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Extensions.rst

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LLVM Extensions

.. toctree::
   :hidden:

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:

  1. It must be the name of a section already defined.
  2. It must differ from the current section.
  3. It must be a COMDAT section.
  4. 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
  ...