NDK Programmer's Guide
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Android NDK r6 added support for the 'x86
' ABI, that allows native code to run on Android-based devices running on CPUs supporting the IA-32 instruction set.
The Android x86 ABI itself is fully specified in docs/CPU-ARCH-ABIS.html.
Generating x86 machine code is simple: just add 'x86' to your APP_ABI definition in your Application.mk file, for example:
APP_ABI := armeabi armeabi-v7a x86
Alternatively, since NDK r7, you can use:
APP_ABI := all
will generate machine code for all supported ABIs with this NDK. Doing so will ensure that your application package contains libraries for all target ABIs. Note that this has an impact on package size, since each ABI will correspond to its own set of native libraries built from the same sources.
The default ABI is still 'armeabi
', if unspecified in your project.
As you would expect, generated libraries will go into $PROJECT/libs/x86/
, and will be embedded into your .apk under /lib/x86/
.
And just like other ABIs, the Android package manager will extract these libraries on a compatible x86-based device automatically at install time, to put them under <dataPath>/lib, where <dataPath> is the application's private data directory.
Similarly, the Google Play server is capable of filtering applications based on the native libraries they embed and your device's target CPU.
Debugging with ndk-gdb should work exactly as described under docs/NDK-GDB.html.
The solution is shaped as C/C++ language header with the same name as standard ARM NEON intrinsics header "arm_neon.h" and is available in all NDK x86 toolchains. It translates NEON intrinsics to native x86 SSE ones.
By default SSE up to SSSE3 is used for porting ARM NEON to Intel SSE.
Current solution covers by default ~93% NEON functions (1869 of total 2009).
The solution
There are few corner cases where x86 implementation produces different results comparing to native execution on ARM. It's been found on NEON tests with total passrate close to 100%. Though these cases are expected to be rare please see below for a complete list of such incompatibilities:
VRECPS/VRECPSQ
VRSQRTS/VRSQRTSQ
VMAX/VMAXQ
VMIN/VMINQ
VRECPE/VRECPEQ
VRSQRTE/VRSQRTEQ
For the major number of cases it is expected to obtain the similar to ARM NEON native perfomance gain for vectorized vs. serial code.
To learn more about it, see here.
In your project add 'x86' to APP_ABI definition and make sure "arm_neon.h" header is included. Your code will be ported to x86 without any other changes necessary.
Look at the "hello-neon" sample in NDK for an example on how ARM NEON porting to x86 SSE works.
It is possible to use the x86 toolchain with NDK r6 in stand-alone mode. See docs/STANDALONE-TOOLCHAIN.html for more details. Briefly speaking, it is now possible to run:
$NDK/build/tools/make-standalone-toolchain.sh --arch=x86 --install-dir=<path>
The toolchain binaries have the i686-linux-android- prefix
.
The minimal native API level provided by official Android x86 platform builds is 9, which corresponds to all the native APIs provided by Android 2.3, i.e. Gingerbread (note also that no new native APIs were introduced by Honeycomb).
You won't have to change anything to your project files if you target an older API level: the NDK build script will automatically select the right set of native platform headers/libraries for you.