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