Help! Sun 3/60+CG4 strangeness

[Gene Spafford]

Here's a really strange problem I'm having.  Any ideas or suggestions
will be greatly appreciated!

First, let me set the scene.  I'm working on putting in speedups to
the X11R3 server.  I'm working with my "Purdue" speedups, and I'm
merging in some new ideas and the "PurduePlus" speedups.  So far, so
good.  People all over the country are using these changes on their
systems and they work great.  However, on some systems, the screen
suffers severe bitrot.  Some extensive debugging and enquiries reveal
the following:

* The bitrot only appears on Sun 3/60+CG4 combinations.  The same code
on 3/60+BW2, 3/50+BW2, 3/50+CG2, and 3/100+CG4 behaves perfectly.  It
appears to be OS independent, but I have no way to be 100% sure.

* The bitrot occurs when trying to execute a "bfins" instruction into
a location in the frame buffer.

* If the destination field is some set of nibbles (bytes) like:
   12345678 ABCDEFGH
and the destination is the bytes 45678ABC, then the insert goes fine, but
nibbles 1 & 2 also get trashed in seemingly random ways (I can't
discern a pattern).  The destruction is always to the first 8 bits,
the stores are always offset more than 8 bits, and it doesn't always
happen.

* All bitfield addresses presented to the "bfins" instruction are
32-bit aligned.

* Use of adb and trace statements show that the correct values are
going into the registers for the "bfins" instruction -- no registers
are getting trashed along the way.  The perfect behavior of the code
on all other Sun 3 systems bears this out.

* Shift and mask instructions to those locations work fine, albeit
much slower.


The end result of all this is that the nicely tuned GCC "asm" macros
in the modified PurduePlus package just won't work for a 3/60+CG4 combo!  

I'm pretty well stumped.  I have tried nearly everything I can think of
to figure out why this is happening and how I might circumvent this
behavior.  If any of you out there have any idea at all why this might
be happening or how I can investigate further, please let me know!
I've got a stack of server optimizations I want to try, but I need a
stable server platform first, and I'd really like this as part of
that platform!

Thanks in advance.
-- 
Gene Spafford
NSF/Purdue/U of Florida  Software Engineering Research Center,
Dept. of Computer Sciences, Purdue University, W. Lafayette IN 47907-2004
Internet:  spaf at cs.purdue.edu	uucp:	...!{decwrl,gatech,ucbvax}!purdue!spaf