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LLVM Extensions
.. contents::
.. toctree::
This document describes extensions to tools and formats LLVM seeks compatibility
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
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
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$S,"rn"
.long 4
.long 242
.long 40
.secrel32 _function_name + 0
.secidx _function_name
``.linkonce`` Directive
``.linkonce [ comdat type ]``
Supported COMDAT types:
Discards duplicate sections with the same COMDAT symbol. This is the default
if no type is specified.
If the symbol is defined multiple times, the linker issues an error.
Duplicates are discarded, but the linker issues an error if any have
different sizes.
Duplicates are discarded, but the linker issues an error if any duplicates
do not have exactly the same content.
Links the largest section from among the duplicates.
Links the newest section from among the duplicates.
.. code-block:: gas
.section .text$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
.long 1
.. code-block:: gas
.section secName, "dr"
.linkonce discard
.globl 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
.long 1
.section secName, "dr", discard, "Symbol2"
.globl 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"
MC supports these flags in the COFF ``.section`` directive:
- ``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.
``.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
.section .text,"ax",@progbits,unique,2
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
.. code-block:: gas
.section .foo,"a",@progbits
.section .bar,"ao",@progbits,.Ltmp
which is equivalent to just
.. code-block:: gas
.section .foo,"a",@progbits
.section .bar,"ao",@progbits,.foo
Target Specific Behaviour
``@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.
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.