他们为什么不使用整数来定义常量而不是使用移位运算符？

Question

In the go source code, the constant bucketCnt is 8. Why is defined in terms of right-shifting 3 times instead of just defining it as 8. I understand that 1 << x implies 2^x.

But, my question is...

Is

// Maximum number of key/value pairs a bucket can hold.
bucketCntBits = 3
bucketCnt     = 1 << bucketCntBits

better than

// Maximum number of key/value pairs a bucket can hold.
bucketCnt     = 8

Answer 1

const (
  // Maximum number of key/value pairs a bucket can hold.
  bucketCntBits = 3
  bucketCnt     = 1 << bucketCntBits
)

The number of key/value pairs a bucket can hold depends on the number of bits used (bucketCntBits = 3). That translates to a bucket count (bucketCnt) of 1 << bucketCntBits or 8. If we change the number of bits to 4 (bucketCntBits = 4) or 2 (bucketCntBits = 2) then bucketCnt is still 1 << bucketCntBits or 16 or 4.

// A map is just a hash table. The data is arranged
// into an array of buckets. Each bucket contains up to
// 8 key/value pairs. The low-order bits of the hash are
// used to select a bucket. Each bucket contains a few
// high-order bits of each hash to distinguish the entries
// within a single bucket.

"The low-order bits of the hash are used to select a bucket."

References:

src/runtime/hashmap.go

Go maps in action

GopherCon 2016: Keith Randall - Inside the Map Implementation

Macro View of Map Internals In Go (2013)