Understanding 64-bit Builds

The build system supports building binaries for two target CPU architectures (64 bit and 32 bit) in the same build. This is known as a multilib build.

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.

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

Product configuration

BoardConfig.mk includes the following variables to configure the second CPU architecture and ABI:

  • TARGET_2ND_ARCH
  • TARGET_2ND_ARCH_VARIANT
  • TARGET_2ND_CPU_VARIANT
  • TARGET_2ND_CPU_ABI
  • TARGET_2ND_CPU_ABI2

You can see an example in build/target/board/generic_arm64/BoardConfig.mk.

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

TARGET_PREFER_32_BIT := true

You can override this setting by using module-specific variables in Android.mk.

In a multilib build, module names in PRODUCT_PACKAGES cover both the 32-bit and 64-bit binaries, as long as they're 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.

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

Module definition in Android.mk

You can use the LOCAL_MULTILIB variable to configure your build for 32 bit/64 bit and override the global TARGET_PREFER_32_BIT variable.

Set LOCAL_MULTILIB to one of the following:

  • "both" builds both 32 bit and 64 bit.
  • "32" builds only 32 bit.
  • "64" builds only 64 bit.
  • "first" builds for only the first architecture (32 bit in 32-bit devices and 64 bit in 64-bit devices).
  • "" is the default. The build system decides which architecture to build based on the module class and other LOCAL_ variables, such as LOCAL_MODULE_TARGET_ARCH and LOCAL_32_BIT_ONLY.

If you want to build your module for specific architectures, use the following variables:

  • LOCAL_MODULE_TARGET_ARCH
    Set this variable to a list of architectures, such as arm x86 arm64. If the architecture being built is in that list, the current module is included by the build system.
  • LOCAL_MODULE_UNSUPPORTED_TARGET_ARCH
    This variable is the opposite of LOCAL_MODULE_TARGET_ARCH. If the architecture being built is not in that list, the current module is included by the build system.

There are minor variants of these two variables:

  • LOCAL_MODULE_TARGET_ARCH_WARN
  • LOCAL_MODULE_UNSUPPORTED_TARGET_ARCH_WARN

The build system warns if the current module is skipped because of the architectures listed.

To set up build flags for a particular architecture, use the architecture-specific LOCAL_ variables. An architecture-specific LOCAL_ variable is a normal LOCAL_ variable with an architecture suffix, for example:

  • LOCAL_SRC_FILES_arm, LOCAL_SRC_FILES_x86,
  • LOCAL_CFLAGS_arm, LOCAL_CFLAGS_arm64,
  • LOCAL_LDFLAGS_arm, LOCAL_LDFLAGS_arm64,

These variables are applied only if a binary is currently being built for that architecture.

Sometimes it’s easier to set up flags based on whether the binary is currently being built for 32 bit or 64 bit. Use the LOCAL_ variable with a _32 or _64 suffix, for example:

  • LOCAL_SRC_FILES_32, LOCAL_SRC_FILES_64,
  • LOCAL_CFLAGS_32, LOCAL_CFLAGS_64,
  • LOCAL_LDFLAGS_32, LOCAL_LDFLAGS_64,

Installing a path

Previously, you could use LOCAL_MODULE_PATH to install a library to a location other than the default one. For example, LOCAL_MODULE_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/hw.

In a multilib build, use LOCAL_MODULE_RELATIVE_PATH instead:

LOCAL_MODULE_RELATIVE_PATH := hw

With this format, both the 64-bit and 32-bit libraries are installed in the correct place.

If you build an executable as both 32 bit and 64 bit, use one of the following variables to distinguish the install path:

  • LOCAL_MODULE_STEM_32, LOCAL_MODULE_STEM_64
    Specifies the installed filename.
  • LOCAL_MODULE_PATH_32, LOCAL_MODULE_PATH_64
    Specifies the install path.

Generated sources

In a multilib build, if you generate source files to $(local-intermediates-dir) (or $(intermediates-dir-for) with explicit variables), it doesn’t work reliably. That’s because the intermediate generated sources are required by both the 32-bit and 64-bit builds, but $(local-intermediates-dir) only points to one of the two intermediate directories.

The build system provides a dedicated, multilib-friendly, intermediate directory for generating sources. You can call $(local-generated-sources-dir) or $(generated-sources-dir-for) to get the directory’s path. Their uses are similar to $(local-intermediates-dir) and $(intermediates-dir-for).

If a source file is generated to this dedicated directory and picked up by LOCAL_GENERATED_SOURCES, it's built for both 32 bit and 64 bit in a multilib build.

Prebuilts

In a multilib build, you can’t use TARGET_ARCH (or together with TARGET_2ND_ARCH) to tell the build system what architecture the prebuilt binary targets. Instead, use the LOCAL_ variables LOCAL_MODULE_TARGET_ARCH or LOCAL_MODULE_UNSUPPORTED_TARGET_ARCH.

With these variables, the build system can choose the corresponding 32-bit prebuilt binary even if it’s working on a 64-bit multilib build.

If you want to use the chosen architecture to compute the source path for the prebuilt binary, call $(get-prebuilt-src-arch).

ODEX file generation

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 architecture. If an app will be launched in both 32-bit and 64-bit processes, use LOCAL_MULTILIB := both to make sure that both 32-bit and 64-bit ODEX files are generated. If the app has any 32-bit or 64-bit JNI libraries, that flag also tells the build system to include them.