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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
------------------
-The following additional relocation type is supported:
+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
.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$S,"rn"
+ .long 4
+ .long 242
+ .long 40
+ .secrel32 _function_name
+ .secidx _function_name
+ ...
+
+``.linkonce`` Directive
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+
+ ``.linkonce [ comdat type ]``
+
+Supported COMDAT types:
+
+``discard``
+ Discards duplicate sections with the same COMDAT symbol. This is the default
+ if no type is specified.
+
+``one_only``
+ If the symbol is defined multiple times, the linker issues an error.
+
+``same_size``
+ Duplicates are discarded, but the linker issues an error if any have
+ different sizes.
+
+``same_contents``
+ Duplicates are discarded, but the linker issues an error if any duplicates
+ do not have exactly the same content.
+
+``largest``
+ Links the largest section from among the duplicates.
+
+``newest``
+ Links the newest section from among the duplicates.
+
+
+.. code-block:: gas
+
+ .section .text$foo
+ .linkonce
+ ...
+
+``.section`` Directive
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+MC supports passing the information in ``.linkonce`` at the end of
+``.section``. For example, these two codes are equivalent
+
+.. 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 exists 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
+
+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``.
+
+.. code-block:: gas
+
+ .section .foo,"bw",discard, "sym"
+ .section .bar,"rd",associative, "sym"
+
+
+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``.
+
+.. code-block:: gas
+
+ .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.
+
+Target Specific Behaviour
+=========================
+
+Windows on ARM
+--------------
+
+Stack Probe Emission
+^^^^^^^^^^^^^^^^^^^^
+
+The reference implementation (Microsoft Visual Studio 2012) emits stack probes
+in the following fashion:
+
+.. code-block:: gas
+
+ 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:
+
+.. code-block:: gas
+
+ 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.
+
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