1 =============== 2 LLVM Extensions 3 =============== 4 5 .. contents:: 6 :local: 7 8 .. toctree:: 9 :hidden: 10 11 Introduction 12 ============ 13 14 This document describes extensions to tools and formats LLVM seeks compatibility 15 with. 16 17 General Assembly Syntax 18 =========================== 19 20 C99-style Hexadecimal Floating-point Constants 21 ---------------------------------------------- 22 23 LLVM's assemblers allow floating-point constants to be written in C99's 24 hexadecimal format instead of decimal if desired. 25 26 .. code-block:: gas 27 28 .section .data 29 .float 0x1c2.2ap3 30 31 Machine-specific Assembly Syntax 32 ================================ 33 34 X86/COFF-Dependent 35 ------------------ 36 37 Relocations 38 ^^^^^^^^^^^ 39 40 The following additional relocation types are supported: 41 42 **@IMGREL** (AT&T syntax only) generates an image-relative relocation that 43 corresponds to the COFF relocation types ``IMAGE_REL_I386_DIR32NB`` (32-bit) or 44 ``IMAGE_REL_AMD64_ADDR32NB`` (64-bit). 45 46 .. code-block:: gas 47 48 .text 49 fun: 50 mov foo@IMGREL(%ebx, %ecx, 4), %eax 51 52 .section .pdata 53 .long fun@IMGREL 54 .long (fun@imgrel + 0x3F) 55 .long $unwind$fun@imgrel 56 57 **.secrel32** generates a relocation that corresponds to the COFF relocation 58 types ``IMAGE_REL_I386_SECREL`` (32-bit) or ``IMAGE_REL_AMD64_SECREL`` (64-bit). 59 60 **.secidx** relocation generates an index of the section that contains 61 the target. It corresponds to the COFF relocation types 62 ``IMAGE_REL_I386_SECTION`` (32-bit) or ``IMAGE_REL_AMD64_SECTION`` (64-bit). 63 64 .. code-block:: gas 65 66 .section .debug$S,"rn" 67 .long 4 68 .long 242 69 .long 40 70 .secrel32 _function_name 71 .secidx _function_name 72 ... 73 74 ``.linkonce`` Directive 75 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 76 77 Syntax: 78 79 ``.linkonce [ comdat type ]`` 80 81 Supported COMDAT types: 82 83 ``discard`` 84 Discards duplicate sections with the same COMDAT symbol. This is the default 85 if no type is specified. 86 87 ``one_only`` 88 If the symbol is defined multiple times, the linker issues an error. 89 90 ``same_size`` 91 Duplicates are discarded, but the linker issues an error if any have 92 different sizes. 93 94 ``same_contents`` 95 Duplicates are discarded, but the linker issues an error if any duplicates 96 do not have exactly the same content. 97 98 ``largest`` 99 Links the largest section from among the duplicates. 100 101 ``newest`` 102 Links the newest section from among the duplicates. 103 104 105 .. code-block:: gas 106 107 .section .text$foo 108 .linkonce 109 ... 110 111 ``.section`` Directive 112 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 113 114 MC supports passing the information in ``.linkonce`` at the end of 115 ``.section``. For example, these two codes are equivalent 116 117 .. code-block:: gas 118 119 .section secName, "dr", discard, "Symbol1" 120 .globl Symbol1 121 Symbol1: 122 .long 1 123 124 .. code-block:: gas 125 126 .section secName, "dr" 127 .linkonce discard 128 .globl Symbol1 129 Symbol1: 130 .long 1 131 132 Note that in the combined form the COMDAT symbol is explicit. This 133 extension exists to support multiple sections with the same name in 134 different COMDATs: 135 136 137 .. code-block:: gas 138 139 .section secName, "dr", discard, "Symbol1" 140 .globl Symbol1 141 Symbol1: 142 .long 1 143 144 .section secName, "dr", discard, "Symbol2" 145 .globl Symbol2 146 Symbol2: 147 .long 1 148 149 In addition to the types allowed with ``.linkonce``, ``.section`` also accepts 150 ``associative``. The meaning is that the section is linked if a certain other 151 COMDAT section is linked. This other section is indicated by the comdat symbol 152 in this directive. It can be any symbol defined in the associated section, but 153 is usually the associated section's comdat. 154 155 The following restrictions apply to the associated section: 156 157 1. It must be a COMDAT section. 158 2. It cannot be another associative COMDAT section. 159 160 In the following example the symobl ``sym`` is the comdat symbol of ``.foo`` 161 and ``.bar`` is associated to ``.foo``. 162 163 .. code-block:: gas 164 165 .section .foo,"bw",discard, "sym" 166 .section .bar,"rd",associative, "sym" 167 168 Target Specific Behaviour 169 ========================= 170 171 Windows on ARM 172 -------------- 173 174 Stack Probe Emission 175 ^^^^^^^^^^^^^^^^^^^^ 176 177 The reference implementation (Microsoft Visual Studio 2012) emits stack probes 178 in the following fashion: 179 180 .. code-block:: gas 181 182 movw r4, #constant 183 bl __chkstk 184 sub.w sp, sp, r4 185 186 However, this has the limitation of 32 MiB (16MiB). In order to accommodate 187 larger binaries, LLVM supports the use of ``-mcode-model=large`` to allow a 4GiB 188 range via a slight deviation. It will generate an indirect jump as follows: 189 190 .. code-block:: gas 191 192 movw r4, #constant 193 movw r12, :lower16:__chkstk 194 movt r12, :upper16:__chkstk 195 blx r12 196 sub.w sp, sp, r4 197 198 Variable Length Arrays 199 ^^^^^^^^^^^^^^^^^^^^^^ 200 201 The reference implementation (Microsoft Visual Studio 2012) does not permit the 202 emission of Variable Length Arrays (VLAs). 203 204 The Windows ARM Itanium ABI extends the base ABI by adding support for emitting 205 a dynamic stack allocation. When emitting a variable stack allocation, a call 206 to ``__chkstk`` is emitted unconditionally to ensure that guard pages are setup 207 properly. The emission of this stack probe emission is handled similar to the 208 standard stack probe emission. 209 210 The MSVC environment does not emit code for VLAs currently. 211 212
