CRC Checksum Algorithm

SSE4.2 and ARM v8 provide their own CRC instructions.

• x86 platform (src/port/pg_crc32c_sse42.c)

  include <nmmintrin.h>
  Use _mm_crc32_u8/16/32/64

• AArch64 platform (src/port/pg_crc32c_armv8.c)

  include <arm_acle.h>
  Use __crc32cb/cw/cd/ch