docs/html-ndk/ndk/guides/cpu-arm-neon.jd

page.title=NEON Support
@jd:body

<div id="qv-wrapper">
    <div id="qv">
      <h2>On this page</h2>

      <ol>
        <li><a href="#ul">Using {@code LOCAL_ARM_NEON}</a></li>
        <li><a href="#uns">Using the {@code .neon} Suffix</a></li>
        <li><a href="#build">Build Requirements</a></li>
        <li><a href="#rd">Runtime Detection</a></li>
        <li><a href="#sc">Sample Code</a></li>
      </ol>
    </div>
  </div>

<p>The NDK supports the ARM Advanced SIMD, an optional instruction-set extension of the ARMv7 spec.
NEON provides a set of scalar/vector instructions and registers (shared with the FPU) comparable to
MMX/SSE/3DNow! in the x86 world. To function, it requires VFPv3-D32 (32 hardware FPU 64-bit
registers, instead of the minimum of 16).</p>

<p>The NDK supports the compilation of modules or even specific source files with support for NEON.
As a result, a specific compiler flag enables the use of GCC ARM NEON intrinsics and VFPv3-D32
at the same time.</p>

<p>Not all ARMv7-based Android devices support NEON, but devices that do may benefit significantly
from its support for scalar/vector instructions. For x86 devices, the NDK can also translate NEON
instructions into SSE, although with several restrictions. For more information, see
<a href="{@docRoot}ndk/guides/x86.html#an">x86 Support for ARM NEON Intrinsics.</a></p>

<h2 id="ul">Using LOCAL_ARM_NEON</h2>
<p>To have the NDK build all its source files with NEON support, include the following line in
your module definition:</p>

<pre class="no-pretty-print">
LOCAL_ARM_NEON := true
</pre>

<p>It can be especially useful to build all source files with NEON support if you want to build a
static or shared library that specifically contains NEON code paths.</p>

<h2 id="uns">Using the .neon Suffix</h2>
<p>When listing source files for your {@code LOCAL_SRC_FILES} variable, you have the option of
using the {@code .neon} suffix to indicate that you want to build binaries with NEON support.
For example, the following example builds one file with {@code .neon} support, and another
without it:</p>

<pre class="no-pretty-print">
LOCAL_SRC_FILES := foo.c.neon bar.c
</pre>

<p>You can combine the {@code .neon} suffix with the {@code .arm} suffix, which specifies the 32-bit
ARM instruction set for non-NEON instructions. In such a definition, {@code arm} must come before
{@code neon}. For example: {@code foo.c.arm.neon} works, but {@code foo.c.neon.arm} does not.</p>

<h2 id="build">Build Requirements</h2>
<p>NEON support only works with the {@code armeabi-v7a} and {@code x86} ABIs. If the NDK build
scripts encounter other ABIs while attempting to build with NEON support, the NDK build scripts
exit. x86 provides <a href="x86.html">partial NEON support</a> via translation header. It is
important to use checks like the following in your <a href="{@docRoot}ndk/guides/android_mk.html">
{@code Android.mk}</a> file:</p>

<pre class="no-pretty-print">
# define a static library containing our NEON code
ifeq ($(TARGET_ARCH_ABI),$(filter $(TARGET_ARCH_ABI), armeabi-v7a x86))
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 || x86
</pre>

<h2 id="rd">Runtime Detection</h2>
<p>Your app must perform runtime detection to confirm that NEON-capable machine code can be run on
the target device. This is because not all ARMv7-based Android devices support NEON. The app can
perform this check using the
<a href="{@docRoot}ndk/guides/cpu-features.html">{@code cpufeatures}</a> library that comes with
this NDK.</p>

<p>You should explicitly check that {@code android_getCpuFamily()} returns {@code
ANDROID_CPU_FAMILY_ARM}, and that {@code android_getCpuFeatures()} returns a value including the
{@code ANDROID_CPU_ARM_FEATURE_NEON flag} set. For example: </p>

<pre class="no-pretty-print">
#include &lt;cpu-features.h&gt;
...
...
if (android_getCpuFamily() == ANDROID_CPU_FAMILY_ARM &amp;&amp;
    (android_getCpuFeatures() &amp; ANDROID_CPU_ARM_FEATURE_NEON) != 0)
{
    // use NEON-optimized routines
    ...
}
else
{
    // use non-NEON fallback routines instead
    ...
}

...
</pre>

<h2 id="sc">Sample Code</h2>
<p>The source code for the NDK's hello-neon sample provides an example of how to use the
{@code cpufeatures} library and NEON intrinsics at the same time. This sample implements a tiny
benchmark for a FIR filter loop using a C version, and a NEON-optimized one for devices that
support it.</p>