Two Hoopl questions

[Edward Z. Yang]

> 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.

In that case, I would recommend taking a look at when -dopt-fuel got removed
and seeing if you can put it back in.  I quite liked this feature and it is
too bad it is gone.

> > • 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?

This is a point of confusion; I got confused about this when
I was working on Hoopl.

What is meant by a transfer function over instruction i is monotonic is:

    if x <= y
    then f_i(x) <= f_i(y)

It says nothing about the "change" in x as you move along the
statements.

Thus, for your example, very_unsafe_stg_call always puts x to be
NAC; this is trivially monotonic since NAC <= NAC.

Edward