【翻譯】(8)CPU ARM Neon

----------------- 英文文檔見android-ndk-r5b的documentation.html 屬於Android Native Development Kit (NDK)的一部分

見http://developer.android.com/sdk/ndk/（需要代理） 翻譯僅個人見解

----------------- Android NDK & ARM NEON instruction set extension support

Android NDK 和 ARM NEON指令集擴充支援

-------------------------------------------------------- Introduction: 介紹：-------------

Android NDK r3 added support for the new 'armeabi-v7a' ARM-based ABI that allows native code to use two useful instruction set extensions:

Android NDK r3添加對新的基於ARM的armeabi-v7a ABI的支援，允許原生代碼使用兩個有用指令集擴充：

- Thumb-2, which provides performance comparable to 32-bit ARM instructions with similar compactness to Thumb-1 - Thumb-2，

它提供與32位ARM指令可比的效能，也提供與Thumb-1相似的緊湊性 - VFPv3,

which provides hardware FPU registers and computations, to boost floating point performance significantly. - VFPv3，

它提供硬體浮點處理單元寄存器和計算能力，顯著提升浮點效能。

More specifically, by default 'armeabi-v7a' only supports VFPv3-D16 which only uses/requires 16 hardware FPU 64-bit registers.

更顯著的是，預設armeabi-v7a只支援VFPv3-D16，VFPv3-D16隻使用/需要16個硬體浮點處理單元64位寄存器。

More information about this can be read in docs/CPU-ARCH-ABIS.html

更多相關資訊可以閱讀docs/CPU-ARCH-ABIS.html。

The ARMv7 Architecture Reference Manual also defines another optional instruction set extension known as "ARM Advanced SIMD", nick-named "NEON". It provides:

ARMv7架構參考手冊還定義另一個可選的指令集合，即ARM進階SIMD，暱稱為NEON。它提供：

- A set of interesting scalar/vector instructions and registers (the latter are mapped to the same chip area as the FPU ones), comparable to MMX/SSE/3DNow! in the x86 world.

- 一組有趣的標量/向量指令（註：標量指令是指處理器每次處理一條資料，而向量指令則相反，允許平行處理多條資料）和寄存器（後來被映射為相同的晶片領域如浮點運算單元寄存器），可以和x86世界的MMX/SSE/3DNow!相比。

- VFPv3-D32 as a requirement (i.e. 32 hardware FPU 64-bit registers, instead of the minimum of 16).

- VFPv3-D32作為一種最低需要（即32個硬體浮點單位64位寄存器，而非至少16個）。

Not all ARMv7-based Android devices will support NEON, but those that do may benefit in significant ways from the scalar/vector instructions.

不是所有基於ARMv7的Android裝置會支援NEON，而那些支援NEON的裝置可以從標向量指令中很有意義地獲得好處。

The NDK supports the compilation of modules or even specific source files with support for NEON. What this means is that a specific compiler flag will be used to enable the use of GCC ARM Neon intrinsics and VFPv3-D32 at the same time. The intrinsics are described here:

NDK支援模組的編譯或甚至是特定的原始碼，擁有對NEON的支援。這意味著將使用一個特定的編譯器開關同時開啟對GCC ARM Neon內建和VFPv3-D32的使用。內建功能在這裡描述：

http://gcc.gnu.org/onlinedocs/gcc/ARM-NEON-Intrinsics.html LOCAL_ARM_NEON: LOCAL_ARM_NEON

--------------- Define LOCAL_ARM_NEON to 'true' in your module definition, and the NDK will build all its source files with NEON support. This can be useful if you want to build a static or shared library that specifically contains NEON code paths.

在你的模組定義中把LOCAL_ARM_NEON定義為true，NDK則會用NEON支援構建所有源檔案。如果你想構建一個特定地包含NEON代碼路徑的靜態或動態庫，這可能有用。

Using the .neon suffix:

使用.neon尾碼：

----------------------- When listing sources files in your LOCAL_SRC_FILES variable, you now have the option of using the .neon suffix to indicate that you want to corresponding source(s) to be built with Neon support. For example:

當在你的LOCAL_SRC_FILES變數中列出源檔案時，你現在擁有使用.neon的選擇，以指出你想把原始碼相應地用Neon支援進行構建。例如：

LOCAL_SRC_FILES := foo.c.neon bar.c

Will only build 'foo.c' with NEON support. 將只對foo.c用NEON支援構建。

Note that the .neon suffix can be used with the .arm suffix too (used to specify the 32-bit ARM instruction set for non-NEON instructions), but must appear after it.

注意.neon尾碼可以同時使用.arm尾碼（用於指明對非NEON指令的32位ARM指令集），但必須出現在後面。

In other words, 'foo.c.arm.neon' works, but 'foo.c.neon.arm' does NOT.

換句話說，foo.c.arm.neon可以，但foo.c.neon.arm不可以。

Build Requirements:

構建需要：

------------------ Neon support only works when targetting the 'armeabi-v7a' ABI, otherwise the NDK build scripts will complain and abort. It is important to use checks like the following in your Android.mk:

Neon支援僅在目標是armeabi-v7a ABI時才可工作，否則NDK構建指令碼將解釋和中止。在你的Android.mk中使用類似如下方式的檢查是很重要的。

# define a static library containing our NEON code

# 定義一個靜態庫，包含我們的NEON代碼

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:

運行時檢測：

------------------ As said previously, NOT ALL ARMv7-BASED ANDROID DEVICES WILL SUPPORT NEON ! It is thus crucial to perform runtime detection to know if the NEON-capable machine code can be run on the target device.

正如前面所說的，不是所有基於ARMv7的Android裝置支援NEON！因此最重要的是執行運行時檢測以知道NEON能力的機器代碼是否能運行在目標裝置上。

To do that, use the 'cpufeatures' library that comes with this NDK. To lean more about it, see docs/CPU-FEATURES.html.

為了做到那一點，可以使用NDK提供的cpufeatures庫。想知道更多相關資訊，請參考docs/CPU-FEATURES.html。

You should explicitly check that android_getCpuFamily() returns ANDROID_CPU_FAMILY_ARM, and that android_getCpuFeatures() returns a value that has the ANDROID_CPU_ARM_FEATURE_NEON flag set, as in:

你應該顯式地檢查android_getCpuFamily()返回ANDROID_CPU_FAMILY_ARM，並且android_getCpuFeatures()返回一個擁有ANDROID_CPU_ARM_FEATURE_NEON標記位設定的值，正如這樣：

#include 

...

...

if (android_getCpuFamily() == ANDROID_CPU_FAMILY_ARM && (android_getCpuFeatures() & ANDROID_CPU_ARM_FEATURE_NEON) != 0)

{ // use NEON-optimized routines // 使用NEON最佳化常式 ...

} else { // use non-NEON fallback routines instead // 改為使用非NEON倒退常式

... } ...

Sample code:

範例程式碼：

------------ Look at the source code for the "hello-neon" sample in this NDK for an example on how to use the 'cpufeatures' library and Neon intrinsics at the same time.

查看這份NDK中hello-neon例子的原始碼以獲得關於如何同時使用cpufeatures庫和Neon內建的例子。

This implements a tiny benchmark for a FIR filter loop using a C version, and a NEON-optimized one for devices that support it.

它實現了一個使用C版本的FIR（註：有限脈衝響應）濾波器迴圈，以及針對支援硬體NEON的裝置的經過NEON最佳化的小型效能比較測試。