Writing a pass for memory optimization

In this recipe, we will briefly discuss a transformation pass that deals with memory optimization.

Getting ready

For this recipe, you will need the opt tool installed.

How to do it…

1. Write the test code on which we will run the memcpy optimization pass:

   $ cat memcopytest.ll
   @cst = internal constant [3 x i32] [i32 -1, i32 -1, i32 -1], align 4
   
   declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind
   declare void @foo(i32*) nounwind
   
   define void @test1() nounwind {
     %arr = alloca [3 x i32], align 4
     %arr_i8 = bitcast [3 x i32]* %arr to i8*
     call void @llvm.memcpy.p0i8.p0i8.i64(i8* %arr_i8, i8* bitcast ([3 x i32]* @cst to i8*), i64 12, i32 4, i1 false)
     %arraydecay = getelementptr inbounds [3 x i32], [3 x i32]* %arr, i64 0, i64 0
     call void @foo(i32* %arraydecay) nounwind
     ret void
   }
   

2. Run the memcpyopt pass on the preceding test case:

   $ opt -memcpyopt -S memcopytest.ll
   ; ModuleID = ' memcopytest.ll'
   
   @cst = internal...