Android build system, NEON and non-NEON builds

Question

I want to build my library for armv6, and there is some neon code that I enable at runtime if the device supports it. The neon code uses neon intrinsics, and to be able to c

Answer 1

I have recently found another way to work around the limitations of NDK. My case was not related to NEON, but for you the same hack could do the job.

The trick is to use the existing "tag" mechanism of NDK to specify special CFLAGS for a bunch of files. This is how you do it:

First, list the neon-specific sources. You cannot use the .neon suffix as described in docs/CPU-ARM-NEON.html because build-binary.mk will find that you are not targeting armeabi-v7a. I use the following technique:

LOCAL_NEON_SRC_FILES := imgproc/neon_utils.c \
                        videoproc/usingneon.cpp
LOCAL_SRC_FILES := main.c \
                   imgproc/img.c \
                   videoproc/video.cpp

LOCAL_SRC_FILES += $(LOCAL_NEON_SRC_FILES)

Now, define the CFLAGS for NEON:

LOCAL_NEON_CFLAGS := -mfloat-abi=softfp -mfpu=neon -march=armv7

Finally, add the following magical line to your Android.mk:

TARGET-process-src-files-tags += $(call add-src-files-target-cflags, $(LOCAL_NEON_SRC_FILES), $(LOCAL_NEON_CFLAGS))

If you have more than one binary to build, you will probably want $(LOCAL_NEON_SRC_FILES) to be reset by

include $(CLEAR_VARS)

For this, add the following to your Android.mk or to Application.mk:

modules-LOCALS += NEON_SRC_FILES

Note: I have not tried this magic for NEON, I needed it for entirely different purposes. You may need some adaptations to achieve the desired compilation options for your files, and for your project. I am using NDK r.8b, and I did not check if this would work on earlier (or later) versions.