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:: text

  .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:: none

  .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.


.. 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 symbol ``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"

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.


ARM64/COFF-Dependent
--------------------

Relocations
^^^^^^^^^^^

The following additional symbol variants are supported:

**:secrel_lo12:** generates a relocation that corresponds to the COFF relocation
types ``IMAGE_REL_ARM64_SECREL_LOW12A`` or ``IMAGE_REL_ARM64_SECREL_LOW12L``.

**:secrel_hi12:** generates a relocation that corresponds to the COFF relocation
type ``IMAGE_REL_ARM64_SECREL_HIGH12A``.

.. code-block:: gas

    add x0, x0, :secrel_hi12:symbol
    ldr x0, [x0, :secrel_lo12:symbol]

    add x1, x1, :secrel_hi12:symbol
    add x1, x1, :secrel_lo12:symbol
    ...


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 ``.section`` 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.

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.

.. code-block:: gas

        .section .foo,"a",@progbits
        .Ltmp:
        .section .bar,"ao",@progbits,.Ltmp

which is equivalent to just

.. code-block:: gas

        .section .foo,"a",@progbits
        .section .bar,"ao",@progbits,.foo

``.linker-options`` Section (linker options)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In order to support passing linker options from the frontend to the linker, a
special section of type ``SHT_LLVM_LINKER_OPTIONS`` (usually named
``.linker-options`` though the name is not significant as it is identified by
the type).  The contents of this section is a simple pair-wise encoding of
directives for consideration by the linker.  The strings are encoded as standard
null-terminated UTF-8 strings.  They are emitted inline to avoid having the
linker traverse the object file for retrieving the value.  The linker is
permitted to not honour the option and instead provide a warning/error to the
user that the requested option was not honoured.

The section has type ``SHT_LLVM_LINKER_OPTIONS`` and has the ``SHF_EXCLUDE``
flag to ensure that the section is treated as opaque by linkers which do not
support the feature and will not be emitted into the final linked binary.

This would be equivalent to the follow raw assembly:

.. code-block:: gas

  .section ".linker-options","e",@llvm_linker_options
  .asciz "option 1"
  .asciz "value 1"
  .asciz "option 2"
  .asciz "value 2"

The following directives are specified:

  - lib

    The parameter identifies a library to be linked against.  The library will
    be looked up in the default and any specified library search paths
    (specified to this point).

  - libpath

    The parameter identifies an additional library search path to be considered
    when looking up libraries after the inclusion of this option.

``SHT_LLVM_DEPENDENT_LIBRARIES`` Section (Dependent Libraries)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This section contains strings specifying libraries to be added to the link by
the linker.

The section should be consumed by the linker and not written to the output.

The strings are encoded as standard null-terminated UTF-8 strings.

For example:

.. code-block:: gas

  .section ".deplibs","MS",@llvm_dependent_libraries,1
  .asciz "library specifier 1"
  .asciz "library specifier 2"

The interpretation of the library specifiers is defined by the consuming linker.

``SHT_LLVM_CALL_GRAPH_PROFILE`` Section (Call Graph Profile)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This section is used to pass a call graph profile to the linker which can be
used to optimize the placement of sections.  It contains a sequence of
(from symbol, to symbol, weight) tuples.

It shall have a type of ``SHT_LLVM_CALL_GRAPH_PROFILE`` (0x6fff4c02), shall
have the ``SHF_EXCLUDE`` flag set, the ``sh_link`` member shall hold the section
header index of the associated symbol table, and shall have a ``sh_entsize`` of
16.  It should be named ``.llvm.call-graph-profile``.

The contents of the section shall be a sequence of ``Elf_CGProfile`` entries.

.. code-block:: c

  typedef struct {
    Elf_Word cgp_from;
    Elf_Word cgp_to;
    Elf_Xword cgp_weight;
  } Elf_CGProfile;

cgp_from
  The symbol index of the source of the edge.

cgp_to
  The symbol index of the destination of the edge.

cgp_weight
  The weight of the edge.

This is represented in assembly as:

.. code-block:: gas

  .cg_profile from, to, 42

``.cg_profile`` directives are processed at the end of the file.  It is an error
if either ``from`` or ``to`` are undefined temporary symbols.  If either symbol
is a temporary symbol, then the section symbol is used instead.  If either
symbol is undefined, then that symbol is defined as if ``.weak symbol`` has been
written at the end of the file.  This forces the symbol to show up in the symbol
table.

``SHT_LLVM_ADDRSIG`` Section (address-significance table)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This section is used to mark symbols as address-significant, i.e. the address
of the symbol is used in a comparison or leaks outside the translation unit. It
has the same meaning as the absence of the LLVM attributes ``unnamed_addr``
and ``local_unnamed_addr``.

Any sections referred to by symbols that are not marked as address-significant
in any object file may be safely merged by a linker without breaking the
address uniqueness guarantee provided by the C and C++ language standards.

The contents of the section are a sequence of ULEB128-encoded integers
referring to the symbol table indexes of the address-significant symbols.

There are two associated assembly directives:

.. code-block:: gas

  .addrsig

This instructs the assembler to emit an address-significance table. Without
this directive, all symbols are considered address-significant.

.. code-block:: gas

  .addrsig_sym sym

If ``sym`` is not otherwise referenced or defined anywhere else in the file,
this directive is a no-op. Otherwise, mark ``sym`` as address-significant.

``SHT_LLVM_SYMPART`` Section (symbol partition specification)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This section is used to mark symbols with the `partition`_ that they
belong to. An ``.llvm_sympart`` section consists of a null-terminated string
specifying the name of the partition followed by a relocation referring to
the symbol that belongs to the partition. It may be constructed as follows:

.. code-block:: gas

  .section ".llvm_sympart","",@llvm_sympart
  .asciz "libpartition.so"
  .word symbol_in_partition

.. _partition: https://lld.llvm.org/Partitions.html

``SHT_LLVM_BB_ADDR_MAP`` Section (basic block address map)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This section stores the binary address of basic blocks along with other related
metadata. This information can be used to map binary profiles (like perf
profiles) directly to machine basic blocks.
This section is emitted with ``-basic-block-address-map`` and will contain
a BB address map table for every function.

The ``SHT_LLVM_BB_ADDR_MAP`` type provides backward compatibility to allow
reading older versions of the BB address map generated by older compilers. Each
function entry starts with a version byte which specifies the encoding version
to use. The following versioning schemes are currently supported.

Version 1 (newest): basic block address offsets are computed relative to the end
of previous blocks.

Example:

.. code-block:: gas

  .section  ".llvm_bb_addr_map","",@llvm_bb_addr_map
  .byte     1                             # version number
  .byte     0                             # feature byte (reserved for future use)
  .quad     .Lfunc_begin0                 # address of the function
  .byte     2                             # number of basic blocks
  # BB record for BB_0
   .uleb128  .Lfunc_beign0-.Lfunc_begin0  # BB_0 offset relative to function entry (always zero)
   .uleb128  .LBB_END0_0-.Lfunc_begin0    # BB_0 size
   .byte     x                            # BB_0 metadata
  # BB record for BB_1
   .uleb128  .LBB0_1-.LBB_END0_0          # BB_1 offset relative to the end of last block (BB_0).
   .uleb128  .LBB_END0_1-.LBB0_1          # BB_1 size
   .byte     y                            # BB_1 metadata

Version 0: basic block address offsets are computed relative to the function
address. This uses the unversioned ``SHT_LLVM_BB_ADDR_MAP_V0`` section type and
is semantically equivalent to using ``SHT_LLVM_BB_ADDR_MAP`` with a zero
version field.

Example:

.. code-block:: gas

  .section  ".llvm_bb_addr_map","",@llvm_bb_addr_map_v0
  .quad     .Lfunc_begin0                 # address of the function
  .byte     2                             # number of basic blocks
  # BB record for BB_0
   .uleb128  .Lfunc_beign0-.Lfunc_begin0  # BB_0 offset relative to the function entry (always zero)
   .uleb128  .LBB_END0_0-.Lfunc_begin0    # BB_0 size
   .byte     x                            # BB_0 metadata
  # BB record for BB_1
   .uleb128  .LBB0_1-.Lfunc_begin0        # BB_1 offset relative to the function entry
   .uleb128  .LBB_END0_1-.LBB0_1          # BB_1 size
   .byte     y                            # BB_1 metadata

PGO Analysis Map
""""""""""""""""

PGO related analysis data can be emitted after each function within the
``SHT_LLVM_BB_ADDR_MAP`` through the optional ``pgo-analysis-map`` flag.
Supported analyses currently are Function Entry Count, Basic Block Frequencies,
and Branch Probabilities.

Each analysis is enabled or disabled via a bit in the feature byte. Currently
those bits are:

#. Function Entry Count - Number of times the function was called as taken
   from a PGO profile. This will always be zero if PGO was not used or the
   function was not encountered in the profile.

#. Basic Block Frequencies - Encoded as raw block frequency value taken from
   MBFI analysis. This value is an integer that encodes the relative frequency
   compared to the entry block. More information can be found in
   'llvm/Support/BlockFrequency.h'.

#. Branch Probabilities - Encoded as raw numerator for branch probability
   taken from MBPI analysis. This value is the numerator for a fixed point ratio
   defined in 'llvm/Support/BranchProbability.h'. It indicates the probability
   that the block is followed by a given successor block during execution.

This extra data requires version 2 or above. This is necessary since successors
of basic blocks won't know their index but will know their BB ID.

Example of BBAddrMap with PGO data:

.. code-block:: gas

  .section  ".llvm_bb_addr_map","",@llvm_bb_addr_map
  .byte     2                             # version number
  .byte     7                             # feature byte - PGO analyses enabled mask
  .quad     .Lfunc_begin0                 # address of the function
  .uleb128  4                             # number of basic blocks
  # BB record for BB_0
   .uleb128  0                            # BB_0 BB ID
   .uleb128  .Lfunc_begin0-.Lfunc_begin0  # BB_0 offset relative to function entry (always zero)
   .uleb128  .LBB_END0_0-.Lfunc_begin0    # BB_0 size
   .byte     0x18                         # BB_0 metadata (multiple successors)
  # BB record for BB_1
   .uleb128  1                            # BB_1 BB ID
   .uleb128  .LBB0_1-.LBB_END0_0          # BB_1 offset relative to the end of last block (BB_0).
   .uleb128  .LBB_END0_1-.LBB0_1          # BB_1 size
   .byte     0x0                          # BB_1 metadata (two successors)
  # BB record for BB_2
   .uleb128  2                            # BB_2 BB ID
   .uleb128  .LBB0_2-.LBB_END1_0          # BB_2 offset relative to the end of last block (BB_1).
   .uleb128  .LBB_END0_2-.LBB0_2          # BB_2 size
   .byte     0x0                          # BB_2 metadata (one successor)
  # BB record for BB_3
   .uleb128  3                            # BB_3 BB ID
   .uleb128  .LBB0_3-.LBB_END0_2          # BB_3 offset relative to the end of last block (BB_2).
   .uleb128  .LBB_END0_3-.LBB0_3          # BB_3 size
   .byte     0x0                          # BB_3 metadata (zero successors)
  # PGO Analysis Map
  .uleb128  1000                          # function entry count (only when enabled)
  # PGO data record for BB_0
   .uleb128  1000                         # BB_0 basic block frequency (only when enabled)
   .uleb128  3                            # BB_0 successors count (only enabled with branch probabilities)
   .uleb128  1                            # BB_0 successor 1 BB ID (only enabled with branch probabilities)
   .uleb128  0x22222222                   # BB_0 successor 1 branch probability (only enabled with branch probabilities)
   .uleb128  2                            # BB_0 successor 2 BB ID (only enabled with branch probabilities)
   .uleb128  0x33333333                   # BB_0 successor 2 branch probability (only enabled with branch probabilities)
   .uleb128  3                            # BB_0 successor 3 BB ID (only enabled with branch probabilities)
   .uleb128  0xaaaaaaaa                   # BB_0 successor 3 branch probability (only enabled with branch probabilities)
  # PGO data record for BB_1
   .uleb128  133                          # BB_1 basic block frequency (only when enabled)
   .uleb128  2                            # BB_1 successors count (only enabled with branch probabilities)
   .uleb128  2                            # BB_1 successor 1 BB ID (only enabled with branch probabilities)
   .uleb128  0x11111111                   # BB_1 successor 1 branch probability (only enabled with branch probabilities)
   .uleb128  3                            # BB_1 successor 2 BB ID (only enabled with branch probabilities)
   .uleb128  0x11111111                   # BB_1 successor 2 branch probability (only enabled with branch probabilities)
  # PGO data record for BB_2
   .uleb128  18                           # BB_2 basic block frequency (only when enabled)
   .uleb128  1                            # BB_2 successors count (only enabled with branch probabilities)
   .uleb128  3                            # BB_2 successor 1 BB ID (only enabled with branch probabilities)
   .uleb128  0xffffffff                   # BB_2 successor 1 branch probability (only enabled with branch probabilities)
  # PGO data record for BB_3
   .uleb128  1000                         # BB_3 basic block frequency (only when enabled)
   .uleb128  0                            # BB_3 successors count (only enabled with branch probabilities)

``SHT_LLVM_OFFLOADING`` Section (offloading data)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This section stores the binary data used to perform offloading device linking
and execution, creating a fat binary. This section is emitted during compilation
of offloading languages such as OpenMP or CUDA. If the data is intended to be
used by the device linker only, it should use the ``SHF_EXCLUDE`` flag so it is
automatically stripped from the final executable or shared library.

The binary data stored in this section conforms to a custom binary format used
for storing offloading metadata. This format is effectively a string table
containing metadata accompanied by a device image.

``SHT_LLVM_LTO`` Section (LLVM bitcode for fat LTO)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This section stores LLVM bitcode used to perform regular LTO or ThinLTO at link
time. This section is generated when the compiler enables fat LTO. This section
has the ``SHF_EXCLUDE`` flag so that it is stripped from the final executable
or shared library.

``SHT_LLVM_JT_SIZES`` Section (Jump table addresses and sizes)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This section stores pairs of (jump table address, number of entries).
This information is useful for tools that need to statically reconstruct
the control flow of executables.

CodeView-Dependent
------------------

``.cv_file`` Directive
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
  ``.cv_file`` *FileNumber FileName* [ *checksum* ] [ *checksumkind* ]

``.cv_func_id`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^
Introduces a function ID that can be used with ``.cv_loc``.

Syntax:
  ``.cv_func_id`` *FunctionId*

``.cv_inline_site_id`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Introduces a function ID that can be used with ``.cv_loc``. Includes
``inlined at`` source location information for use in the line table of the
caller, whether the caller is a real function or another inlined call site.

Syntax:
  ``.cv_inline_site_id`` *FunctionId* ``within`` *Function* ``inlined_at`` *FileNumber Line* [ *Column* ]

``.cv_loc`` Directive
^^^^^^^^^^^^^^^^^^^^^
The first number is a file number, must have been previously assigned with a
``.file`` directive, the second number is the line number and optionally the
third number is a column position (zero if not specified).  The remaining
optional items are ``.loc`` sub-directives.

Syntax:
  ``.cv_loc`` *FunctionId FileNumber* [ *Line* ] [ *Column* ] [ *prologue_end* ] [ ``is_stmt`` *value* ]

``.cv_linetable`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
  ``.cv_linetable`` *FunctionId* ``,`` *FunctionStart* ``,`` *FunctionEnd*

``.cv_inline_linetable`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
  ``.cv_inline_linetable`` *PrimaryFunctionId* ``,`` *FileNumber Line FunctionStart FunctionEnd*

``.cv_def_range`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^^
The *GapStart* and *GapEnd* options may be repeated as needed.

Syntax:
  ``.cv_def_range`` *RangeStart RangeEnd* [ *GapStart GapEnd* ] ``,`` *bytes*

``.cv_stringtable`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``.cv_filechecksums`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``.cv_filechecksumoffset`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
  ``.cv_filechecksumoffset`` *FileNumber*

``.cv_fpo_data`` Directive
^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
  ``.cv_fpo_data`` *procsym*

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:

.. code-block:: gas

  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.


**@GOTPCREL_NORELAX** can be used in place of ``@GOTPCREL`` to guarantee that
the assembler emits an ``R_X86_64_GOTPCREL`` relocation instead of a relaxable
``R_X86_64[_REX]_GOTPCRELX`` relocation.

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 ``-mcmodel=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.

Windows on ARM64
----------------

Stack Probe Emission
^^^^^^^^^^^^^^^^^^^^

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

.. code-block:: gas

  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 ``-mcmodel=large`` to allow a 8GiB
(±4GiB) range via a slight deviation.  It will generate an indirect jump as
follows:

.. code-block:: gas

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