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arm: add cpu flag detection

For Linux (and Android) use getauxval(). For ios and windows on arm
assume NEON is available.

--- a/meson.build

+++ b/meson.build

@@ -132,6 +132,12 @@

     cdata.set('HAVE_ALIGNED_MALLOC', 1)

 endif

+if (host_machine.cpu_family() == 'aarch64' or

+    host_machine.cpu_family().startswith('arm'))

+    if cc.has_function('getauxval', prefix : '#include <sys/auxv.h>', args : test_args)

+        cdata.set('HAVE_GETAUXVAL', 1)

+    endif

+endif

 # Compiler flag tests

--- a/src/arm/cpu.c

+++ b/src/arm/cpu.c

@@ -25,8 +25,30 @@

  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

  */

+#include "config.h"

+

 #include "src/arm/cpu.h"

+#if HAVE_GETAUXVAL

+#include <sys/auxv.h>

+

+#if ARCH_AARCH64

+#define NEON_HWCAP HWCAP_ASIMD

+#elif ARCH_ARM

+#define NEON_HWCAP HWCAP_ARM_NEON

+#endif

+#endif

+

 unsigned dav1d_get_cpu_flags_arm(void) {

-    return DAV1D_ARM_CPU_FLAG_NEON;

+    unsigned flags = 0;

+#if HAVE_GETAUXVAL

+    unsigned long hw_cap = getauxval(AT_HWCAP);

+    flags |= (hw_cap & NEON_HWCAP) ? DAV1D_ARM_CPU_FLAG_NEON : 0;

+#elif __APPLE__

+    flags |= DAV1D_ARM_CPU_FLAG_NEON;

+#elif defined(_WIN32)

+    flags |= DAV1D_ARM_CPU_FLAG_NEON;

+#endif

+

+    return flags;

 }