Using JIT compilation for direct execution

Running LLVM IR directly is the first idea that comes to mind when thinking about a JIT compiler. This is what the lli tool, the LLVM interpreter, and the dynamic compiler do. We will explore the lli tool in the next section, and subsequently implement a similar tool on our own.

Exploring the lli tool

Let's try the lli tool with a very simple example. Store the following source as a hello.ll file. It is the equivalent of a C hello world application. It declares the prototype for the printf() function from the C library. The hellostr constant contains the message to be printed. Inside the main() function, a pointer to the first character of the message is calculated via the getelementptr instruction, and this value is passed to the printf() function. The application always returns 0. The complete source code is as follows:

declare i32 @printf(i8*, ...)
@hellostr = private unnamed_addr constant [13 x i8] c"Hello   ...