// A header for a Go map. type hmap struct { // Note: the format of the hmap is also encoded in cmd/compile/internal/gc/reflect.go. // Make sure this stays in sync with the compiler's definition. count int// # live cells == size of map. Must be first (used by len() builtin) flags uint8 B uint8// log_2 of # of buckets (can hold up to loadFactor * 2^B items) noverflow uint16// approximate number of overflow buckets; see incrnoverflow for details hash0 uint32// hash seed
buckets unsafe.Pointer // array of 2^B Buckets. may be nil if count==0. oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing nevacuate uintptr// progress counter for evacuation (buckets less than this have been evacuated)
extra *mapextra // optional fields }
可以看到这是一个map的头部结构，其中有几个关键结构，分别是
count 当前map的元素个数
buckets 桶的数量，一般是2^B个
oldbuckets 扩容前的buckets
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// mapextra holds fields that are not present on all maps. type mapextra struct { // If both key and elem do not contain pointers and are inline, then we mark bucket // type as containing no pointers. This avoids scanning such maps. // However, bmap.overflow is a pointer. In order to keep overflow buckets // alive, we store pointers to all overflow buckets in hmap.extra.overflow and hmap.extra.oldoverflow. // overflow and oldoverflow are only used if key and elem do not contain pointers. // overflow contains overflow buckets for hmap.buckets. // oldoverflow contains overflow buckets for hmap.oldbuckets. // The indirection allows to store a pointer to the slice in hiter. overflow *[]*bmap oldoverflow *[]*bmap
// nextOverflow holds a pointer to a free overflow bucket. nextOverflow *bmap }
简单来说，这个可以忽略。
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// A bucket for a Go map. type bmap struct { // tophash generally contains the top byte of the hash value // for each key in this bucket. If tophash[0] < minTopHash, // tophash[0] is a bucket evacuation state instead. tophash [bucketCnt]uint8 // Followed by bucketCnt keys and then bucketCnt elems. // NOTE: packing all the keys together and then all the elems together makes the // code a bit more complicated than alternating key/elem/key/elem/... but it allows // us to eliminate padding which would be needed for, e.g., map[int64]int8. // Followed by an overflow pointer. }