A CIL Tutorial - Department of Computer Science - ETH Zürich

CHAPTER 3. DATAFLOW ANALYSIS 27
and oekind of unop (vml : varmap list) (u : unop) (e : exp) : oekind =
match u with
| Neg → oekind of exp vml e
| BNot → e | > oekind of exp vml | > oekind neg
| LNot → Top
The function oekind of binop determines whether the result of a binary operation is Odd or Even.
In this example we are handling only the cases for addition, subtraction, and multiplication, but it
is also possible to handle other cases.
and oekind of binop (vml : varmap list) (b : binop) (e1 : exp) (e2 : exp) : oekind =
let k1, k2 = oekind of exp vml e1, oekind of exp vml e2 in
match b with
| PlusA → begin
match k1, k2 with
| Even, Even → Even
| Odd, Odd → Even
| Even, Odd → Odd
| Odd, Even → Odd
| , → Top
end
(∗ ... ∗)
| → Top
If there is a write to memory or a function call, we must destroy anything we knew about a local
variable whose address has been taken. In the case of a function call, if the analysis were to also
handle global variables, it would also have to clear their state even if their addresses haven't been
taken (but a global whose address hasn't been taken would still be safe from a memory write.) The
function varmap list kill moves the state of variables whose address has been taken to Top in
the resulting varmap.
let varmap list kill (vml : varmap list) : varmap list =
L.map (fun (vid, (vi, k)) →
if vi.vaddrof then (vid, (vi, Top)) else (vid, (vi, k)))
vml
The function varmap list handle inst implements the transfer function for our dataow analysis.
That is it looks at an instruction and a state, and calculates what the state should be after the
instruction is run. On a simple assignment to a local variable, we replace its mapping in the input
state with the oekind of the right-hand-side of the assignment. On memory writes, and function
calls, we use varmap list kill to move the aected variables to Top.