LLVM Extensions¶

Introduction
General Assembly Syntax
- C99-style Hexadecimal Floating-point Constants
Machine-specific Assembly Syntax
- X86/COFF-Dependent

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 type is 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

`.linkonce` Directive ¶

Syntax:

.linkonce [ comdat type [ section identifier ] ]

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.

associative

Links the section if a certain other COMDAT section is linked. This other section is indicated by its section identifier following the comdat type. The following restrictions apply to the associated section:

It must be the name of a section already defined.
It must differ from the current section.
It must be a COMDAT section.
It cannot be another associative COMDAT section.

largest

Links the largest section from among the duplicates.

newest

Links the newest section from among the duplicates.

.section .text$foo
.linkonce
  ...

.section .xdata$foo
.linkonce associative .text$foo
  ...

`.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 explict. This extension exits 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