93369c2470
+ Add a small cleanup script (build/tools/cleanup-apps.sh) + Minor formatting of hello-gl2 sample.
128 lines
4.1 KiB
Plaintext
128 lines
4.1 KiB
Plaintext
Android NDK & ARM NEON instruction set extension support
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--------------------------------------------------------
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Introduction:
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-------------
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Android NDK r3 added support for the new 'armeabi-v7a' ARM-based ABI
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that allows native code to use two useful instruction set extenstions:
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- Thumb-2, which provides performance comparable to 32-bit ARM
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instructions with similar compactness to Thumb-1
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- VFPv3, which provides hardware FPU registers and computations,
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to boost floating point performance significantly.
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More specifically, by default 'armeabi-v7a' only supports
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VFPv3-D16 which only uses/requires 16 hardware FPU 64-bit registers.
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More information about this can be read in docs/CPU-ARCH-ABIS.TXT
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The ARMv7 Architecture Reference Manual also defines another optional
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instruction set extension known as "ARM Advanced SIMD", nick-named
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"NEON". It provides:
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- A set of interesting scalar/vector instructions and registers
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(the latter are mapped to the same chip area than the FPU ones),
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comparable to MMX/SSE/3DNow! in the x86 world.
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- VFPv3-D32 as a requirement (i.e. 32 hardware FPU 64-bit registers,
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instead of the minimum of 16).
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Not all ARMv7-based Android devices will support NEON, but those that
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do may benefit in significant ways from the scalar/vector instructions.
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The NDK supports the compilation of modules or even specific source
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files with support for NEON. What this means is that a specific compiler
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flag will be used to enable the use of GCC ARM Neon intrinsics and
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VFPv3-D32 at the same time. The intrinsics are described here:
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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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---------------
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Define LOCAL_ARM_NEON to 'true' in your module definition, and the NDK
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will build all its source files with NEON support. This can be useful if
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you want to build a static or shared library that specifically contains
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NEON code paths.
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Using the .neon suffix:
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-----------------------
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When listing sources files in your LOCAL_SRC_FILES variable, you now have
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the option of using the .neon suffix to indicate that you want to
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corresponding source(s) to be built with Neon support. For example:
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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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Note that the .neon suffix can be used with the .arm suffix too (used to
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specify the 32-bit ARM instruction set for non-NEON instructions), but must
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appear after it.
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In other words, 'foo.c.arm.neon' works, but 'foo.c.neon.arm' does NOT.
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Build Requirements:
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------------------
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Neon support only works when targetting the 'armeabi-v7a' ABI, otherwise the
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NDK build scripts will complain and abort. It is important to use checks like
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the following in your Android.mk:
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# define a static library containing our NEON code
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ifeq ($(TARGET_ARCH_ABI),armeabi-v7a)
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include $(CLEAR_VARS)
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LOCAL_MODULE := mylib-neon
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LOCAL_SRC_FILES := mylib-neon.c
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LOCAL_ARM_NEON := true
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include $(BUILD_STATIC_LIBRARY)
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endif # TARGET_ARCH_ABI == armeabi-v7a
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Runtime Detection:
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------------------
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As said previously, NOT ALL ARMv7-BASED ANDROID DEVICES WILL SUPPORT NEON !
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It is thus crucial to perform runtime detection to know if the NEON-capable
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machine code can be run on the target device.
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To do that, use the 'cpufeatures' library that comes with this NDK. To lean
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more about it, see docs/CPU-FEATURES.TXT.
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You should explicitely check that android_getCpuFamily() returns
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ANDROID_CPU_FAMILY_ARM, and that android_getCpuFeatures() returns a value
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that has the ANDROID_CPU_ARM_FEATURE_NEON flag set, as in:
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#include <cpu-features.h>
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...
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...
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if (android_getCpuFamily() == ANDROID_CPU_FAMILY_ARM &&
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(android_getCpuFeatures() & ANDROID_CPU_ARM_FEATURE_NEON) != 0)
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{
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// use NEON-optimized routines
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...
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}
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else
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{
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// use non-NEON fallback routines instead
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...
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}
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...
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Sample code:
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------------
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Look at the source code for the "hello-neon" sample in this NDK for an example
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on how to use the 'cpufeatures' library and Neon intrinsics at the same time.
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This implements a tiny benchmark for a FIR filter loop using a C version, and
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a NEON-optimized one for devices that support it.
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