Machine IR (MIR) Format Reference Manual¶

Introduction
Overview
MIR Testing Guide
- Testing Individual Code Generation Passes
  - Simplifying MIR files
- Limitations
High Level Structure
- Embedded Module
- Machine Functions
Machine Instructions Format Reference

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(es), and output the resulting MIR code.

You can generate an input MIR file for the test by using the -stop-after or -stop-before 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

If the same pass is run multiple times, a run index can be included after the name with a comma.

llc -stop-after=dead-mi-elimination,1 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 -o - %s -mtriple=x86_64-- -run-pass=postrapseudos | FileCheck %s

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

Simplifying MIR files ¶

The MIR code coming out of -stop-after/-stop-before is very verbose; Tests are more accessible and future proof when simplified:

Use the -simplify-mir option with llc.
Machine function attributes often have default values or the test works just as well with default values. Typical candidates for this are: alignment:, exposesReturnsTwice, legalized, regBankSelected, selected. The whole frameInfo section is often unnecessary if there is no special frame usage in the function. tracksRegLiveness on the other hand is often necessary for some passes that care about block livein lists.
The (global) liveins: list is typically only interesting for early instruction selection passes and can be removed when testing later passes. The per-block liveins: on the other hand are necessary if tracksRegLiveness is true.
Branch probability data in block successors: lists can be dropped if the test doesn’t depend on it. Example: successors: %bb.1(0x40000000), %bb.2(0x40000000) can be replaced with successors: %bb.1, %bb.2.
MIR code contains a whole IR module. This is necessary because there are no equivalents in MIR for global variables, references to external functions, function attributes, metadata, debug info. Instead some MIR data references the IR constructs. You can often remove them if the test doesn’t depend on them.
Alias Analysis is performed on IR values. These are referenced by memory operands in MIR. Example: :: (load 8 from %ir.foobar, !alias.scope !9). If the test doesn’t depend on (good) alias analysis the references can be dropped: :: (load 8)
MIR blocks can reference IR blocks for debug printing, profile information or debug locations. Example: bb.42.myblock in MIR references the IR block myblock. It is usually possible to drop the .myblock reference and simply use bb.42.
If there are no memory operands or blocks referencing the IR then the IR function can be replaced by a parameterless dummy function like define @func() { ret void }.
It is possible to drop the whole IR section of the MIR file if it only contains dummy functions (see above). The .mir loader will create the IR functions automatically in this case.

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 don’t support temporary or local symbols.
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 temporary or local 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: '$rdi' }
callSites:
  - { bb: 0, offset: 3, fwdArgRegs:
      - { arg: 0, reg: '$edi' } }
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
    CALL64pcrel32 @foo <regmask...>
    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.

The attribute callSites is a representation of call site information which keeps track of call instructions and registers used to transfer call arguments.

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>

Example:

%bb.0

The following syntax is also supported, but the former syntax is preferred for block references:

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

Example:

%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, IsInlineAsmBrIndirectTarget 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>
bb.4 (inlineasm-br-indirect-target):
  <instructions>

Alignment is specified in bytes, and must be a power of two.

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 or frame-destroy can be specified before the instruction’s name:

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

$x21, $x20 = frame-destroy LDPXi $sp

Bundled Instructions ¶

The syntax for bundled instructions is the following:

BUNDLE implicit-def $r0, implicit-def $r1, implicit $r2 {
  $r0 = SOME_OP $r2
  $r1 = ANOTHER_OP internal $r0
}

The first instruction is often a bundle header. The instructions between { and } are bundled with the first instruction.

Registers ¶

Registers are one of the key primitives in the machine instructions serialization language. They are primarily 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 and by the ‘$’ prefix sigil. 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 and by the ‘%’ sigil. 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 eighteen different kinds of machine operands, and all of them can be serialized.

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

An immediate operand is also used to represent a subregister index when the machine instruction has one of the following opcodes:

EXTRACT_SUBREG
INSERT_SUBREG
REG_SEQUENCE
SUBREG_TO_REG

In case this is true, the Machine Operand is printed according to the target.

For example:

In AArch64RegisterInfo.td:

def sub_32 : SubRegIndex<32>;

If the third operand is an immediate with the value 15 (target-dependent value), based on the instruction’s opcode and the operand’s index the operand will be printed as %subreg.sub_32:

%1:gpr64 = SUBREG_TO_REG 0, %0, %subreg.sub_32

For integers > 64bit, we use a special machine operand, MO_CImmediate, which stores the immediate in a ConstantInt using an APInt (LLVM’s arbitrary precision integers).

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`
`renamable`	`RegState::Renamable`

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.

Constant Pool Indices ¶

A constant pool index (CPI) operand is printed using its index in the function’s MachineConstantPool and an offset.

For example, a CPI with the index 1 and offset 8:

%1:gr64 = MOV64ri %const.1 + 8

For a CPI with the index 0 and offset -12:

%1:gr64 = MOV64ri %const.0 - 12

A constant pool entry is bound to a LLVM IR Constant or a target-specific MachineConstantPoolValue. When serializing all the function’s constants the following format is used:

constants:
  - id:               <index>
    value:            <value>
    alignment:        <alignment>
    isTargetSpecific: <target-specific>

where:

<index> is a 32-bit unsigned integer;
<value> is a LLVM IR Constant;
<alignment> is a 32-bit unsigned integer specified in bytes, and must be power of two;
<target-specific> is either true or false.

Example:

constants:
  - id:               0
    value:            'double 3.250000e+00'
    alignment:        8
  - id:               1
    value:            'g-(LPC0+8)'
    alignment:        4
    isTargetSpecific: true

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.

Target-dependent Index Operands ¶

A target index operand is a target-specific index and an offset. The target-specific index is printed using target-specific names and a positive or negative offset.

For example, the amdgpu-constdata-start is associated with the index 0 in the AMDGPU backend. So if we have a target index operand with the index 0 and the offset 8:

$sgpr2 = S_ADD_U32 _, target-index(amdgpu-constdata-start) + 8, implicit-def _, implicit-def _

Jump-table Index Operands ¶

A jump-table index operand with the index 0 is printed as following:

tBR_JTr killed $r0, %jump-table.0

A machine jump-table entry contains a list of MachineBasicBlocks. When serializing all the function’s jump-table entries, the following format is used:

jumpTable:
  kind:             <kind>
  entries:
    - id:             <index>
      blocks:         [ <bbreference>, <bbreference>, ... ]

where <kind> is describing how the jump table is represented and emitted (plain address, relocations, PIC, etc.), and each <index> is a 32-bit unsigned integer and blocks contains a list of machine basic block references.

Example:

jumpTable:
  kind:             inline
  entries:
    - id:             0
      blocks:         [ '%bb.3', '%bb.9', '%bb.4.d3' ]
    - id:             1
      blocks:         [ '%bb.7', '%bb.7', '%bb.4.d3', '%bb.5' ]

External Symbol Operands ¶

An external symbol operand is represented using an identifier with the & prefix. The identifier is surrounded with ““‘s and escaped if it has any special non-printable characters in it.

Example:

CALL64pcrel32 &__stack_chk_fail, csr_64, implicit $rsp, implicit-def $rsp

MCSymbol Operands ¶

A MCSymbol operand is holding a pointer to a MCSymbol. For the limitations of this operand in MIR, see limitations.

The syntax is:

EH_LABEL <mcsymbol Ltmp1>

Debug Instruction Reference Operands ¶

A debug instruction reference operand is a pair of indices, referring to an instruction and an operand within that instruction respectively; see Instruction referencing locations.

The example below uses a reference to Instruction 1, Operand 0:

DBG_INSTR_REF !123, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0), debug-location !456

CFIIndex Operands ¶

A CFI Index operand is holding an index into a per-function side-table, MachineFunction::getFrameInstructions(), which references all the frame instructions in a MachineFunction. A CFI_INSTRUCTION may look like it contains multiple operands, but the only operand it contains is the CFI Index. The other operands are tracked by the MCCFIInstruction object.

The syntax is:

CFI_INSTRUCTION offset $w30, -16

which may be emitted later in the MC layer as:

.cfi_offset w30, -16

IntrinsicID Operands ¶

An Intrinsic ID operand contains a generic intrinsic ID or a target-specific ID.

The syntax for the returnaddress intrinsic is:

$x0 = COPY intrinsic(@llvm.returnaddress)

Predicate Operands ¶

A Predicate operand contains an IR predicate from CmpInst::Predicate, like ICMP_EQ, etc.

For an int eq predicate ICMP_EQ, the syntax is:

%2:gpr(s32) = G_ICMP intpred(eq), %0, %1

Comments ¶

Machine operands can have C/C++ style comments, which are annotations enclosed between /* and */ to improve readability of e.g. immediate operands. In the example below, ARM instructions EOR and BCC and immediate operands 14 and 0 have been annotated with their condition codes (CC) definitions, i.e. the always and eq condition codes:

dead renamable $r2, $cpsr = tEOR killed renamable $r2, renamable $r1, 14 /* CC::always */, $noreg
t2Bcc %bb.4, 0 /* CC:eq */, killed $cpsr

As these annotations are comments, they are ignored by the MI parser. Comments can be added or customized by overriding InstrInfo’s hook createMIROperandComment().

Debug-Info constructs ¶

Most of the debugging information in a MIR file is to be found in the metadata of the embedded module. Within a machine function, that metadata is referred to by various constructs to describe source locations and variable locations.

Source locations ¶

Every MIR instruction may optionally have a trailing reference to a DILocation metadata node, after all operands and symbols, but before memory operands:

$rbp = MOV64rr $rdi, debug-location !12

The source location attachment is synonymous with the !dbg metadata attachment in LLVM-IR. The absence of a source location attachment will be represented by an empty DebugLoc object in the machine instruction.

Fixed variable locations ¶

There are several ways of specifying variable locations. The simplest is describing a variable that is permanently located on the stack. In the stack or fixedStack attribute of the machine function, the variable, scope, and any qualifying location modifier are provided:

- { id: 0, name: offset.addr, offset: -24, size: 8, alignment: 8, stack-id: default,
 4  debug-info-variable: '!1', debug-info-expression: '!DIExpression()',
    debug-info-location: '!2' }

Where:

debug-info-variable identifies a DILocalVariable metadata node,
debug-info-expression adds qualifiers to the variable location,
debug-info-location identifies a DILocation metadata node.

These metadata attributes correspond to the operands of a llvm.dbg.declare IR intrinsic, see the source level debugging documentation.

Varying variable locations ¶

Variables that are not always on the stack or change location are specified with the DBG_VALUE meta machine instruction. It is synonymous with the llvm.dbg.value IR intrinsic, and is written:

DBG_VALUE $rax, $noreg, !123, !DIExpression(), debug-location !456

The operands to which respectively:

Identifies a machine location such as a register, immediate, or frame index,
Is either $noreg, or immediate value zero if an extra level of indirection is to be added to the first operand,
Identifies a DILocalVariable metadata node,
Specifies an expression qualifying the variable location, either inline or as a metadata node reference,

While the source location identifies the DILocation for the scope of the variable. The second operand (IsIndirect) is deprecated and to be deleted. All additional qualifiers for the variable location should be made through the expression metadata.

Instruction referencing locations ¶

This experimental feature aims to separate the specification of variable values from the program point where a variable takes on that value. Changes in variable value occur in the same manner as DBG_VALUE meta instructions but using DBG_INSTR_REF. Variable values are identified by a pair of instruction number and operand number. Consider the example below:

$rbp = MOV64ri 0, debug-instr-number 1, debug-location !12
DBG_INSTR_REF !123, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0), debug-location !456

Instruction numbers are directly attached to machine instructions with an optional debug-instr-number attachment, before the optional debug-location attachment. The value defined in $rbp in the code above would be identified by the pair <1, 0>.

The 3rd operand of the DBG_INSTR_REF above records the instruction and operand number <1, 0>, identifying the value defined by the MOV64ri. The first two operands to DBG_INSTR_REF are identical to DBG_VALUE_LIST, and the DBG_INSTR_REF s position records where the variable takes on the designated value in the same way.

More information about how these constructs are used is available in Instruction referencing for debug info. The related documents Source Level Debugging with LLVM and How to Update Debug Info: A Guide for LLVM Pass Authors may be useful as well.