llvm/docs/Extensions.rst

===============
LLVM Extensions
===============

.. contents::
   :local:

.. 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.

.. code-block:: gas

  .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).

.. code-block:: gas

  .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).

.. code-block:: gas

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

``.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.


.. code-block:: gas

  .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

.. code-block:: gas

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

.. code-block:: gas

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

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


.. code-block:: gas

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

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