NEON Support

The NDK supports ARM Advanced SIMD, commonly known as NEON, an optional instruction set extension for ARMv7 and ARMv8. NEON provides scalar/vector instructions and registers (shared with the FPU) comparable to MMX/SSE/3DNow! in the x86 world. To function, it requires VFPv3-D32 (32 hardware FPU 64-bit registers, instead of the minimum of 16).

Not all ARMv7-based Android devices support NEON, but devices that do may benefit significantly from its support for scalar/vector instructions.

The NDK supports the compilation of modules or even specific source files with support for NEON.

Using LOCAL_ARM_NEON

To have the NDK build all source files assuming NEON support, include the following line in your module definition: makefile LOCAL_ARM_NEON := true

It can be especially useful to build all source files with NEON support if you want to build a static or shared library that specifically contains NEON-only code.

Using the .neon Suffix

When listing source files for your LOCAL_SRC_FILES variable, you have the option of using the .neon suffix to indicate that you want to build individual files with NEON support. For example, the following builds one file (foo.c) with NEON support and another (bar.c) without: makefile LOCAL_SRC_FILES := foo.c.neon bar.c

You can combine the .neon suffix with the .arm suffix, which specifies the 32-bit ARM instruction set (instead of Thumb2) for non-NEON instructions. In such a case, .arm must come before .neon. For example: foo.c.arm.neon works, but foo.c.neon.arm does not.

Build Requirements

NEON support works with the armeabi-v7a and arm64-v8a ABIs. If the NDK build scripts encounter other ABIs while attempting to build with NEON support, the NDK build scripts exit. It is important to use checks like the following in your Android.mk file:

# define a static library containing our NEON code
ifeq ($(TARGET_ARCH_ABI),armeabi-v7a)
include $(CLEAR_VARS)
LOCAL_MODULE    := mylib-neon
LOCAL_SRC_FILES := mylib-neon.c
LOCAL_ARM_NEON  := true
include $(BUILD_STATIC_LIBRARY)
endif # TARGET_ARCH_ABI == armeabi-v7a

Runtime Detection

Your app must perform runtime detection to confirm that NEON-capable machine code can be run on the target device. This is because not all ARMv7-based Android devices support NEON. The app can perform this check using the cpufeatures library that comes with the NDK.

You should check that android_getCpuFamily() returns ANDROID_CPU_FAMILY_ARM and that android_getCpuFeatures() returns a value with the ANDROID_CPU_ARM_FEATURE_NEON flag set. For example:

#include <cpu-features.h>
...
...
if (android_getCpuFamily() == ANDROID_CPU_FAMILY_ARM &&
    (android_getCpuFeatures() & ANDROID_CPU_ARM_FEATURE_NEON) != 0)
{
    // use NEON-optimized routines
    ...
}
else
{
    // use non-NEON fallback routines instead
    ...
}

...

Cross-Platform Support for x86

NDK supports cross-platform compilation of your existing ARM SIMD (NEON) instrinsic functions into x86 SSE code, through the use of the third-party NEON_2_SSE.h. For more information on this topic, see From ARM NEON to Intel SSE-the automatic porting solution, tips and tricks.

Sample Code

The hello-neon sample provides an example of how to use the cpufeatures library and NEON intrinsics at the same time. This sample implements a tiny benchmark for a FIR filter loop with a C version and a NEON-optimized version for devices that support NEON.