【翻译】(8)CPU ARM Neon

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英文文档见android-ndk-r5b的documentation.html

属于Android Native Development Kit (NDK)的一部分

见http://developer.android.com/sdk/ndk/（需要代理）

翻译仅个人见解

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Android NDK & ARM NEON instruction set extension support

 

Android NDK 和 ARM NEON指令集扩展支持

--------------------------------------------------------

 

Introduction:

 

介绍：

-------------

 

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添加对新的基于ARM的armeabi-v7a ABI的支持，允许原生代码使用两个有用指令集扩展：

 

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

 

- Thumb-2，它提供与32位ARM指令可比的性能，也提供与Thumb-1相似的紧凑性

 

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

 

- VFPv3，它提供硬件浮点处理单元寄存器和计算能力，显著提升浮点性能。

 

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

 

  更显著的是，默认armeabi-v7a只支持VFPv3-D16，VFPv3-D16只使用/需要16个硬件浮点处理单元64位寄存器。

 

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

 

更多相关信息可以阅读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架构参考手册还定义另一个可选的指令集合，即ARM高级SIMD，昵称为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.

 

- 一组有趣的标量/矢量指令（注：标量指令是指处理器每次处理一条数据，而矢量指令则相反，允许并行处理多条数据）和寄存器（后来被映射为相同的芯片领域如浮点运算单元寄存器），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作为一种最低需要（即32个硬件浮点单元64位寄存器，而非至少16个）。

 

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

 

不是所有基于ARMv7的Android设备会支持NEON，而那些支持NEON的设备可以从标矢量指令中很有意义地获得好处。

 

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支持模块的编译或甚至是特定的源代码，拥有对NEON的支持。这意味着将使用一个特定的编译器开关同时打开对GCC ARM Neon内建和VFPv3-D32的使用。内建功能在这里描述：

 

    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.

 

在你的模块定义中把LOCAL_ARM_NEON定义为true，NDK则会用NEON支持构建所有源文件。如果你想构建一个特定地包含NEON代码路径的静态或动态库，这可能有用。

 

Using the .neon suffix:

 

使用.neon后缀：

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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:

 

当在你的LOCAL_SRC_FILES变量中列出源文件时，你现在拥有使用.neon的选择，以指出你想把源代码相应地用Neon支持进行构建。例如：

 

  LOCAL_SRC_FILES := foo.c.neon bar.c

 

Will only build 'foo.c' with NEON support.

 

将只对foo.c用NEON支持构建。

 

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后缀可以同时使用.arm后缀（用于指明对非NEON指令的32位ARM指令集），但必须出现在后面。

 

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

 

换句话说，foo.c.arm.neon可以，但foo.c.neon.arm不可以。

 

Build Requirements:

 

构建需要：

------------------

 

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支持仅在目标是armeabi-v7a ABI时才可工作，否则NDK构建脚本将解释和中止。在你的Android.mk中使用类似如下方式的检查是很重要的。

 

   # define a static library containing our NEON code

   # 定义一个静态库，包含我们的NEON代码

   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:

 

运行时检测：

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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.

 

正如前面所说的，不是所有基于ARMv7的Android设备支持NEON！因此最重要的是执行运行时检测以知道NEON能力的机器代码是否能运行在目标设备上。

 

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

 

为了做到那一点，可以使用NDK提供的cpufeatures库。想知道更多相关信息，请参考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:

 

你应该显式地检查android_getCpuFamily()返回ANDROID_CPU_FAMILY_ARM，并且android_getCpuFeatures()返回一个拥有ANDROID_CPU_ARM_FEATURE_NEON标记位设置的值，正如这样：

 

    #include <cpu-features.h>

 

    ...

    ...

 

    if (android_getCpuFamily() == ANDROID_CPU_FAMILY_ARM &&

        (android_getCpuFeatures() & ANDROID_CPU_ARM_FEATURE_NEON) != 0)

    {

        // use NEON-optimized routines

        // 使用NEON优化例程

        ...

    }

    else

    {

        // use non-NEON fallback routines instead

        // 改为使用非NEON倒退例程

        ...

    }

 

    ...

 

Sample code:

 

示例代码：

------------

 

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.

 

查看这份NDK中hello-neon例子的源代码以获得关于如何同时使用cpufeatures库和Neon内建的例子。

 

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

 

它实现了一个使用C版本的FIR（注：有限脉冲响应）滤波器循环，以及针对支持硬件NEON的设备的经过NEON优化的小型性能比较测试。