【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
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Android NDK 和 ARM NEON指令集扩展支持
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Introduction:
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介绍：
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Android NDK r3 added support for the new 'armeabi-v7a' ARM-based ABI that allows native code to use two useful instruction set extensions:
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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：
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- Thumb-2, which provides performance comparable to 32-bit ARM instructions with similar compactness to Thumb-1
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- 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
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- VFPv3, which provides hardware FPU registers and computations, to boost floating point performance significantly.
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- 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。
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??More specifically, by default 'armeabi-v7a' only supports VFPv3-D16 which only uses/requires 16 hardware FPU 64-bit registers.
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??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。
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More information about this can be read in docs/CPU-ARCH-ABIS.html
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更多相关信息可以阅读docs/CPU-ARCH-ABIS.html。
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The ARMv7 Architecture Reference Manual also defines another optional instruction set extension known as "ARM Advanced SIMD", nick-named "NEON". It provides:
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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：
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- 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.
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- 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。
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- VFPv3-D32 as a requirement (i.e. 32 hardware FPU 64-bit registers, instead of the minimum of 16).
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- 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）。
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Not all ARMv7-based Android devices will support NEON, but those that do may benefit in significant ways from the scalar/vector instructions.
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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。
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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:
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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：
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?? ?http://gcc.gnu.org/onlinedocs/gcc/ARM-NEON-Intrinsics.html
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LOCAL_ARM_NEON:
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LOCAL_ARM_NEON
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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.
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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。
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Using the .neon suffix:
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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:
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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：
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??LOCAL_SRC_FILES := foo.c.neon bar.c
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Will only build 'foo.c' with NEON support.
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将只对foo.c用NEON支持构建。
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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.
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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。
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In other words, 'foo.c.arm.neon' works, but 'foo.c.neon.arm' does NOT.
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换句话说，foo.c.arm.neon可以，但foo.c.neon.arm不可以。
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Build Requirements:
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构建需要：
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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:
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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。
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?? # 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
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Runtime Detection:
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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.
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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。
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To do that, use the 'cpufeatures' library that comes with this NDK. To lean more about it, see docs/CPU-FEATURES.html.
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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。
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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:
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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：
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?? ?#include <cpu-features.h>
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?? ?...
?? ?...
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?? ?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
?? ? ? ?...
?? ?}
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?? ?...
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Sample code:
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示例代码：
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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.
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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。
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This implements a tiny benchmark for a FIR filter loop using a C version, and a NEON-optimized one for devices that support it.
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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。
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