LLVM Cookbook

Transforming and optimizing loops

In this recipe, we will see how we can transform and optimize loops to get shorter execution times. We will mainly be looking into the Loop-Invariant Code Motion (LICM) optimization technique, and see how it works and how it transforms the code. We will also look at a relatively simpler technique called loop deletion, where we eliminate loops with non-infinite, computable trip counts that have no side effects on a function's return value.

Getting ready

You must have the opt tool built for this recipe.

How to do it…

1. Write the test cases for the LICM pass:

   $ cat testlicm.ll
   define void @testfunc(i32 %i) {
   ; <label>:0
     br label %Loop
   Loop:        ; preds = %Loop, %0
    %j = phi i32 [ 0, %0 ], [ %Next, %Loop ] ; <i32> [#uses=1] ...