Android build system, NEON and non-NEON builds

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 compile it, I must enable armeabi-v7a, but that affects regular c-code (it becomes broken on some low-end devices).

So, if the android build system wasn't excessively intrusive, I wouldn't have to ask questions, but it seems that there is no way for me to compile one file for armv6 and the other file for arm7-neon.

Can anybody give any clues if that's doable?

Edit
Before trying to reply and wasting internet-ink, it should be clear that these are the main points:
1) make only ONE lib.
2) make build that runs on armv6 (pre neon devices, e.g. armeabi).
3) allow this build to also contain NEON code (which could be executed based on run-time cpu detection; cpu detection is outside the scope of the question).
4) NEON code comes from a c/cpp file and is written using neon intrinsics.

Omitting any part of these requirements totally loses the point of the question

Solution 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.

Solution 2:

If you put the NEON code in a separate module (static library or shared library), you can manually tune CFLAGS for that module in your Android.mk to your wanting.

If you have C files that use #ifdef __ARM_NEON__ around intrinsics, your best choice would be to put these files in shared libraries - one compiled for v6, another for neon.

I usually load such "supplimentary" libraries directly from Java, so that the main native code does not care about these changes at all.

Update: here is a simple example that uses static library:

Android.mk

LOCAL_PATH:= $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := neon_utils
LOCAL_SRC_FILES := neon_add.c
LOCAL_CFLAGS += -mfloat-abi=softfp -mfpu=neon -march=armv7
include $(BUILD_STATIC_LIBRARY)

NDK_PATH:=$(call my-dir)/../..

include $(CLEAR_VARS)
LOCAL_MODULE    := test_conditional_load
LOCAL_C_INCLUDES := $(NDK_PATH)/sources/cpufeatures
LOCAL_SRC_FILES := main.c
LOCAL_STATIC_LIBRARIES  :=  neon_utils cpufeatures

include $(BUILD_EXECUTABLE)

include $(NDK_PATH)/sources/cpufeatures/Android.mk

main.c

#include <stdio.h>
#include <cpu-features.h>

void neon_add(int32_t * ptr);

int main()
{
    int32_t int32_4[] = {2,3,4,5};

    if (android_getCpuFeatures() & ANDROID_CPU_ARM_FEATURE_NEON)
    {
        neon_add(int32_4);
        printf("neon\n");
    }
    else
    {
        printf("not neon\n");
    }
    printf("values = %d, %d, %d, %d\n", int32_4[0], int32_4[1], int32_4[2], int32_4[3]);
    return 0;
}

neon_add.c

#include <arm_neon.h>

void neon_add(int32_t * ptr)
{
    int32x4_t vin = vld1q_s32(ptr);
    int32x4_t vout = vaddq_s32(vin, vin);
    vst1q_s32(ptr, vout);
}

Solution 3:

Using this link http://www.kandroid.org/ndk/docs/CPU-ARM-NEON.html

You can selectively build certain source files with NEON support using the .neon file suffix. Hopefully you can work out whether or not this applies to the ABI selection...

You may be able to put your NEON code path in one file, and using the make file conditional as shown above select to build the correct file based on which ABI you are targeting

Solution 4:

This snippet from the documentation will probably help:

   # define a static library containing our NEON code
   ifeq ($(TARGET_ARCH_ABI),armeabi-v7a)
      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

That only compiles mylib-neon when using armeabi-v7a even if you use the same Android.mk file for various ABIs. For armeabi v6 it will skip over it completely.