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 endianess 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.TXT 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.TXT to see how to perform these runtime
  checks, and docs/CPU-ARM-NEON.TXT 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.


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.
