Multi-Core Devices & Xamarin.Android

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Android can run on several different computer architectures. This document discusses the different CPU architectures that may be employed for a Xamarin.Android application. This document will also explain how Android applications are packaged to support different CPU architectures. The Application Binary Interface (ABI) will be introduced, and guidance will be provided regarding which ABIs to use in a Xamarin.Android application.

Overview

Android allows for the creation of "fat binaries," a single .apk file that contains machine code that will support multiple, different CPU architectures. This is accomplished by associating each piece of machine code with an Application Binary Interface. The ABI is used to control which machine code will run on a given hardware device. For example, for an Android application to run on an x86 device, it is necessary to include x86 ABI support when compiling the application.

Specifically, each Android application will support at least one embedded-application binary interface (EABI). EABI are conventions specific to embedded software programs. A typical EABI will describe things such as:

  • The CPU instruction set.

  • The endianness of memory stores and loads at run time.

  • The binary format of object files and program libraries, as well as which type of content is allowed or supported in these files and libraries.

  • The various conventions used to pass data between application code and the system (for example: how registers and/or the stack are used when functions are called, alignment constraints, etc.)

  • Alignment and size constraints for enum types, structures, fields, and arrays.

  • The list of function symbols available to your machine code at run time, generally from a very specific selected set of libraries.

armeabi and Thread Safety

The Application Binary Interface will be discussed in detail below, but it is important to remember that the armeabi runtime used by Xamarin.Android is not thread safe. If an application that has armeabi support is deployed to an armeabi-v7a device, many strange and unexplainable exceptions will occur.

Due to a bug in Android 4.0.0, 4.0.1, 4.0.2, and 4.0.3, the native libraries will be picked up from the armeabi directory even though there is an armeabi-v7a directory present and the device is an armeabi-v7a device.

Note: Xamarin.Android will ensure that .so are added to the APK in the correct order. This bug should not be an issue for users of Xamarin.Android.

ABI Descriptions

Each ABI supported by Android is identified by a unique name.

armeabi

This is the name of an EABI for ARM-based CPUs that support at least the ARMv5TE instruction set. Android follows the little-endian ARM GNU/Linux ABI. This ABI does not support hardware-assisted floating-point computations. All FP operations are performed by software helper functions that come from the compiler's libgcc.a static library. SMP devices are not supported by armeabi.

Note: Xamarin.Android's armeabi code is not thread safe and should not be used on multi-CPU armeabi-v7adevices (described below). Using aremabi code on a single-core armeabi-v7a device is safe.

armeabi-v7a

This is another ARM-based CPU instruction set that extends the armeabi EABI described above. The armeabi-v7a EABI has support for hardware floating-point operations and multiple CPU (SMP) devices. An application that uses the armeabi-v7a EABI can expect substantial performance improvements over an application that uses armeabi.

Note: armeabi-v7a machine code will not run on ARMv5 devices.

arm64-v8a

This is a 64-bit instruction set that is based on the ARMv8 CPU architecture. This architecture is used in the Nexus 9. Xamarin.Android 5.1 provides experimental support for this architecture (for more information, see Experimental Features).

x86

This is the name of an ABI for CPUs that support the instruction set commonly named x86 or IA-32. This ABI corresponds to instructions for the Pentium Pro instruction set, including the MMX, SSE, SSE2, and SSE3 instruction sets. It does not include any other optional IA-32 instruction set extensions such as:

  • the MOVBE instruction.
  • Supplemental SSE3 extension (SSSE3).
  • any variant of SSE4.

Note: Google TV, although it runs on x86, is not supported by Android's NDK or by Xamarin.Android.

x86_64

This is the name of an ABI for CPUs that support the 64-bit x86 instruction set (also referred to as x64 or AMD64). Xamarin.Android 5.1 provides experimental support for this architecture (for more information, see Experimental Features).

mips

This is the name of an ABI for MIPS-based CPUs that support at least the MIPS32r1 instruction set. Neither MIPS 16 nor micromips are supported by Android.

Note: MIPS devices are not currently supported by Xamarin.Android, but will be in a future release.

APK File Format

The Android Application Package is the file format that holds all of the code, assets, resources, and certificates necessary for an Android application. It is a .zip file, but uses the .apk file name extension. When expanded, the contents of an .apk created by Xamarin.Android can be seen in the screenshot below:

A quick description of the contents of the .apk file:

  • AndroidManifest.xml – This is the AndroidManifest.xml file, in binary XML format.

  • classes.dex – This contains the application code, compiled into the dex file format that is used by the Android runtime VM.

  • resources.arsc – This file contains all of the precompiled resources for the application.

  • lib – This directory holds the compiled code for each ABI. It will contain one subfolder for each ABI that was described in the previous section. In the screenshot above, the .apk in question has native libraries for both armeabi-v7a and for x86 .

  • META-INF – This directory (if present) is used to store signing information, package, and extension configuration data.

  • res – This directory holds the resources that were not compiled into resources.arsc .

Note: The file libmonodroid.so is the native library required by all Xamarin.Android applications.

Android Device ABI Support

Each Android device supports executing native code in up to two ABIs:

  • The "primary" ABI – This corresponds to the machine code used in the system image.

  • A "secondary" ABI – This is an optional ABI that is also supported by the system image.

For example, a typical ARMv5TE device will only have a primary ABI of armeabi, while an ARMv7 device would specify a primary ABI of armeabi-v7a and a secondary ABI of armeabi. A typical x86 device would only specify a primary ABI of x86.

Android Native Library Installation

At package installation time, native libraries within the .apk are extracted into the app's native library directory, typically /data/data/<package-name>/lib, and are thereafter referred to as $APP/lib.

Android's native library installation behavior varies dramatically between Android versions.

Installing Native Libraries: Pre-Android 4.0

Android prior to 4.0 Ice Cream Sandwich will only extract native libraries from a single ABI within the .apk. Android apps of this vintage will first try to extract all native libraries for the primary ABI, and if no such libraries exist, Android will then extract all native libraries for the secondary ABI. No "merging" is done.

For example, consider a situation where an application is installed on an armeabi-v7a device. The .apk, which supports both armeabi and armeabi-v7a, has the following ABI lib directories and files in it:

lib/armeabi/libone.so
lib/armeabi/libtwo.so
lib/armeabi-v7a/libtwo.so

After installation, the native library directory will contain:

$APP/lib/libtwo.so # from the armeabi-v7a directory in the apk

In other words, no libone.so is installed. This will cause problems, as libone.so is not present for the application to load at run time. This behavior, while unexpected, has been logged as a bug and reclassified as "working as intended."

Consequently, when targeting Android versions prior to 4.0, it is necessary to provide all native libraries for each ABI that the application will support, that is, the .apk should contain:

lib/armeabi/libone.so
lib/armeabi/libtwo.so
lib/armeabi-v7a/libone.so
lib/armeabi-v7a/libtwo.so

Installing Native Libraries: Android 4.0 – Android 4.0.3

Android 4.0 Ice Cream Sandwich changes the extraction logic. It will enumerate all native libraries, see if the file's basename has already been extracted, and if both of the following conditions are met, then the library will be extracted:

  • It hasn't already been extracted.

  • The native library's ABI matches the target's primary or secondary ABI.

Meeting these conditions allows "merging" behavior; that is, if we have an .apk with the following contents:

lib/armeabi/libone.so
lib/armeabi/libtwo.so
lib/armeabi-v7a/libtwo.so

Then after installation, the native library directory will contain:

$APP/lib/libone.so
$APP/lib/libtwo.so

Unfortunately, this behavior is order dependent, as described in the following document - Issue 24321: Galaxy Nexus 4.0.2 uses armeabi native code when both armeabi and armeabi-v7a is included in apk.

The native libraries are processed "in order" (as listed by, for example, unzip), and the first match is extracted. Since the .apk contains armeabi and armeabi-v7a versions of libtwo.so, and the armeabi is listed first, it's the armeabi version that is extracted, not the armeabi-v7a version:

$APP/lib/libone.so # armeabi
$APP/lib/libtwo.so # armeabi, NOT armeabi-v7a!

Furthermore, even if both armeabi and armeabi-v7a ABIs are specified (as described below in the section Declaring Supported ABIs), Xamarin.Android will create the following element in the . csproj:

<AndroidSupportedAbis>armeabi,armeabi-v7a</AndroidSupportedAbis>

Consequently, the armeabi libmonodroid.so will be found first within the .apk, and the armeabi libmonodroid.so will be the one that is extracted, even though the armeabi-v7a libmonodroid.so is present and optimized for the target. This can also result in obscure run-time errors, as armeabi is not SMP safe.

Installing Native Libraries: Android 4.0.4 and later

Android 4.0.4 changes the extraction logic: it will enumerate all native libraries, read the file's basename, then extract the primary ABI version (if present), or the secondary ABI (if present). This allows "merging" behavior; that is, if we have an .apk with the following contents:

lib/armeabi/libone.so
lib/armeabi/libtwo.so
lib/armeabi-v7a/libtwo.so

Then after installation, the native library directory will contain:

$APP/lib/libone.so # from armeabi
$APP/lib/libtwo.so # from armeabi-v7a

Xamarin.Android and ABIs

Xamarin.Android supports the following architectures:

  • armeabi
  • armeabi-v7a
  • x86

Xamarin.Android provides experimental support for the following architectures:

  • arm64-v8a
  • x86_64

Note that 64-bit runtimes are not required in order to run your app on 64-bit devices. For more information about experimental features in Xamarin.Android 5.1, see Experimental Features.

Xamarin.Android does not currently provide support for mips.

Declaring Supported ABI's

By default, Xamarin.Android will default to armeabi-v7a for Release builds, and to armeabi-v7a and x86 for Debug builds.

Support for different ABIs can be set through the Project Options for a Xamarin.Android project. In Visual Studio, this can be set in the Android Options page of project Properties, under the Advanced tab, as shown in the following screenshot:

In Xamarin Studio, the supported architectures may be selected on the Android Build page of Project Options, under the Advanced tab, as shown in the following screenshot:

There are some situations when it may be necessary to declare additional ABI support such as when:

  • Deploying the application to an x86 device.

  • Deploying the application to an armeabi-v7a device to ensure thread safety.

Summary

This document discussed the different CPU architectures that an Android application may run on. It introduced the Application Binary Interface and how it is used by Android to support disparate CPU architectures. It then went on to discuss how to specify ABI support in a Xamarin.Android application and highlighted the issues that arise when using Xamarin.Android applications on an armeabi-v7a device that are intended only for armeabi.

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