/*
 * Allow CFLAGS to override the configured / deduced cache line size
 */
#ifndef CLIB_LOG2_CACHE_LINE_BYTES

/* Default cache line size of 64 bytes. */
#ifndef CLIB_LOG2_CACHE_LINE_BYTES
#define CLIB_LOG2_CACHE_LINE_BYTES 6
#endif

#endif /* CLIB_LOG2_CACHE_LINE_BYTES defined */

#if (CLIB_LOG2_CACHE_LINE_BYTES >= 9)
#error Cache line size 512 bytes or greater
#endif

#define CLIB_CACHE_LINE_BYTES (1 << CLIB_LOG2_CACHE_LINE_BYTES)
#define CLIB_CACHE_LINE_ALIGN_MARK(mark) u8 mark[0] __attribute__((aligned(CLIB_CACHE_LINE_BYTES)))

在编译器没有指定cache line大小的情况下，我们将它指定成64字节。不允许编译器指定的cache line超过256字节。

cache line bytes指的是一次性从内存读入到CPU缓存中的字节数目。CPU访问自己内部的缓存比访问内存的效率高得多。

cache line对齐的目的是，减少cache失效的次数，避免无畏的性能降低。

/* Default cache line fill buffers. */
#ifndef CLIB_N_PREFETCHES
#define CLIB_N_PREFETCHES 16
#endif

/* Read/write arguments to __builtin_prefetch. */
#define CLIB_PREFETCH_READ 0
#define CLIB_PREFETCH_LOAD 0	/* alias for read */
#define CLIB_PREFETCH_WRITE 1
#define CLIB_PREFETCH_STORE 1	/* alias for write */

#define _CLIB_PREFETCH(n,size,type)				\
  if ((size) > (n)*CLIB_CACHE_LINE_BYTES)			\
    __builtin_prefetch (_addr + (n)*CLIB_CACHE_LINE_BYTES,	\
			CLIB_PREFETCH_##type,			\
			/* locality */ 3);

#define CLIB_PREFETCH(addr,size,type)		\
do {						\
  void * _addr = (addr);			\
						\
  ASSERT ((size) <= 4*CLIB_CACHE_LINE_BYTES);	\
  _CLIB_PREFETCH (0, size, type);		\
  _CLIB_PREFETCH (1, size, type);		\
  _CLIB_PREFETCH (2, size, type);		\
  _CLIB_PREFETCH (3, size, type);		\
} while (0)

#undef _

对指定的内存块，进行预先读取，或者预先写入。提升效率。