Question

I’m writing an app that needs some JNI code.
This code involves floating point processing, so I want it to be as fast as it can be. Therefore I want to support ARM5 and ARM7 architectures. In my Application.mk it says:

APP_ABI := all

… and the build process compiles the module for ARM5, ARM7 and x86. I verified that all .so files end up in the .apk file.

Now the question is: Will Android automatically load the “best” .so file depending on the platform it is running on? In other words: Will it load the ARM5 module on ARM5 devices and ARM7 on ARM7 devices?

Answer 1

Yes.

On an ARMv5 or ARMv6 device (CPU), the ARMv5 binary will be used (APP_ABI = armeabi)
On an ARMv7 device, the system will see the ARMv7 binary is available and will use it (APP_ABI = armeabi-v7a)

In the NDK documentation, we can read :

III.2. Android Platform ABI support:
------------------------------------

The Android system knows at runtime which ABI(s) it supports. More
precisely, up to two build-specific system properties are used to
indicate:

- the 'primary' ABI for the device, corresponding to the machine
  code used in the system image itself.

- an optional 'secondary' ABI, corresponding to another ABI that
  is also supported by the system image.

For example, a typical ARMv5TE-based device would only define
the primary ABI as 'armeabi' and not define a secondary one.

On the other hand, a typical ARMv7-based device would define the
primary ABI to 'armeabi-v7a' and the secondary one to 'armeabi'
since it can run application native binaries generated for both
of them.

A typical x86-based device only defines a primary abi named 'x86'.