LLVM Extensions — LLVM 5 documentation

# LLVM Extensions

- Introduction
- General Assembly Syntax
  - C99-style Hexadecimal Floating-point Constants
- Machine-specific Assembly Syntax
  - X86/COFF-Dependent
    - Relocations
    - `.linkonce` Directive
    - `.section` Directive
  - ELF-Dependent
    - `.section` Directive
- Target Specific Behaviour
  - X86
    - Relocations
  - Windows on ARM
    - Stack Probe Emission
    - Variable Length Arrays

## Introduction

This document describes extensions to tools and formats LLVM seeks compatibility with.

## General Assembly Syntax

### C99-style Hexadecimal Floating-point Constants

LLVM’s assemblers allow floating-point constants to be written in C99’s hexadecimal format instead of decimal if desired.

    .section .data
    .float 0x1c2.2ap3

## Machine-specific Assembly Syntax

### X86/COFF-Dependent

#### Relocations

The following additional relocation types are supported:

@IMGREL (AT&T syntax only) generates an image-relative relocation that corresponds to the COFF relocation types `IMAGE_REL_I386_DIR32NB` (32-bit) or `IMAGE_REL_AMD64_ADDR32NB` (64-bit).

    .text
    fun:
      mov foo@IMGREL(%ebx, %ecx, 4), %eax

    .section .pdata
      .long fun@IMGREL
      .long (fun@imgrel + 0x3F)
      .long $unwind$fun@imgrel

.secrel32 generates a relocation that corresponds to the COFF relocation types `IMAGE_REL_I386_SECREL` (32-bit) or `IMAGE_REL_AMD64_SECREL` (64-bit).

.secidx relocation generates an index of the section that contains the target. It corresponds to the COFF relocation types `IMAGE_REL_I386_SECTION` (32-bit) or `IMAGE_REL_AMD64_SECTION` (64-bit).

    .section .debug$S,"rn"
      .long 4
      .long 242
      .long 40
      .secrel32 _function_name + 0
      .secidx   _function_name
      ...

#### `.linkonce` Directive

Syntax:

> `.linkonce [ comdat type ]`

Supported COMDAT types:

`discard`
  Discards duplicate sections with the same COMDAT symbol. This is the default if no type is specified.
`one_only`
  If the symbol is defined multiple times, the linker issues an error.
`same_size`
  Duplicates are discarded, but the linker issues an error if any have different sizes.
`same_contents`
  Duplicates are discarded, but the linker issues an error if any duplicates do not have exactly the same content.
`largest`
  Links the largest section from among the duplicates.
`newest`
  Links the newest section from among the duplicates.

    .section .text$foo
    .linkonce
      ...

#### `.section` Directive

MC supports passing the information in `.linkonce` at the end of `.section`. For example, these two codes are equivalent

    .section secName, "dr", discard, "Symbol1"
    .globl Symbol1
    Symbol1:
    .long 1

    .section secName, "dr"
    .linkonce discard
    .globl Symbol1
    Symbol1:
    .long 1

Note that in the combined form the COMDAT symbol is explicit. This extension exists to support multiple sections with the same name in different COMDATs:

    .section secName, "dr", discard, "Symbol1"
    .globl Symbol1
    Symbol1:
    .long 1

    .section secName, "dr", discard, "Symbol2"
    .globl Symbol2
    Symbol2:
    .long 1

In addition to the types allowed with `.linkonce`, `.section` also accepts `associative`. The meaning is that the section is linked if a certain other COMDAT section is linked. This other section is indicated by the comdat symbol in this directive. It can be any symbol defined in the associated section, but is usually the associated section’s comdat.

> The following restrictions apply to the associated section:

> 1. It must be a COMDAT section.
> 2. It cannot be another associative COMDAT section.

In the following example the symobl `sym` is the comdat symbol of `.foo` and `.bar` is associated to `.foo`.

    .section        .foo,"bw",discard, "sym"
    .section        .bar,"rd",associative, "sym"

MC supports these flags in the COFF `.section` directive:

> - `b`: BSS section (`IMAGE_SCN_CNT_INITIALIZED_DATA`)
> - `d`: Data section (`IMAGE_SCN_CNT_UNINITIALIZED_DATA`)
> - `n`: Section is not loaded (`IMAGE_SCN_LNK_REMOVE`)
> - `r`: Read-only
> - `s`: Shared section
> - `w`: Writable
> - `x`: Executable section
> - `y`: Not readable
> - `D`: Discardable (`IMAGE_SCN_MEM_DISCARDABLE`)

These flags are all compatible with gas, with the exception of the `D` flag, which gnu as does not support. For gas compatibility, sections with a name starting with “.debug” are implicitly discardable.

### ELF-Dependent

#### `.section` Directive

In order to support creating multiple sections with the same name and comdat, it is possible to add an unique number at the end of the `.seciton` directive. For example, the following code creates two sections named `.text`.

    .section        .text,"ax",@progbits,unique,1
    nop

    .section        .text,"ax",@progbits,unique,2
    nop

The unique number is not present in the resulting object at all. It is just used in the assembler to differentiate the sections.

The ‘o’ flag is mapped to SHF_LINK_ORDER. If it is present, a symbol must be given that identifies the section to be placed is the .sh_link.

    .section .foo,"a",@progbits
    .Ltmp:
    .section .bar,"ao",@progbits,.Ltmp

which is equivalent to just

    .section .foo,"a",@progbits
    .section .bar,"ao",@progbits,.foo

## Target Specific Behaviour

### X86

#### Relocations

`@ABS8` can be applied to symbols which appear as immediate operands to instructions that have an 8-bit immediate form for that operand. It causes the assembler to use the 8-bit form and an 8-bit relocation (e.g. `R_386_8` or `R_X86_64_8`) for the symbol.

For example:

    cmpq $foo@ABS8, %rdi

This causes the assembler to select the form of the 64-bit `cmpq` instruction that takes an 8-bit immediate operand that is sign extended to 64 bits, as opposed to `cmpq $foo, %rdi` which takes a 32-bit immediate operand. This is also not the same as `cmpb $foo, %dil`, which is an 8-bit comparison.

### Windows on ARM

#### Stack Probe Emission

The reference implementation (Microsoft Visual Studio 2012) emits stack probes in the following fashion:

    movw r4, #constant
    bl __chkstk
    sub.w sp, sp, r4

However, this has the limitation of 32 MiB (±16MiB). In order to accommodate larger binaries, LLVM supports the use of `-mcode-model=large` to allow a 4GiB range via a slight deviation. It will generate an indirect jump as follows:

    movw r4, #constant
    movw r12, :lower16:__chkstk
    movt r12, :upper16:__chkstk
    blx r12
    sub.w sp, sp, r4

#### Variable Length Arrays

The reference implementation (Microsoft Visual Studio 2012) does not permit the emission of Variable Length Arrays (VLAs).

The Windows ARM Itanium ABI extends the base ABI by adding support for emitting a dynamic stack allocation. When emitting a variable stack allocation, a call to `__chkstk` is emitted unconditionally to ensure that guard pages are setup properly. The emission of this stack probe emission is handled similar to the standard stack probe emission.

The MSVC environment does not emit code for VLAs currently.