134  * presence or not of -mfpu=neon on the GCC command line, it is possible to do
 135  * this in $(CC), e.g. "CC=gcc -mfpu=neon", but people who build libpng rarely
 136  * do this.
 137  */
 138 #ifndef PNG_ARM_NEON_OPT
 139    /* ARM NEON optimizations are being controlled by the compiler settings,
 140     * typically the target FPU.  If the FPU has been set to NEON (-mfpu=neon
 141     * with GCC) then the compiler will define __ARM_NEON__ and we can rely
 142     * unconditionally on NEON instructions not crashing, otherwise we must
 143     * disable use of NEON instructions.
 144     *
 145     * NOTE: at present these optimizations depend on 'ALIGNED_MEMORY', so they
 146     * can only be turned on automatically if that is supported too.  If
 147     * PNG_ARM_NEON_OPT is set in CPPFLAGS (to >0) then arm/arm_init.c will fail
 148     * to compile with an appropriate #error if ALIGNED_MEMORY has been turned
 149     * off.
 150     *
 151     * Note that gcc-4.9 defines __ARM_NEON instead of __ARM_NEON__, so we
 152     * check both variants.
 153     */
 154 #  if (defined(__ARM_NEON__) || defined(__ARM_NEON)) && \
 155    defined(PNG_ALIGNED_MEMORY_SUPPORTED)
 156 #     define PNG_ARM_NEON_OPT 2
 157 #  else
 158 #     define PNG_ARM_NEON_OPT 0
 159 #  endif
 160 #endif
 161 
 162 #if PNG_ARM_NEON_OPT > 0
 163    /* NEON optimizations are to be at least considered by libpng, so enable the
 164     * callbacks to do this.
 165     */
 166 #  define PNG_FILTER_OPTIMIZATIONS png_init_filter_functions_neon
 167 
 168    /* By default the 'intrinsics' code in arm/filter_neon_intrinsics.c is used
 169     * if possible - if __ARM_NEON__ is set and the compiler version is not known
 170     * to be broken.  This is controlled by PNG_ARM_NEON_IMPLEMENTATION which can
 171     * be:
 172     *
 173     *    1  The intrinsics code (the default with __ARM_NEON__)
 174     *    2  The hand coded assembler (the default without __ARM_NEON__)
 175     *

 134  * presence or not of -mfpu=neon on the GCC command line, it is possible to do
 135  * this in $(CC), e.g. "CC=gcc -mfpu=neon", but people who build libpng rarely
 136  * do this.
 137  */
 138 #ifndef PNG_ARM_NEON_OPT
 139    /* ARM NEON optimizations are being controlled by the compiler settings,
 140     * typically the target FPU.  If the FPU has been set to NEON (-mfpu=neon
 141     * with GCC) then the compiler will define __ARM_NEON__ and we can rely
 142     * unconditionally on NEON instructions not crashing, otherwise we must
 143     * disable use of NEON instructions.
 144     *
 145     * NOTE: at present these optimizations depend on 'ALIGNED_MEMORY', so they
 146     * can only be turned on automatically if that is supported too.  If
 147     * PNG_ARM_NEON_OPT is set in CPPFLAGS (to >0) then arm/arm_init.c will fail
 148     * to compile with an appropriate #error if ALIGNED_MEMORY has been turned
 149     * off.
 150     *
 151     * Note that gcc-4.9 defines __ARM_NEON instead of __ARM_NEON__, so we
 152     * check both variants.
 153     */
 154 #  if ((defined(__ARM_NEON__) || defined(__ARM_NEON)) && \
 155    defined(PNG_ALIGNED_MEMORY_SUPPORTED) && defined(__arm__))
 156 #     define PNG_ARM_NEON_OPT 2
 157 #  else
 158 #     define PNG_ARM_NEON_OPT 0
 159 #  endif
 160 #endif
 161 
 162 #if PNG_ARM_NEON_OPT > 0
 163    /* NEON optimizations are to be at least considered by libpng, so enable the
 164     * callbacks to do this.
 165     */
 166 #  define PNG_FILTER_OPTIMIZATIONS png_init_filter_functions_neon
 167 
 168    /* By default the 'intrinsics' code in arm/filter_neon_intrinsics.c is used
 169     * if possible - if __ARM_NEON__ is set and the compiler version is not known
 170     * to be broken.  This is controlled by PNG_ARM_NEON_IMPLEMENTATION which can
 171     * be:
 172     *
 173     *    1  The intrinsics code (the default with __ARM_NEON__)
 174     *    2  The hand coded assembler (the default without __ARM_NEON__)
 175     *