page.title=Understanding 64-bit Builds
@jd:body

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    <h2>In this document</h2>
    <ol id="auto-toc">
    </ol>
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<h2 id=overview>Overview</h2>

<p>From the build system’s perspective, the most prominent change is that now it
supports building binaries for two target CPU architectures (64-bit and 32-bit)
in the same build. That’s also known as <em>Multilib build</em>.</p>

<p>For native static libraries and shared libraries, the build system sets up
rules to build binaries for both architectures. The product configuration
(PRODUCT_PACKAGES), together with the dependency graph, determines which
binaries are built and installed to the system image.</p>

<p>For executables and apps, the build system builds only the 64-bit version by
default, but you can override this setting by using a global
<code>BoardConfig.mk</code> variable or a module-scoped variable.</p>

<p class="caution"><strong>Caution:</strong> If an app exposes an API to other
apps that can be either 32- or 64-bit, the app must have the
<code>android:multiarch</code> property set to a value of <code>true</code>
within its manifest to avoid potential errors.</p>

<h2 id=product_configuration>Product Configuration</h2>


<p>In <code>BoardConfig.mk</code>, we added the following variables to
configure the second CPU architecture and ABI:</p>

<pre class=prettyprint>
TARGET_2ND_ARCH
TARGET_2ND_ARCH_VARIANT
TARGET_2ND_CPU_VARIANT
TARGET_2ND_CPU_ABI
TARGET_2ND_CPU_ABI2
</pre>


<p>You can see an example in <code>build/target/board/generic_arm64/BoardConfig.mk</code>.</p>

<p>If you want the build system to build 32-bit executables and apps by default,
set the following variable:</p>

<pre class=prettyprint>
TARGET_PREFER_32_BIT := true
</pre>

<p>However, you can override this setting  by using module-specific variables in
<code>Android.mk</code>.</p>

<p>In a Multilib build, module names in <code>PRODUCT_PACKAGES</code> cover
both the 32-bit and 64-bit binaries, as long as they are defined by the build
system. For libraries pulled in by dependency, a 32-bit library is installed
only if it’s required by another 32-bit library or executable. The same is true
for 64-bit libraries.</p>

<p>However, module names on the <code>make</code> command line cover only the
64-bit version. For example, after running <code>lunch
aosp_arm64-eng</code>,<code>make libc</code> builds only the 64-bit libc. To
build the 32-bit libc, you need to run <code>make libc_32</code>.</p>

<h2 id=module_definition_in_android_mk>Module Definition in Android.mk</h2>

<p>You can use the <code>LOCAL_MULTILIB</code> variable to configure your build
for 32-bit and/or 64-bit and override the global
<code>TARGET_PREFER_32_BIT</code>.</p>

<p>Set <code>LOCAL_MULTILIB</code> to one of the following:</p>

<ul>
  <li>"both”: build both 32-bit and 64-bit.</li>
  <li>“32”: build only 32-bit.</li>
  <li>“64”: build only 64-bit.</li>
  <li>“first”: build for only the first arch (32-bit in 32-bit devices and 64-bit
in 64-bit devices).</li>
  <li>“”: the default; the build system decides what arch to build based on the
module class and other LOCAL_ variables, such as LOCAL_MODULE_TARGET_ARCH,
LOCAL_32_BIT_ONLY, etc.</li>
</ul>

<p>In a Multilib build, conditionals like <code>ifeq $(TARGET_ARCH)</code> don’t work any
more. </p>

<p>If you want to build your module for some specific arch(s), the following
variables can help you:</p>

<ul>
  <li>LOCAL_MODULE_TARGET_ARCH<br>It can be set to a list of archs, something
like “arm x86 arm64”. Only if the arch being built is among that list will the
current module be included by the build system.</li>

  <li>LOCAL_MODULE_UNSUPPORTED_TARGET_ARCH<br>The opposite of
LOCAL_MODULE_TARGET_ARCH. Only if the arch being built is not among the list,
the current module will be included.</li>
</ul>

<p>There are minor variants of the above two variables:</p>

<ul>
  <li>LOCAL_MODULE_TARGET_ARCH_WARN</li>
  <li>LOCAL_MODULE_UNSUPPORTED_TARGET_ARCH_WARN</li>
</ul>

<p>The build system will give warning if the current module is skipped due to
archs limited by them.</p>

<p>To set up arch-specific build flags, use the arch-specific LOCAL_ variables. An
arch-specific LOCAL_ variable is a normal LOCAL_ variable with an arch suffix,
for example:</p>

<ul>
  <li> <code>LOCAL_SRC_FILES_arm, LOCAL_SRC_FILES_x86,</code>
  <li> <code>LOCAL_CFLAGS_arm, LOCAL_CFLAGS_arm64,</code>
  <li> <code>LOCAL_LDFLAGS_arm, LOCAL_LDFLAGS_arm64,</code>
</ul>

<p>Those variables will be applied only if a binary is  currently being built for
that arch.</p>

<p>Sometimes it’s more convenient to set up flags based on whether the binary is
currently being built for 32-bit or 64-bit. In that case you can use the LOCAL_
variable with a _32 or _64 suffix, for example:</p>

<ul>
  <li> <code>LOCAL_SRC_FILES_32, LOCAL_SRC_FILES_64,</code>
  <li> <code>LOCAL_CFLAGS_32, LOCAL_CFLAGS_64,</code>
  <li> <code>LOCAL_LDFLAGS_32, LOCAL_LDFLAGS_64,</code>
</ul>

<p>Note that not all of the LOCAL_ variables support the arch-specific variants.
For an up-to-date list of such variables, refer to <code>build/core/clear_vars.mk</code>.</p>

<h2 id=install_path>Install path</h2>


<p>In the past, you could use LOCAL_MODULE_PATH to install a library to a
location other than the default one. For example, <code>LOCAL_MODULE_PATH :=
$(TARGET_OUT_SHARED_LIBRARIES)/hw</code>.</p>

<p>In Multilib build, use LOCAL_MODULE_RELATIVE_PATH instead:</p>

<pre class=prettyprint>
LOCAL_MODULE_RELATIVE_PATH := hw
</pre>


<p>so that both the 64-bit and 32-bit libraries can be installed to the right
place.</p>

<p>If you build an executable as both 32-bit and 64-bit, you’ll need to use one of
the following variables to distinguish the install path:</p>

<ul>
  <li><code>LOCAL_MODULE_STEM_32, LOCAL_MODULE_STEM_64</code><br>Specifies the installed file name.
  <li><code>LOCAL_MODULE_PATH_32, LOCAL_MODULE_PATH_64</code><br>Specifies the install path.
</ul>

<h2 id=generated_sources>Generated sources </h2>

<p>In a Multilib build,  if you generate source files to
<code>$(local-intermediates-dir)</code> (or <code>$(intermediates-dir-for)
</code>with explicit variables), it won’t reliably work any more. That’s
because the intermediate generated sources will be required by both 32-bit and
64-bit build, but <code>$(local-intermediates-dir)</code> only points to one of
the two intermediate directories.</p>

<p>Happily, the build system now provides a dedicated, Multilib-friendly,
intermediate directory for generating sources. You can call<code>
$(local-generated-sources-dir)</code> or
<code>$(generated-sources-dir-for)</code> to get the directory’s path. Their
usages are similar to <code>$(local-intermediates-dir)</code> and
<code>$(intermediates-dir-for)</code>. </p>

<p>If a source file is generated to the new dedicated directory and picked up
by <code>LOCAL_GENERATED_SOURCES</code>, it is built for both 32-bit and 64-bit
in multilib build.</p>

<h2 id=prebuilts>Prebuilts</h2>


<p>In Multilib, you can’t use <code>TARGET_ARCH</code> (or together with
<code>TARGET_2ND_ARCH</code>) to tell the build system what arch the prebuilt
binary is targeted for. Use the aforementioned <code>LOCAL_</code> variable
<code>LOCAL_MODULE_TARGET_ARCH</code> or
<code>LOCAL_MODULE_UNSUPPORTED_TARGET_ARCH</code> instead.</p>

<p>With these variables, the build system can choose the corresponding 32-bit
prebuilt binary even if it’s currently doing a 64-bit Multilib build.</p>

<p>If you want to use the chosen arch to compute the source path for the prebuilt
binary , you can call<code> $(get-prebuilt-src-arch)</code>.</p>

<h2 id=dex-preopt>Dex-preopt</h2>


<p>For 64-bit devices, by default we generate both 32-bit and 64-bit odex files
for the boot image and any Java libraries. For APKs, by default we generate
odex only for the primary 64-bit arch. If an app will be launched in both
32-bit and 64-bit processes, please use <code>LOCAL_MULTILIB := both</code> to make sure
both 32-bit and 64-bit odex files are generated. That flag also tells the build
system to include both 32-bit and 64-bit JNI libraries, if the app has any.</p>