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Date: Mon, 10 Aug 2009 10:19:07 -0700
Message-ID: <dd6fe68a0908101019x5449c72axc794bdd56ab87977@mail.gmail.com>
Subject: Re: [9fans] Unexpected 8c warning message
From: Russ Cox <rsc@swtch.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
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This problem is uncomputable, so trying to
handle every case that comes up is problematic.
There has to be a line somewhere.  Saying that
the compiler could figure out does not imply
that it must.

I think it's perfectly reasonable that a compiler,
when presented with a program like

    int x;

    for(___; ___; ___) {
        x = ___;
        ___;
    }
    if(!x)
        ___;

should complain about a possible used-but-not-set
of x.  As a person reading that code, I can't look at
the overall structure of the code and see that x is
obviously initialized at the if statement, unless I analyze
the various abstracted-away ___ pieces.  And if one of
those changes, then there is now a real error.

SET is hard to use correctly and not portable.
You are better off with a simple assignment,
simpler than the one you used in your example:
just zero the variable before the loop.

    int x;

    x = 0;
    for(___; ___; ___) {
        x = ___;
        ___;
    }
    if(!x)
        ___;

Now at least the people reading the code can see
that x is initialized, for sure.  It's very hard for me to
see how "x = 0" is a medicine worse than the disease.
I do agree that the "SD = SDList[0];" you had is not
a good solution, because it makes it look like that
value is important, but "SD = nil;" avoids that issue.

Alternately, since the if(!x) is really the continuation
of the last iteration of the loop, you could move it
inside the loop:

    int x;

    for(___; ___; ___) {
        x = ___;
        ___;
        if(last iteration && !x)
            ___;
    }

which makes it clear to both people and compilers
that x is set before it is used.

It is not the compiler's job to enable you to be as
clever as possible.  If this really matters to you and
you don't want the compiler warnings, you can
always turn them off.

Russ