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From: muller@src.dec.com (Eric Muller)
Newsgroups: comp.lang.modula3
Subject: Re: optimization and garbage collection
Message-ID: <1991Jan9.184352.5678@src.dec.com>
Date: 10 Jan 91 02:43:52 GMT
References: <91Jan2.131022pst.3117@arcturia.parc.xerox.com> <5142@exodus.Eng.Sun.COM> <39976@super.ORG> <5293@exodus.Eng.Sun.COM>
Sender: news@src.dec.com (News)
Reply-To: muller@src.dec.com (Eric Muller)
Organization: DEC Systems Research Center
Lines: 32

David Chase argues that an aggressive C optimizer can disturb Modula-3
programs (in the case of a Modula-3 compiler that generates C, of
course). In particular, garbage collection can be a real trouble, as
roots can be "hidden".

While such occurrences are possible, I would not rule out a bug in the
Modula-3 compiler or in the C optimizer to conclude right away in
favor of an unwanted side-effect of aggressive optimization.  If I
were to give probabilities to:

   1. SRC Modula-3 generates incorrect C code
   2. the C compiler is incorrect
   3. it is one of those things we cannot do because we generate C

I would say: p(1) = 99.9%, p(2) = 0.099%, p(3) = 0.001%.

Of course, we make assumptions when generating code and in the garbage
collector, and they may not be satisfied. As for the generated code, we
try to use no more than what ANSI-C says. For the garbage collector,
we assume that if a traced heap object (i.e. a traced OBJECT or REF)
is accessible by the program, then there is somewhere in the stacks or
the registers a reference to one of the pages that contains that
object. If it turns out that this is not reasonable, we can increase
the level of conservatism, for example that saying that the pointer
can be as far as x bytes from object.

However, we have yet to see an example of 3. In the mean time, keep
these bug reports coming so that we can achieve p(1) = 50%.

Eric Muller.
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