=============== 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 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: - ``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. 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.