# Machine IR (MIR) Format Reference Manual

- Introduction
- Overview
- MIR Testing Guide
  - Testing Individual Code Generation Passes
  - Limitations
- High Level Structure
  - Embedded Module
  - Machine Functions
- Machine Instructions Format Reference
  - Machine Basic Blocks
    - Block References
    - Successors
    - Live In Registers
    - Miscellaneous Attributes
  - Machine Instructions
    - Instruction Flags
  - Registers
  - Machine Operands
    - Immediate Operands
    - Register Operands
      - Register Flags
      - Subregister Indices
    - Global Value Operands

Warning

This is a work in progress.

## Introduction

This document is a reference manual for the Machine IR (MIR) serialization format. MIR is a human readable serialization format that is used to represent LLVM’s machine specific intermediate representation.

The MIR serialization format is designed to be used for testing the code generation passes in LLVM.

## Overview

The MIR serialization format uses a YAML container. YAML is a standard data serialization language, and the full YAML language spec can be read at yaml.org.

A MIR file is split up into a series of YAML documents. The first document can contain an optional embedded LLVM IR module, and the rest of the documents contain the serialized machine functions.

## MIR Testing Guide

You can use the MIR format for testing in two different ways:

- You can write MIR tests that invoke a single code generation pass using the `run-pass` option in llc.
- You can use llc’s `stop-after` option with existing or new LLVM assembly tests and check the MIR output of a specific code generation pass.

### Testing Individual Code Generation Passes

The `run-pass` option in llc allows you to create MIR tests that invoke just a single code generation pass. When this option is used, llc will parse an input MIR file, run the specified code generation pass, and print the resulting MIR to the standard output stream.

You can generate an input MIR file for the test by using the `stop-after` option in llc. For example, if you would like to write a test for the post register allocation pseudo instruction expansion pass, you can specify the machine copy propagation pass in the `stop-after` option, as it runs just before the pass that we are trying to test:

> `llc -stop-after machine-cp bug-trigger.ll > test.mir`

After generating the input MIR file, you’ll have to add a run line that uses the `-run-pass` option to it. In order to test the post register allocation pseudo instruction expansion pass on X86-64, a run line like the one shown below can be used:

> `# RUN: llc -run-pass postrapseudos -march=x86-64 %s -o /dev/null | FileCheck %s`

The MIR files are target dependent, so they have to be placed in the target specific test directories. They also need to specify a target triple or a target architecture either in the run line or in the embedded LLVM IR module.

### Limitations

Currently the MIR format has several limitations in terms of which state it can serialize:

- The target-specific state in the target-specific `MachineFunctionInfo` subclasses isn’t serialized at the moment.
- The target-specific `MachineConstantPoolValue` subclasses (in the ARM and SystemZ backends) aren’t serialized at the moment.
- The `MCSymbol` machine operands are only printed, they can’t be parsed.
- A lot of the state in `MachineModuleInfo` isn’t serialized - only the CFI instructions and the variable debug information from MMI is serialized right now.

These limitations impose restrictions on what you can test with the MIR format. For now, tests that would like to test some behaviour that depends on the state of certain `MCSymbol` operands or the exception handling state in MMI, can’t use the MIR format. As well as that, tests that test some behaviour that depends on the state of the target specific `MachineFunctionInfo` or `MachineConstantPoolValue` subclasses can’t use the MIR format at the moment.

## High Level Structure

### Embedded Module

When the first YAML document contains a YAML block literal string, the MIR parser will treat this string as an LLVM assembly language string that represents an embedded LLVM IR module. Here is an example of a YAML document that contains an LLVM module:

    define i32 @inc(i32* %x) {
    entry:
      %0 = load i32, i32* %x
      %1 = add i32 %0, 1
      store i32 %1, i32* %x
      ret i32 %1
    }

### Machine Functions

The remaining YAML documents contain the machine functions. This is an example of such YAML document:

    ---
    name:            inc
    tracksRegLiveness: true
    liveins:
      - { reg: '%rdi' }
    body: |
      bb.0.entry:
        liveins: %rdi

        %eax = MOV32rm %rdi, 1, _, 0, _
        %eax = INC32r killed %eax, implicit-def dead %eflags
        MOV32mr killed %rdi, 1, _, 0, _, %eax
        RETQ %eax
    ...

The document above consists of attributes that represent the various properties and data structures in a machine function.

The attribute `name` is required, and its value should be identical to the name of a function that this machine function is based on.

The attribute `body` is a YAML block literal string. Its value represents the function’s machine basic blocks and their machine instructions.

## Machine Instructions Format Reference

The machine basic blocks and their instructions are represented using a custom, human readable serialization language. This language is used in the YAML block literal string that corresponds to the machine function’s body.

A source string that uses this language contains a list of machine basic blocks, which are described in the section below.

### Machine Basic Blocks

A machine basic block is defined in a single block definition source construct that contains the block’s ID. The example below defines two blocks that have an ID of zero and one:

    bb.0:
      <instructions>
    bb.1:
      <instructions>

A machine basic block can also have a name. It should be specified after the ID in the block’s definition:

    bb.0.entry:       ; This block's name is "entry"
       <instructions>

The block’s name should be identical to the name of the IR block that this machine block is based on.

#### Block References

The machine basic blocks are identified by their ID numbers. Individual blocks are referenced using the following syntax:

    %bb.<id>[.<name>]

Examples:

    %bb.0
    %bb.1.then

#### Successors

The machine basic block’s successors have to be specified before any of the instructions:

    bb.0.entry:
      successors: %bb.1.then, %bb.2.else
      <instructions>
    bb.1.then:
      <instructions>
    bb.2.else:
      <instructions>

The branch weights can be specified in brackets after the successor blocks. The example below defines a block that has two successors with branch weights of 32 and 16:

    bb.0.entry:
      successors: %bb.1.then(32), %bb.2.else(16)

#### Live In Registers

The machine basic block’s live in registers have to be specified before any of the instructions:

    bb.0.entry:
      liveins: %edi, %esi

The list of live in registers and successors can be empty. The language also allows multiple live in register and successor lists - they are combined into one list by the parser.

#### Miscellaneous Attributes

The attributes `IsAddressTaken`, `IsLandingPad` and `Alignment` can be specified in brackets after the block’s definition:

    bb.0.entry (address-taken):
      <instructions>
    bb.2.else (align 4):
      <instructions>
    bb.3(landing-pad, align 4):
      <instructions>

### Machine Instructions

A machine instruction is composed of a name, machine operands, instruction flags, and machine memory operands.

The instruction’s name is usually specified before the operands. The example below shows an instance of the X86 `RETQ` instruction with a single machine operand:

    RETQ %eax

However, if the machine instruction has one or more explicitly defined register operands, the instruction’s name has to be specified after them. The example below shows an instance of the AArch64 `LDPXpost` instruction with three defined register operands:

    %sp, %fp, %lr = LDPXpost %sp, 2

The instruction names are serialized using the exact definitions from the target’s `*InstrInfo.td` files, and they are case sensitive. This means that similar instruction names like `TSTri` and `tSTRi` represent different machine instructions.

#### Instruction Flags

The flag `frame-setup` can be specified before the instruction’s name:

    %fp = frame-setup ADDXri %sp, 0, 0

### Registers

Registers are one of the key primitives in the machine instructions serialization language. They are primarly used in the register machine operands, but they can also be used in a number of other places, like the basic block’s live in list.

The physical registers are identified by their name. They use the following syntax:

    %<name>

The example below shows three X86 physical registers:

    %eax
    %r15
    %eflags

The virtual registers are identified by their ID number. They use the following syntax:

    %<id>

Example:

    %0

The null registers are represented using an underscore (‘`_`’). They can also be represented using a ‘`%noreg`’ named register, although the former syntax is preferred.

### Machine Operands

There are seventeen different kinds of machine operands, and all of them, except the `MCSymbol` operand, can be serialized. The `MCSymbol` operands are just printed out - they can’t be parsed back yet.

#### Immediate Operands

The immediate machine operands are untyped, 64-bit signed integers. The example below shows an instance of the X86 `MOV32ri` instruction that has an immediate machine operand `-42`:

    %eax = MOV32ri -42

#### Register Operands

The register primitive is used to represent the register machine operands. The register operands can also have optional register flags, a subregister index, and a reference to the tied register operand. The full syntax of a register operand is shown below:

    [<flags>] <register> [ :<subregister-idx-name> ] [ (tied-def <tied-op>) ]

This example shows an instance of the X86 `XOR32rr` instruction that has 5 register operands with different register flags:

    dead %eax = XOR32rr undef %eax, undef %eax, implicit-def dead %eflags, implicit-def %al

##### Register Flags

The table below shows all of the possible register flags along with the corresponding internal `llvm::RegState` representation:

Flag | Internal Value
--- | ---
`implicit` | `RegState::Implicit`
`implicit-def` | `RegState::ImplicitDefine`
`def` | `RegState::Define`
`dead` | `RegState::Dead`
`killed` | `RegState::Kill`
`undef` | `RegState::Undef`
`internal` | `RegState::InternalRead`
`early-clobber` | `RegState::EarlyClobber`
`debug-use` | `RegState::Debug`

##### Subregister Indices

The register machine operands can reference a portion of a register by using the subregister indices. The example below shows an instance of the `COPY` pseudo instruction that uses the X86 `sub_8bit` subregister index to copy 8 lower bits from the 32-bit virtual register 0 to the 8-bit virtual register 1:

    %1 = COPY %0:sub_8bit

The names of the subregister indices are target specific, and are typically defined in the target’s `*RegisterInfo.td` file.

#### Global Value Operands

The global value machine operands reference the global values from the embedded LLVM IR module. The example below shows an instance of the X86 `MOV64rm` instruction that has a global value operand named `G`:

    %rax = MOV64rm %rip, 1, _, @G, _

The named global values are represented using an identifier with the ‘@’ prefix. If the identifier doesn’t match the regular expression [-a-zA-Z$._][-a-zA-Z$._0-9]*, then this identifier must be quoted.

The unnamed global values are represented using an unsigned numeric value with the ‘@’ prefix, like in the following examples: `@0`, `@989`.