    Android Native CPU ABI Management

    Introduction:
    =============

    Every piece of native code generated with the Android NDK matches a given
    "Application Binary Interface" (ABI) that defines exactly how your
    application's machine code is expected to interact with the system at
    runtime.

    A typical ABI describes things in *excruciating* details, and will typically
    include the following information:

      - the CPU instruction set that the machine code should use

      - the endianness of memory stores and loads at runtime

      - the format of executable binaries (shared libraries, programs, etc...)
        and what type of content is allowed/supported in them.

      - various conventions used to pass data between your code and
        the system (e.g. how registers and/or the stack are used when functions
        are called, alignment constraints, etc...)

      - alignment and size constraints for enum types, structure fields and
        arrays.

      - the list of function symbols available to your machine code at runtime,
        generally from a very specific selected set of libraries.

    This document lists the exact ABIs supported by the Android NDK and the
    official Android platform releases.

    I. Supported ABIs:
    ==================

    Each supported ABI is identified by a unique name.

     I.1. 'armeabi'
     --------------

      This is the name of an ABI for ARM-based CPUs that support *at* *least*
      the ARMv5TE instruction set. Please refer to following documentation for
      more details:

       - ARM Architecture Reference manual                (a.k.a  ARMARM)
       - Procedure Call Standard for the ARM Architecture (a.k.a. AAPCS)
       - ELF for the ARM Architecture                     (a.k.a. ARMELF)
       - ABI for the ARM Architecture                     (a.k.a. BSABI)
       - Base Platform ABI for the ARM Architecture       (a.k.a. BPABI)
       - C Library ABI for the ARM Architecture           (a.k.a. CLIABI)
       - C++ ABI for the ARM Architecture                 (a.k.a. CPPABI)
       - Runtime ABI for the ARM Architecture             (a.k.a. RTABI)

       - ELF System V Application Binary Interface
         (DRAFT - 24 April 2001)

       - Generic C++ ABI  (http://www.codesourcery.com/public/cxx-abi/abi.html)

      Note that the AAPCS standard defines 'EABI' as a moniker used to specify
      a _family_ of similar but distinct ABIs. Android follows the little-endian
      ARM GNU/Linux ABI as documented in the following document:

          http://www.codesourcery.com/gnu_toolchains/arm/arm_gnu_linux_abi.pdf

      With the exception that wchar_t is only one byte. This should not matter
      in practice since wchar_t is simply *not* really supported by the Android
      platform anyway.

      This ABI does *not* support hardware-assisted floating point computations.
      Instead, all FP operations are performed through software helper functions
      that come from the compiler's libgcc.a static library.

      Thumb (a.k.a. Thumb-1) instructions are supported. Note that the NDK
      will generate thumb code by default, unless you define LOCAL_ARM_MODE
      in your Android.mk (see docs/ANDROID-MK.html for all details).

     I.2. 'armeabi-v7a'
     ------------------

      This is the name of another ARM-based CPU ABI that *extends* 'armeabi' to
      include a few CPU instruction set extensions as described in the following
      document:

      - ARM Architecture v7-a Reference Manual

      The instruction extensions supported by this Android-specific ABI are:

         - The Thumb-2 instruction set extension.
         - The VFP hardware FPU instructions.

      More specifically, VFPv3-D16 is being used, which corresponds to 16
      dedicated 64-bit floating point registers provided by the CPU.

      Other extensions described by the v7-a ARM like Advanced SIMD (a.k.a. NEON),
      VFPv3-D32 or ThumbEE are optional to this ABI, which means that developers
      should check *at* *runtime* whether the extensions are available and provide
      alternative code paths if this is not the case.

      (Just like one typically does on x86 systems to check/use MMX/SSE2/etc...
       specialized instructions).

      You can check docs/CPU-FEATURES.html to see how to perform these runtime
      checks, and docs/CPU-ARM-NEON.html to learn about the NDK's support for
      building NEON-capable machine code too.

      IMPORTANT NOTE: This ABI enforces that all double values are passed during
      function calls in 'core' register pairs, instead of dedicated FP ones.
      However, all internal computations can be performed with the FP registers
      and will be greatly sped up.

      This little constraint, while resulting in a slight decrease of
      performance, ensures binary compatibility with all existing 'armeabi'
      binaries.

      IMPORTANT NOTE: The 'armeabi-v7a' machine code will *not* run on ARMv5 or
                      ARMv6 based devices.

     I.3. 'x86'
     ----------

      This is the name of an ABI for CPUs supporting the instruction set
      commonly named 'x86' or 'IA-32'. More specifically, this targets the
      instruction sets commonly referenced as 'Pentium Pro' plus MMX, SSE,
      SSE2 and SSE3.  Please refer to following documentations for
      more details:

        Intel IA-32 Intel Architecture Software Developer's Manual
          volume 2: Instruction Set Reference
        Amendment to System V Application Binary Interface
          Intel386 Processor Architecture Supplement

      IMPORTANT IMPORTANT IMPORTANT IMPORTANT IMPORTANT IMPORTANT:

          THE 'x86' ABI IS AN EXPERIMENTAL FEATURE THAT IS NOT FULLY
          SUPPORTED YET BY THIS NDK RELEASE. TRYING TO USE IT WILL RESULT
          IN AN ERROR DURING THE BUILD PROCESS.

      Note that optional features like MOVBE must be explicitly tested at
      runtime by the generated machine code and cannot be assumed to be
      everywhere.

    II. Generating code for a specific ABI:
    =======================================

    By default, the NDK will generate machine code for the 'armeabi' ABI.
    You can however add the following line to your Application.mk to generate
    ARMv7-a compatible machine code instead:

       APP_ABI := armeabi-v7a

    It is also possible to build machine code for *two* distinct ABIs by using:

       APP_ABI := armeabi armeabi-v7a

    This will instruct the NDK to build two versions of your machine code: one for
    each ABI listed on this line. Both libraries will be copied to your application
    project path and will be ultimately packaged into your .apk.

    Such a package is called a "fat binary" in Android speak since it contains
    machine code for more than one CPU architecture. At installation time, the
    package manager will only unpack the most appropriate machine code for the
    target device. See below for details.

    III. ABI Management on the Android platform:
    ============================================

    This section provides specific details about how the Android platform manages
    native code in application packages.

      III.1. Native code in Application Packages:
      -------------------------------------------

        It is expected that shared libraries generated with the NDK are stored in
        the final application package (.apk) at locations of the form:

           lib/<abi>/lib<name>.so

        Where <abi> is one of the ABI names listed in section II above, and <name>
        is a name that can be used when loading the shared library from the VM
        as in:

            System.loadLibrary("<name>");

        Since .apk files are just zip files, you can trivially list their content
        with a command like:

            unzip -l <apk>

        to verify that the native shared libraries you want are indeed at the
        proper location. You can also place native shared libraries at other
        locations within the .apk, but they will be ignored by the system, or more
        precisely by the steps described below; you will need to extract/install
        them manually in your application.

        In the case of a "fat" binary, two distinct libraries are thus placed in
        the  .apk, for example at:

            lib/armeabi/libfoo.so
            lib/armeabi-v7a/libfoo.so

      III.2. Android Platform ABI support:
      ------------------------------------

        The Android system knows at runtime which ABI(s) it supports. More
        precisely, up to two build-specific system properties are used to
        indicate:

        - the 'primary' ABI for the device, corresponding to the machine
          code used in the system image itself.

        - an optional 'secondary' ABI, corresponding to another ABI that
          is also supported by the system image.

        For example, a typical ARMv5TE-based device would only define
        the primary ABI as 'armeabi' and not define a secondary one.

        On the other hand, a typical ARMv7-based device would define the
        primary ABI to 'armeabi-v7a' and the secondary one to 'armeabi'
        since it can run application native binaries generated for both
        of them.

      III.3. Automatic extraction of native code at install time:
      -----------------------------------------------------------

        When installing an application, the package manager service will scan
        the .apk and look for any shared library of the form:

             lib/<primary-abi>/lib<name>.so

        If one is found, then it is copied under $APPDIR/lib/lib<name>.so,
        where $APPDIR corresponds to the application's specific data directory.

        If none is found, and a secondary ABI is defined, the service will
        then scan for shared libraries of the form:

            lib/<secondary-abi>/lib<name>.so

        If anything is found, then it is copied under $APPDIR/lib/lib<name>.so

        This mechanism ensures that the best machine code for the target
        device is automatically extracted from the package at installation
        time.