4. Mu Intermediate Representation

The client generates code in the format of Mu Intermediate Representation, or Mu IR for short. The IR is the language in which programs are represented in the Mu micro VM. The Mu IR is defined by the Mu IR chapter of the Mu specification.

The structure of the Mu IR is an AST. Mu bundle has a text form for human readability. A bundle can also be built using the IR building API which calls into Mu to build an AST inside Mu. The IR building API is designed for productional setting. This tutorial will use the text-form API.

4.1. Bundle

The client submits one bundle (a piece of Mu IR code) at a time to the Mu micro VM.

    // insert your bundle here

A bundle defines many types, function signatures, constants, global cell and functions. The client may submit multiple bundles one after another.

After submitting, Mu knows about those things. Types can be used, global cells are allocated, and functions are callable.

A bundle looks like this:

// Type
.typedef @i64 = int<64>

// Function signature
.funcsig @i_to_i = (@i64) -> (@i64)

// Constant
.const @I64_1 <@i64> = 1
.const @I64_2 <@i64> = 2

// Global cell
.global @g_foo <@i64>

// Function declaration (no body)
.funcdecl @factorial <@i_to_i>

// Function definition (with body)
.funcdef @fibonacci VERSION %v1 <@i_to_i> {
    %entry(<@i64> %n):                          // Basic block
        %lt = SLT <@i64> %n @I64_2                  // Instructions
        BRANCH2 %lt %small(%n) %big(%n)             // Instructions

    %small(<@i64> %n):                          // Basic block
        RET %n                                      // Interaction

    %big(<@i64> %n):                            // Basic block
        %nm1 = SUB <@i64> %n @I64_1                 // Instruction
        %nm2 = SUB <@i64> %n @I64_2                 // Instruction
        %v1  = CALL <@i_to_i> @fibonacci (%nm1)     // Instruction
        %v2  = CALL <@i_to_i> @fibonacci (%nm2)     // Instruction
        %rv  = ADD <@i64> %v1 %v2                   // Instruction
        RET %rv                                     // Instruction

If you have used LLVM before, the Mu IR is the counterpart of LLVM modules.

4.2. Names

You may have noticed that there are names for almost all entities. In the Mu IR, there are two kinds of names: global names and local names. Global names start with @ and local names start with %. The allowed characters in names are [a-zA-Z0-9_.].

In fact, local names are just syntax sugars of some global names, that is, anything that has a name has a global name. This will be discussed later.

At this moment, you only need to know what a bundle may contain. Their details will be discussed in the following chapters.