1<html><body><pre>Android NDK & ARM NEON instruction set extension support 2-------------------------------------------------------- 3 4Introduction: 5------------- 6 7Android NDK r3 added support for the new 'armeabi-v7a' ARM-based ABI 8that allows native code to use two useful instruction set extensions: 9 10- Thumb-2, which provides performance comparable to 32-bit ARM 11 instructions with similar compactness to Thumb-1 12 13- VFPv3, which provides hardware FPU registers and computations, 14 to boost floating point performance significantly. 15 16 More specifically, by default 'armeabi-v7a' only supports 17 VFPv3-D16 which only uses/requires 16 hardware FPU 64-bit registers. 18 19More information about this can be read in docs/CPU-ARCH-ABIS.html 20 21The ARMv7 Architecture Reference Manual also defines another optional 22instruction set extension known as "ARM Advanced SIMD", nick-named 23"NEON". It provides: 24 25- A set of interesting scalar/vector instructions and registers 26 (the latter are mapped to the same chip area as the FPU ones), 27 comparable to MMX/SSE/3DNow! in the x86 world. 28 29- VFPv3-D32 as a requirement (i.e. 32 hardware FPU 64-bit registers, 30 instead of the minimum of 16). 31 32Not all ARMv7-based Android devices will support NEON, but those that 33do may benefit in significant ways from the scalar/vector instructions. 34 35The NDK supports the compilation of modules or even specific source 36files with support for NEON. What this means is that a specific compiler 37flag will be used to enable the use of GCC ARM Neon intrinsics and 38VFPv3-D32 at the same time. The intrinsics are described here: 39 40 http://gcc.gnu.org/onlinedocs/gcc/ARM-NEON-Intrinsics.html 41 42 43LOCAL_ARM_NEON: 44--------------- 45 46Define LOCAL_ARM_NEON to 'true' in your module definition, and the NDK 47will build all its source files with NEON support. This can be useful if 48you want to build a static or shared library that specifically contains 49NEON code paths. 50 51 52Using the .neon suffix: 53----------------------- 54 55When listing sources files in your LOCAL_SRC_FILES variable, you now have 56the option of using the .neon suffix to indicate that you want to 57corresponding source(s) to be built with Neon support. For example: 58 59 LOCAL_SRC_FILES := foo.c.neon bar.c 60 61Will only build 'foo.c' with NEON support. 62 63Note that the .neon suffix can be used with the .arm suffix too (used to 64specify the 32-bit ARM instruction set for non-NEON instructions), but must 65appear after it. 66 67In other words, 'foo.c.arm.neon' works, but 'foo.c.neon.arm' does NOT. 68 69 70Build Requirements: 71------------------ 72 73Neon support only works when targeting the 'armeabi-v7a' ABI, otherwise the 74NDK build scripts will complain and abort. It is important to use checks like 75the following in your Android.mk: 76 77 # define a static library containing our NEON code 78 ifeq ($(TARGET_ARCH_ABI),armeabi-v7a) 79 include $(CLEAR_VARS) 80 LOCAL_MODULE := mylib-neon 81 LOCAL_SRC_FILES := mylib-neon.c 82 LOCAL_ARM_NEON := true 83 include $(BUILD_STATIC_LIBRARY) 84 endif # TARGET_ARCH_ABI == armeabi-v7a 85 86 87Runtime Detection: 88------------------ 89 90As said previously, NOT ALL ARMv7-BASED ANDROID DEVICES WILL SUPPORT NEON ! 91It is thus crucial to perform runtime detection to know if the NEON-capable 92machine code can be run on the target device. 93 94To do that, use the 'cpufeatures' library that comes with this NDK. To learn 95more about it, see docs/CPU-FEATURES.html. 96 97You should explicitly check that android_getCpuFamily() returns 98ANDROID_CPU_FAMILY_ARM, and that android_getCpuFeatures() returns a value 99that has the ANDROID_CPU_ARM_FEATURE_NEON flag set, as in: 100 101 #include <cpu-features.h> 102 103 ... 104 ... 105 106 if (android_getCpuFamily() == ANDROID_CPU_FAMILY_ARM && 107 (android_getCpuFeatures() & ANDROID_CPU_ARM_FEATURE_NEON) != 0) 108 { 109 // use NEON-optimized routines 110 ... 111 } 112 else 113 { 114 // use non-NEON fallback routines instead 115 ... 116 } 117 118 ... 119 120Sample code: 121------------ 122 123Look at the source code for the "hello-neon" sample in this NDK for an example 124on how to use the 'cpufeatures' library and Neon intrinsics at the same time. 125 126This implements a tiny benchmark for a FIR filter loop using a C version, and 127a NEON-optimized one for devices that support it. 128</pre></body></html> 129