LLVM Extensions¶

Introduction
General Assembly Syntax
- C99-style Hexadecimal Floating-point Constants
Machine-specific Assembly Syntax
- X86/COFF-Dependent
- ELF-Dependent
  - .section Directive
Target Specific Behaviour

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:

It must be a COMDAT section.

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.

Windows on ARM64 ¶

Stack Probe Emission ¶

The reference implementation (Microsoft Visual Studio 2017) emits stack probes in the following fashion:

mov x15, #constant
bl __chkstk
sub sp, sp, x15, lsl #4

However, this has the limitation of 256 MiB (±128MiB). In order to accommodate larger binaries, LLVM supports the use of -mcode-model=large to allow a 8GiB (±4GiB) range via a slight deviation. It will generate an indirect jump as follows:

mov x15, #constant
adrp x16, __chkstk
add x16, x16, :lo12:__chkstk
blr x16
sub sp, sp, x15, lsl #4