Using LLVM utility passes

In this recipe, you will learn about LLVM's utility passes. As the name signifies, they are of much utility to users who want to understand certain things about LLVM that are not easy to understand by going through code. We will look into two utility passes that represent the CFG of a program.

Getting ready

You need to build and install LLVM, and install the graphviz tool. You can download graphviz from http://www.graphviz.org/Download.php, or install it from your machine's package manager, if it is in the list of available packages.

How to do it...

Perform the following steps:

1. Write the test code required for running the utility passes. This test code consists of if blocks, it will create a new edge in the CFG:

   $ cat utility.ll
   declare double @foo()
   
   declare double @bar()
   
   define double @baz(double %x) {
   entry:
     %ifcond = fcmp one double %x, 0.000000e+00
     br i1 %ifcond, label %then, label %else
   
   then:       ; preds = %entry
     %calltmp = call double @foo...