1.偏移

#define SHIFT_NANO_QUANTUM		4
#define NANO_REGIME_QUANTA_SIZE	(1 << SHIFT_NANO_QUANTUM)	// 16

static MALLOC_INLINE size_t
segregated_size_to_fit(nanozone_t *nanozone, size_t size, size_t *pKey)
{
	size_t k, slot_bytes;

	if (0 == size) {
		size = NANO_REGIME_QUANTA_SIZE; // Historical behavior
	}
	k = (size + NANO_REGIME_QUANTA_SIZE - 1) >> SHIFT_NANO_QUANTUM; // round up and shift for number of quanta
	slot_bytes = k << SHIFT_NANO_QUANTUM;							// multiply by power of two quanta size
	*pKey = k - 1;													// Zero-based!

	return slot_bytes;
}

2. 加对齐字节数减1，&（~字节数减一）

#   define WORD_MASK 7UL
static inline uint32_t word_align(uint32_t x) {
    // 7+8 = 15
    // 0000 1111
    // 0000 1000
    //&
    // 1111 1000 ~7
    // 0000 1000 8
    
    // 0000 0111
    //
    // x + 7
    // 8
    // 8 二阶
    // (x + 7) >> 3 << 3
    return (x + WORD_MASK) & ~WORD_MASK;
}