that something uses GP
addressing, it's a little difficult for me to do much about it.  So:

    (1) Is there a way to deal with this that I don't know about?
	Maybe a program which, somewhat in the manner of pixie, mungs
	.o files to remove any GP-relative addressing (how feasible is
	it to use $at for this sort of thing?)

    (2) Failing that, how have other people dealt with porting systems
	that want to load foreign functions at runtime?  I assume
	that this should arise in some of the LISP and Prolog systems
	that were on John Mashey's list of third-party software for
	MIPS boxes (true?). 

    (3)	The good folks at MIPS normally have pretty convincing numbers
	to back their decisions.  Question: how much slower would
	things be if the system-wide default was -G 0 rather than -G 8?
	(Yes, I realize this is as ill-formed a question as "so, how
	many MIPS does it do?").  Wouldn't -G 0 be a better default,
	with -G 8 left for the speed freaks?

The 26-bit addresses in jumps are also a bit of a nuisance, but it
looks as though one can get around this by building the original
(non-incremental) executable with -N (OMAGIC) or perhaps with clever
choice of -T and -D values.

If anybody cares, the rest of SICSTUS 0.6 seems to work pretty
happily, although we haven't tested it a whole lot. The save/restore
code needed some tweaking to find the start of the (writable) data
segment, but that was about all.

Thanks for any light you can shed,
    Thomas Maslen				    maslen at polya.stanford.edu