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're looking for a simpler implementation:

First, ensure all NEON-capable code is conditionally compiled only for ABI armeabi-v7a, and additionally will only be executed if at runtime it's running on an ARMv7 implementation which includes NEON:

/* 
   bar_better_on_neon.c
 */

#ifdef HAVE_ARMV7
#  include 
#  ifdef ANDROID
#    include "cpu-features.h"
#  endif
#endif

#ifdef HAVE_ARMV7 
static int check_for_neon(void)
{
#  ifdef ANDROID
    // Not all Android devices with ARMv7 are guaranteed to have NEON, so check.
    uint64_t features = android_getCpuFeatures();
    return (features & ANDROID_CPU_ARM_FEATURE_ARMv7) && (features & ANDROID_CPU_ARM_FEATURE_NEON);
#  elif defined(__APPLE__)
    return 1;
#  else
    return 0;
#  endif
}
#endif

void bar(void)
{
#ifdef HAVE_ARMV7
    if (check_for_neon()) {

        /* here put neon code */

    } else {
#endif

        /* here put non-neon code */

#ifdef HAVE_ARMV7
    }
#endif
}

Here, the check_for_neon() uses the NDK's cpufeatures library. Then, in your Android.mk file:

LOCAL_SRC_FILES := foo.c bar_better_on_neon.c
ifeq ($(TARGET_ARCH_ABI),armeabi-v7a)
  # Compile the one file in NEON mode.
  LOCAL_SRC_FILES := $(subst bar_better_on_neon.c, bar_better_on_neon.c.neon,$(LOCAL_SRC_FILES))
  LOCAL_CFLAGS += -DHAVE_ARMV7=1
endif