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The armv7-windows-itanium environment is nearly identical to the MSVC ABI. It has a few divergences, mostly revolving around the use of the Itanium ABI for C++. VLA support is one of the extensions that are amongst the set of the extensions. This adds support for proper VLA emission for this environment. This is somewhat similar to the handling for __chkstk emission on X86 and the large stack frame emission for ARM. The invocation style for chkstk is still controlled via the -mcmodel flag to clang. Make an explicit note that this is an extension. llvm-svn: 210489
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===============
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LLVM Extensions
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===============
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.. contents::
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:local:
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.. toctree::
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:hidden:
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Introduction
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============
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This document describes extensions to tools and formats LLVM seeks compatibility
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with.
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General Assembly Syntax
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===========================
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C99-style Hexadecimal Floating-point Constants
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----------------------------------------------
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LLVM's assemblers allow floating-point constants to be written in C99's
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hexadecimal format instead of decimal if desired.
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.. code-block:: gas
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.section .data
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.float 0x1c2.2ap3
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Machine-specific Assembly Syntax
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================================
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X86/COFF-Dependent
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------------------
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Relocations
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^^^^^^^^^^^
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The following additional relocation types are supported:
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**@IMGREL** (AT&T syntax only) generates an image-relative relocation that
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corresponds to the COFF relocation types ``IMAGE_REL_I386_DIR32NB`` (32-bit) or
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``IMAGE_REL_AMD64_ADDR32NB`` (64-bit).
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.. code-block:: gas
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.text
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fun:
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mov foo@IMGREL(%ebx, %ecx, 4), %eax
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.section .pdata
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.long fun@IMGREL
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.long (fun@imgrel + 0x3F)
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.long $unwind$fun@imgrel
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**.secrel32** generates a relocation that corresponds to the COFF relocation
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types ``IMAGE_REL_I386_SECREL`` (32-bit) or ``IMAGE_REL_AMD64_SECREL`` (64-bit).
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**.secidx** relocation generates an index of the section that contains
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the target. It corresponds to the COFF relocation types
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``IMAGE_REL_I386_SECTION`` (32-bit) or ``IMAGE_REL_AMD64_SECTION`` (64-bit).
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.. code-block:: gas
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.section .debug$S,"rn"
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.long 4
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.long 242
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.long 40
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.secrel32 _function_name
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.secidx _function_name
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...
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``.linkonce`` Directive
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Syntax:
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``.linkonce [ comdat type ]``
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Supported COMDAT types:
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``discard``
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Discards duplicate sections with the same COMDAT symbol. This is the default
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if no type is specified.
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``one_only``
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If the symbol is defined multiple times, the linker issues an error.
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``same_size``
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Duplicates are discarded, but the linker issues an error if any have
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different sizes.
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``same_contents``
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Duplicates are discarded, but the linker issues an error if any duplicates
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do not have exactly the same content.
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``largest``
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Links the largest section from among the duplicates.
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``newest``
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Links the newest section from among the duplicates.
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.. code-block:: gas
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.section .text$foo
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.linkonce
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...
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``.section`` Directive
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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MC supports passing the information in ``.linkonce`` at the end of
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``.section``. For example, these two codes are equivalent
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.. code-block:: gas
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.section secName, "dr", discard, "Symbol1"
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.globl Symbol1
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Symbol1:
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.long 1
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.. code-block:: gas
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.section secName, "dr"
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.linkonce discard
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.globl Symbol1
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Symbol1:
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.long 1
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Note that in the combined form the COMDAT symbol is explicit. This
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extension exists to support multiple sections with the same name in
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different COMDATs:
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.. code-block:: gas
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.section secName, "dr", discard, "Symbol1"
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.globl Symbol1
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Symbol1:
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.long 1
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.section secName, "dr", discard, "Symbol2"
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.globl Symbol2
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Symbol2:
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.long 1
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In addition to the types allowed with ``.linkonce``, ``.section`` also accepts
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``associative``. The meaning is that the section is linked if a certain other
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COMDAT section is linked. This other section is indicated by the comdat symbol
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in this directive. It can be any symbol defined in the associated section, but
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is usually the associated section's comdat.
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The following restrictions apply to the associated section:
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1. It must be a COMDAT section.
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2. It cannot be another associative COMDAT section.
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In the following example the symobl ``sym`` is the comdat symbol of ``.foo``
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and ``.bar`` is associated to ``.foo``.
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.. code-block:: gas
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.section .foo,"bw",discard, "sym"
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.section .bar,"rd",associative, "sym"
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Target Specific Behaviour
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=========================
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Windows on ARM
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--------------
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Stack Probe Emission
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^^^^^^^^^^^^^^^^^^^^
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The reference implementation (Microsoft Visual Studio 2012) emits stack probes
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in the following fashion:
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.. code-block:: gas
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movw r4, #constant
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bl __chkstk
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sub.w sp, sp, r4
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However, this has the limitation of 32 MiB (±16MiB). In order to accommodate
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larger binaries, LLVM supports the use of ``-mcode-model=large`` to allow a 4GiB
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range via a slight deviation. It will generate an indirect jump as follows:
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.. code-block:: gas
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movw r4, #constant
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movw r12, :lower16:__chkstk
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movt r12, :upper16:__chkstk
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blx r12
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sub.w sp, sp, r4
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Variable Length Arrays
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^^^^^^^^^^^^^^^^^^^^^^
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The reference implementation (Microsoft Visual Studio 2012) does not permit the
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emission of Variable Length Arrays (VLAs).
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The Windows ARM Itanium ABI extends the base ABI by adding support for emitting
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a dynamic stack allocation. When emitting a variable stack allocation, a call
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to ``__chkstk`` is emitted unconditionally to ensure that guard pages are setup
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properly. The emission of this stack probe emission is handled similar to the
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standard stack probe emission.
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The MSVC environment does not emit code for VLAs currently.
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