Debuggers

[jo...@ima.uucp]

About 7 years ago I did some very preliminary studies involving the
BLISS-32 code generator while I was at Digital. I managed to convince
myself that it was possible to derive source code from the optimized
object code, however, one would have to make extensive use of
heuristics.

Unfortunately, I was unable to convince management that there was
sufficient payback to warrant a project. (I used the concept of filtering
optimized object code through a source generator, and then filtering
that back through the optimizer. They weren't interested: the existing
optimizations were good enough, and I was proposing orders of
magnitude more time to do a compile.)

The heuristics would only be good for a certain class of compilers
and their code generators. The peephole optimizers and cross jumping
done by different compilers would create an enormous explosion of
possible source codes.

I eventually came to the conclusion that there are two possible uses
of 'compiling object into source'. 1) As a study in a given compiler's
effectiveness at handling particular classes of programs. and 2)
as an analysis tool to see how a computer would execute a program.
The original thought of increasing code optimization would turn out
to be an impractical use; with a good optimizer in the first place,
you would not obtain practical results after the second iteration.

If you're interested, I'll try to dig up my notes.

--Bruce Dawson
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[jo...@ima.uucp]

You should order the paper "Interactive Source-Level Debugging of Optimized
Programs", by Polle Zellweger from the folloowing address:

Xerox Corporation, Palo Alto Research Center
Communications Group
3333 Coyote Hill Road, Palo Alto, CA 94304.

I heard Polle discuss her paper. As I recall, the upshot was that
optimized debugging wasn't worth the trouble.

But then that's just one opinion....
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