This document describes extensions to tools and formats LLVM seeks compatibility with.
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).
.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).
.section .debug$S,"rn" .long 4 .long 242 .long 40 .secrel32 _function_name .secidx _function_name ...
MC supports passing the information in .linkonce at the end of .section. For example, these two codes are equivalent
.section secName, "dr", discard, "Symbol1" .globl Symbol1 Symbol1: .long 1
.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:
.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:
- It must be a COMDAT section.
- 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.
.section .foo,"bw",discard, "sym" .section .bar,"rd",associative, "sym"
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.
.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 reference implementation (Microsoft Visual Studio 2012) emits stack probes in the following fashion:
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:
movw r4, #constant movw r12, :lower16:__chkstk movt r12, :upper16:__chkstk blx r12 sub.w sp, sp, r4
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.