Thank you Edward. I am aware of these requirements - my problem is writing the
code in which these
will always hold (I'm optimizing Cmm and hand-written Cmm files tend to cause
many problems that
don't appear in automatically generated Cmm). Having a debugging tool in form
of Fuel would be
helpful for me, because instead of getting a freeze and seeing no output from
-ddump-cmm I would
see incorrectly transformed Cmm that would allow me to track bugs more easily.
> > • The transfer function must be monotonic: given a more infor-
> > mative fact in, it must produce a more informative fact out.
I spent some considerable time thinking about this. Consider a simple example
of copy propagation.
Let's assume that { x = NAC }, i.e. we know that x has been defined earlier but
is Not-A-Constant
and so we cannot rewrite it. Now we have something llike this:
x := 3;
y := x;
Here we are allowed to rewrite y := x with y := 3, because after first
statement we learn that { x
= 3 }. Now consider this:
x := 3;
x := very_unsafe_stg_call();
y := x;
Here, after the first statement we learn that { x = 3 }, but after the second
one we learn once
again that x is NAC and so we are not allowed to rewrite statement y := x. So
the value of x can
change from NAC to a constant and from constant to a NAC. Is such a transfer
function monotonic?
Janek
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