A CIL Tutorial - Department of Computer Science - ETH Zürich

Chapter 4
Instrumentation
In Chapters 1 and 2, we saw how to use the AST visitor to modify the program by removing
an instruction. It was probably easy to see how to change that example to add statements and
instructions, instead. However, adding function calls is a bit trickier. In particular, the Call
constructor of the type Cil.instr requires a Cil.exp expression for the function. We can build an
expression for the function out of a varinfo for the function, which we can nd simply by iterating
over the globals of a Cil.file. In this example we'll look at some handy patterns for accomplishing
that, and for getting the instrumentation calls into the code.
It almost goes without saying that the ability insert function calls is imensely useful in the
dynamic analysis of programs and the implementation of new language runtime features.
4.1 tut4.ml
First, we'll add calls to functions that will do some proling of loops. At the end of each loop, the
instrumented program will print the source location of the loop and the number of iterations that
ran. First, we'll set up a quick way to refer to the instrumentation calls. Then, we'll write a visitor
to wrap the calls around loops.
4.1.1 Setup of instrumentation calls
It isn't strictly necessary to wrap up the varinfos for the instrumentation calls in a record type.
They could just be global variables of this module, but I nd this helps me stay a bit more organized.
Also, if you add a new eld to the record, the OCaml compiler gives you error messages if you forget
some part of the initialization.
Additionally, in larger projects, I tend to put all of this set-up code in its own module, but in
this small example we should be okay.
36