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

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 
#include 

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 

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