    /* arm_init.c - NEON optimised filter functions
     *
     * Copyright (c) 2013 Glenn Randers-Pehrson
     * Written by Mans Rullgard, 2011.
     * Last changed in libpng 1.6.6 [September 16, 2013]
     *
     * This code is released under the libpng license.
     * For conditions of distribution and use, see the disclaimer
     * and license in png.h
     */
    /* Below, after checking __linux__, various non-C90 POSIX 1003.1 functions are
     * called.
     */
    #define _POSIX_SOURCE 1

    #include "../pngpriv.h"

    #ifdef PNG_READ_SUPPORTED
    #if PNG_ARM_NEON_OPT > 0
    #ifdef PNG_ARM_NEON_CHECK_SUPPORTED /* Do run-time checks */
    #include <signal.h> /* for sig_atomic_t */

    #ifdef __ANDROID__
    /* Linux provides access to information about CPU capabilites via
     * /proc/self/auxv, however Android blocks this while still claiming to be
     * Linux.  The Andoid NDK, however, provides appropriate support.
     *
     * Documentation: http://www.kandroid.org/ndk/docs/CPU-ARM-NEON.html
     */
    #include <cpu-features.h>

    static int
    png_have_neon(png_structp png_ptr)
    {
       /* This is a whole lot easier than the mess below, however it is probably
        * implemented as below, therefore it is better to cache the result (these
        * function calls may be slow!)
        */
       PNG_UNUSED(png_ptr)
       return android_getCpuFamily() == ANDROID_CPU_FAMILY_ARM &&
          (android_getCpuFeatures() & ANDROID_CPU_ARM_FEATURE_NEON) != 0;
    }
    #elif defined(__linux__)
    /* The generic __linux__ implementation requires reading /proc/self/auxv and
     * looking at each element for one that records NEON capabilities.
     */
    #include <unistd.h> /* for POSIX 1003.1 */
    #include <errno.h>  /* for EINTR */

    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <elf.h>
    #include <asm/hwcap.h>

    /* A read call may be interrupted, in which case it returns -1 and sets errno to
     * EINTR if nothing was done, otherwise (if something was done) a partial read
     * may result.
     */
    static size_t
    safe_read(png_structp png_ptr, int fd, void *buffer_in, size_t nbytes)
    {
       size_t ntotal = 0;
       char *buffer = png_voidcast(char*, buffer_in);

       while (nbytes > 0)
       {
          unsigned int nread;
          int iread;

          /* Passing nread > INT_MAX to read is implementation defined in POSIX
           * 1003.1, therefore despite the unsigned argument portable code must
           * limit the value to INT_MAX!
           */
          if (nbytes > INT_MAX)
             nread = INT_MAX;

          else
             nread = (unsigned int)/*SAFE*/nbytes;

          iread = read(fd, buffer, nread);

          if (iread == -1)
          {
             /* This is the devil in the details, a read can terminate early with 0
              * bytes read because of EINTR, yet it still returns -1 otherwise end
              * of file cannot be distinguished.
              */
             if (errno != EINTR)
             {
                png_warning(png_ptr, "/proc read failed");
                return 0; /* I.e. a permanent failure */
             }
          }

          else if (iread < 0)
          {
             /* Not a valid 'read' result: */
             png_warning(png_ptr, "OS /proc read bug");
             return 0;
          }

          else if (iread > 0)
          {
             /* Continue reading until a permanent failure, or EOF */
             buffer += iread;
             nbytes -= (unsigned int)/*SAFE*/iread;
             ntotal += (unsigned int)/*SAFE*/iread;
          }

          else
             return ntotal;
       }

       return ntotal; /* nbytes == 0 */
    }

    static int
    png_have_neon(png_structp png_ptr)
    {
       int fd = open("/proc/self/auxv", O_RDONLY);
       Elf32_auxv_t aux;

       /* Failsafe: failure to open means no NEON */
       if (fd == -1)
       {
          png_warning(png_ptr, "/proc/self/auxv open failed");
          return 0;
       }

       while (safe_read(png_ptr, fd, &aux, sizeof aux) == sizeof aux)
       {
          if (aux.a_type == AT_HWCAP && (aux.a_un.a_val & HWCAP_NEON) != 0)
          {
             close(fd);
             return 1;
          }
       }

       close(fd);
       return 0;
    }
    #else
       /* We don't know how to do a run-time check on this system */
    #  error "no support for run-time ARM NEON checks"
    #endif /* OS checks */
    #endif /* PNG_ARM_NEON_CHECK_SUPPORTED */

    #ifndef PNG_ALIGNED_MEMORY_SUPPORTED
    #  error "ALIGNED_MEMORY is required; set: -DPNG_ALIGNED_MEMORY_SUPPORTED"
    #endif

    void
    png_init_filter_functions_neon(png_structp pp, unsigned int bpp)
    {
       /* The switch statement is compiled in for ARM_NEON_API, the call to
        * png_have_neon is compiled in for ARM_NEON_CHECK.  If both are defined
        * the check is only performed if the API has not set the NEON option on
        * or off explicitly.  In this case the check controls what happens.
        *
        * If the CHECK is not compiled in and the option is UNSET the behavior prior
        * to 1.6.7 was to use the NEON code - this was a bug caused by having the
        * wrong order of the 'ON' and 'default' cases.  UNSET now defaults to OFF,
        * as documented in png.h
        */
    #ifdef PNG_ARM_NEON_API_SUPPORTED
       switch ((pp->options >> PNG_ARM_NEON) & 3)
       {
          case PNG_OPTION_UNSET:
             /* Allow the run-time check to execute if it has been enabled -
              * thus both API and CHECK can be turned on.  If it isn't supported
              * this case will fall through to the 'default' below, which just
              * returns.
              */
    #endif /* PNG_ARM_NEON_API_SUPPORTED */
    #ifdef PNG_ARM_NEON_CHECK_SUPPORTED
             {
                static volatile sig_atomic_t no_neon = -1; /* not checked */

                if (no_neon < 0)
                   no_neon = !png_have_neon(pp);

                if (no_neon)
                   return;
             }
    #ifdef PNG_ARM_NEON_API_SUPPORTED
             break;
    #endif
    #endif /* PNG_ARM_NEON_CHECK_SUPPORTED */

    #ifdef PNG_ARM_NEON_API_SUPPORTED
          default: /* OFF or INVALID */
             return;

          case PNG_OPTION_ON:
             /* Option turned on */
             break;
       }
    #endif

       /* IMPORTANT: any new external functions used here must be declared using
        * PNG_INTERNAL_FUNCTION in ../pngpriv.h.  This is required so that the
        * 'prefix' option to configure works:
        *
        *    ./configure --with-libpng-prefix=foobar_
        *
        * Verify you have got this right by running the above command, doing a build
        * and examining pngprefix.h; it must contain a #define for every external
        * function you add.  (Notice that this happens automatically for the
        * initialization function.)
        */
       pp->read_filter[PNG_FILTER_VALUE_UP-1] = png_read_filter_row_up_neon;

       if (bpp == 3)
       {
          pp->read_filter[PNG_FILTER_VALUE_SUB-1] = png_read_filter_row_sub3_neon;
          pp->read_filter[PNG_FILTER_VALUE_AVG-1] = png_read_filter_row_avg3_neon;
          pp->read_filter[PNG_FILTER_VALUE_PAETH-1] =
             png_read_filter_row_paeth3_neon;
       }

       else if (bpp == 4)
       {
          pp->read_filter[PNG_FILTER_VALUE_SUB-1] = png_read_filter_row_sub4_neon;
          pp->read_filter[PNG_FILTER_VALUE_AVG-1] = png_read_filter_row_avg4_neon;
          pp->read_filter[PNG_FILTER_VALUE_PAETH-1] =
              png_read_filter_row_paeth4_neon;
       }
    }
    #endif /* PNG_ARM_NEON_OPT > 0 */
    #endif /* PNG_READ_SUPPORTED */