1 <html><body><pre>Android Native CPU ABI Management 2 3 4 Introduction: 5 ============= 6 7 Every piece of native code generated with the Android NDK matches a given 8 "Application Binary Interface" (ABI) that defines exactly how your 9 application's machine code is expected to interact with the system at 10 runtime. 11 12 A typical ABI describes things in *excruciating* details, and will typically 13 include the following information: 14 15 - the CPU instruction set that the machine code should use 16 17 - the endianness of memory stores and loads at runtime 18 19 - the format of executable binaries (shared libraries, programs, etc...) 20 and what type of content is allowed/supported in them. 21 22 - various conventions used to pass data between your code and 23 the system (e.g. how registers and/or the stack are used when functions 24 are called, alignment constraints, etc...) 25 26 - alignment and size constraints for enum types, structure fields and 27 arrays. 28 29 - the list of function symbols available to your machine code at runtime, 30 generally from a very specific selected set of libraries. 31 32 This document lists the exact ABIs supported by the Android NDK and the 33 official Android platform releases. 34 35 36 I. Supported ABIs: 37 ================== 38 39 Each supported ABI is identified by a unique name. 40 41 42 I.1. 'armeabi' 43 -------------- 44 45 This is the name of an ABI for ARM-based CPUs that support *at* *least* 46 the ARMv5TE instruction set. Please refer to following documentation for 47 more details: 48 49 - ARM Architecture Reference manual (a.k.a ARMARM) 50 - Procedure Call Standard for the ARM Architecture (a.k.a. AAPCS) 51 - ELF for the ARM Architecture (a.k.a. ARMELF) 52 - ABI for the ARM Architecture (a.k.a. BSABI) 53 - Base Platform ABI for the ARM Architecture (a.k.a. BPABI) 54 - C Library ABI for the ARM Architecture (a.k.a. CLIABI) 55 - C++ ABI for the ARM Architecture (a.k.a. CPPABI) 56 - Runtime ABI for the ARM Architecture (a.k.a. RTABI) 57 58 - ELF System V Application Binary Interface 59 (DRAFT - 24 April 2001) 60 61 - Generic C++ ABI (http://www.codesourcery.com/public/cxx-abi/abi.html) 62 63 Note that the AAPCS standard defines 'EABI' as a moniker used to specify 64 a _family_ of similar but distinct ABIs. Android follows the little-endian 65 ARM GNU/Linux ABI as documented in the following document: 66 67 http://www.codesourcery.com/gnu_toolchains/arm/arm_gnu_linux_abi.pdf 68 69 With the exception that wchar_t is only one byte. This should not matter 70 in practice since wchar_t is simply *not* really supported by the Android 71 platform anyway. 72 73 This ABI does *not* support hardware-assisted floating point computations. 74 Instead, all FP operations are performed through software helper functions 75 that come from the compiler's libgcc.a static library. 76 77 Thumb (a.k.a. Thumb-1) instructions are supported. Note that the NDK 78 will generate thumb code by default, unless you define LOCAL_ARM_MODE 79 in your Android.mk (see docs/ANDROID-MK.html for all details). 80 81 82 I.2. 'armeabi-v7a' 83 ------------------ 84 85 This is the name of another ARM-based CPU ABI that *extends* 'armeabi' to 86 include a few CPU instruction set extensions as described in the following 87 document: 88 89 - ARM Architecture v7-a Reference Manual 90 91 The instruction extensions supported by this Android-specific ABI are: 92 93 - The Thumb-2 instruction set extension. 94 - The VFP hardware FPU instructions. 95 96 More specifically, VFPv3-D16 is being used, which corresponds to 16 97 dedicated 64-bit floating point registers provided by the CPU. 98 99 Other extensions described by the v7-a ARM like Advanced SIMD (a.k.a. NEON), 100 VFPv3-D32 or ThumbEE are optional to this ABI, which means that developers 101 should check *at* *runtime* whether the extensions are available and provide 102 alternative code paths if this is not the case. 103 104 (Just like one typically does on x86 systems to check/use MMX/SSE2/etc... 105 specialized instructions). 106 107 You can check docs/CPU-FEATURES.html to see how to perform these runtime 108 checks, and docs/CPU-ARM-NEON.html to learn about the NDK's support for 109 building NEON-capable machine code too. 110 111 IMPORTANT NOTE: This ABI enforces that all double values are passed during 112 function calls in 'core' register pairs, instead of dedicated FP ones. 113 However, all internal computations can be performed with the FP registers 114 and will be greatly sped up. 115 116 This little constraint, while resulting in a slight decrease of 117 performance, ensures binary compatibility with all existing 'armeabi' 118 binaries. 119 120 IMPORTANT NOTE: The 'armeabi-v7a' machine code will *not* run on ARMv5 or 121 ARMv6 based devices. 122 123 124 I.3. 'x86' 125 ---------- 126 127 This is the name of an ABI for CPUs supporting the instruction set 128 commonly named 'x86' or 'IA-32'. More specifically, this ABI corresponds 129 to the following: 130 131 - instructions normally generated by GCC with the following compiler 132 flags: 133 134 -march=i686 -msse3 -mstackrealign -mfpmath=sse 135 136 which targets Pentium Pro instruction set, according to the GCC 137 documentation, plus the MMX, SSE, SSE2 and SSE3 instruction set 138 extensions. 139 140 - using the standard Linux x86 32-bit calling convention (e.g. section 6, 141 "Register Usage" of the "Calling conventions..." document below), not 142 the SVR4 one. 143 144 The ABI does *not* include any other optional IA-32 instruction set 145 extension, including, but not limited to: 146 147 - the MOVBE instruction 148 - the SSSE3 "supplemental SSE3" extension 149 - any variant of "SSE4" 150 151 You can still use these, as long as you use runtime feature probing to 152 enable them, and provide fallbacks for devices that do not support them. 153 154 Please refer to the following documents for more details: 155 156 http://gcc.gnu.org/onlinedocs/gcc/i386-and-x86_002d64-Options.html 157 158 Calling conventions for different C++ compilers and operating systems 159 http://www.agner.org/optimize/calling_conventions.pdf 160 161 Intel IA-32 Intel Architecture Software Developer's Manual 162 volume 2: Instruction Set Reference 163 164 Intel IA-32 Intel Architecture Software Developer's Manual 165 volume 3: System Programming 166 167 Amendment to System V Application Binary Interface 168 Intel386 Processor Architecture Supplement 169 170 171 II. Generating code for a specific ABI: 172 ======================================= 173 174 By default, the NDK will generate machine code for the 'armeabi' ABI. 175 You can however add the following line to your Application.mk to generate 176 ARMv7-a compatible machine code instead: 177 178 APP_ABI := armeabi-v7a 179 180 It is also possible to build machine code for *two* distinct ABIs by using: 181 182 APP_ABI := armeabi armeabi-v7a 183 184 This will instruct the NDK to build two versions of your machine code: one for 185 each ABI listed on this line. Both libraries will be copied to your application 186 project path and will be ultimately packaged into your .apk. 187 188 Such a package is called a "fat binary" in Android speak since it contains 189 machine code for more than one CPU architecture. At installation time, the 190 package manager will only unpack the most appropriate machine code for the 191 target device. See below for details. 192 193 194 195 III. ABI Management on the Android platform: 196 ============================================ 197 198 This section provides specific details about how the Android platform manages 199 native code in application packages. 200 201 202 III.1. Native code in Application Packages: 203 ------------------------------------------- 204 205 It is expected that shared libraries generated with the NDK are stored in 206 the final application package (.apk) at locations of the form: 207 208 lib/<abi>/lib<name>.so 209 210 Where <abi> is one of the ABI names listed in section II above, and <name> 211 is a name that can be used when loading the shared library from the VM 212 as in: 213 214 System.loadLibrary("<name>"); 215 216 Since .apk files are just zip files, you can trivially list their content 217 with a command like: 218 219 unzip -l <apk> 220 221 to verify that the native shared libraries you want are indeed at the 222 proper location. You can also place native shared libraries at other 223 locations within the .apk, but they will be ignored by the system, or more 224 precisely by the steps described below; you will need to extract/install 225 them manually in your application. 226 227 In the case of a "fat" binary, up to three distinct libraries can be placed 228 in the .apk, for example at: 229 230 lib/armeabi/libfoo.so 231 lib/armeabi-v7a/libfoo.so 232 lib/x86/libfoo.so 233 234 235 III.2. Android Platform ABI support: 236 ------------------------------------ 237 238 The Android system knows at runtime which ABI(s) it supports. More 239 precisely, up to two build-specific system properties are used to 240 indicate: 241 242 - the 'primary' ABI for the device, corresponding to the machine 243 code used in the system image itself. 244 245 - an optional 'secondary' ABI, corresponding to another ABI that 246 is also supported by the system image. 247 248 For example, a typical ARMv5TE-based device would only define 249 the primary ABI as 'armeabi' and not define a secondary one. 250 251 On the other hand, a typical ARMv7-based device would define the 252 primary ABI to 'armeabi-v7a' and the secondary one to 'armeabi' 253 since it can run application native binaries generated for both 254 of them. 255 256 A typical x86-based device only defines a primary abi named 'x86'. 257 258 259 III.3. Automatic extraction of native code at install time: 260 ----------------------------------------------------------- 261 262 When installing an application, the package manager service will scan 263 the .apk and look for any shared library of the form: 264 265 lib/<primary-abi>/lib<name>.so 266 267 If one is found, then it is copied under $APPDIR/lib/lib<name>.so, 268 where $APPDIR corresponds to the application's specific data directory. 269 270 If none is found, and a secondary ABI is defined, the service will 271 then scan for shared libraries of the form: 272 273 lib/<secondary-abi>/lib<name>.so 274 275 If anything is found, then it is copied under $APPDIR/lib/lib<name>.so 276 277 This mechanism ensures that the best machine code for the target 278 device is automatically extracted from the package at installation 279 time. 280 </pre></body></html> 281
