07 October 2019

本文学习参考自:map

本文内容基于go1.13.1源码。

在阅读Go map的实现代码时,最好先了解哈希表这种数据结构实现的算法思想,对理解Go map的实现会有帮助,我这里简单总结下:

  • map 内部采用的是数组存储 KV,每个数组元素可以认为是一个桶
  • key 经过哈希算法后再与 map的数组长度取模映射到某个桶中
  • 如果多个 key 映射到了相同的桶,就意味着出现了哈希冲突,解决冲突的方式有两种:开放寻址法和链表法
  • 当 KV 过多时,map 就需要扩容(因为数组是固定大小的),扩容的策略是新分配一个更大的数组,然后在插入和删除 key 的时候,将对应的桶的数据搬移到新分配的数组的桶中。这种方式把扩容所需要的 O(n) 时间开销均摊到了 O(1) 的插入和删除操作中。
  • map 中用装载因子(map中元素的个数 / map的容量)来表示空闲位置的情况。装载因子越大,说明空闲位置越少,冲突越多。

调试用的go代码map.go:

package main

func main() {
    myMap := make(map[string]int, 53)
    myMap["key"] = 1
    print(myMap["key"])
    delete(myMap, "key")
}
  • 当 make 的 hint <= 8 时,会直接在栈上分配一个 bucket,一个 bucket 可以存储8对 KV(笔者测试了下将value的类型由int换为[8192]string,也是这样子)
  • 当 make 的 hint > 8 && hint <= 52 时,会在堆上分配 bucket,此时不会分配 overflow bucket
  • 当 make 的 hint > 52 时,会在堆上分配 bucket 和 overflow bucket

通过对比 hint <= 8 和 hint > 8 生成的"".main汇编代码 go tool compile -N -l -S map.go

  • hint <= 8 直接在"".main的栈上初始化 hmap 结构体和一个 bucket
  • hint > 8 会通过调用runtime.makemap在栈上初始化 hmap 结构体,并在堆上分配bucket

通过dlv debug,可以单步调试代码,并且可以使用si命令,单步执行汇编代码

hmap 结构体如下:

// 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
}

// 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
}

初始化

func makemap(t *maptype, hint int, h *hmap) *hmap {
    // 判断 hint 是否合法
    mem, overflow := math.MulUintptr(uintptr(hint), t.bucket.size)
    if overflow || mem > maxAlloc {
        hint = 0
    }

    // initialize Hmap
    if h == nil {
        h = new(hmap)
    }
    h.hash0 = fastrand()

    // Find the size parameter B which will hold the requested # of elements.
    // For hint < 0 overLoadFactor returns false since hint < bucketCnt.
    // 找到满足 loadFactor * 2^B >= hint 的 B,其中 loadFactor = loadFactorNum / loadFactorDen = 13 / 2 = 6.5
    B := uint8(0)
    for overLoadFactor(hint, B) {
        B++
    }
    h.B = B

    // allocate initial hash table
    // if B == 0, the buckets field is allocated lazily later (in mapassign)
    // If hint is large zeroing this memory could take a while.
    if h.B != 0 {
        var nextOverflow *bmap
        // makeBucketArray 中会判断 h.B 是否 >= 4,如果是,则会分配 nextOverflow,即overflow bucket
        h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
        if nextOverflow != nil {
            h.extra = new(mapextra)
            h.extra.nextOverflow = nextOverflow
        }
    }

    return h
}

overflow bucket 的作用是用于存储哈希冲突的KV(go map 采用链表法的方式解决哈希冲突))。当 hint = 53 时,分配的 bucket 情况如下:

赋值

func mapassign_faststr(t *maptype, h *hmap, s string) unsafe.Pointer {
    if h == nil {
        panic(plainError("assignment to entry in nil map"))
    }
    if raceenabled {
        callerpc := getcallerpc()
        racewritepc(unsafe.Pointer(h), callerpc, funcPC(mapassign_faststr))
    }
    // 不允许并发写
    if h.flags&hashWriting != 0 {
        throw("concurrent map writes")
    }
    key := stringStructOf(&s)
    // 调用key类型对应的hash算法
    hash := t.key.alg.hash(noescape(unsafe.Pointer(&s)), uintptr(h.hash0))

    // Set hashWriting after calling alg.hash for consistency with mapassign.
    // 异或操作,设置写标记位
    h.flags ^= hashWriting

    if h.buckets == nil {
        h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
    }

again:
    // 计算key存储在哪个bucket
    bucket := hash & bucketMask(h.B)
    if h.growing() {
        // 如果map正在扩容,需要确保bucket已经被搬运到hmap.buckets中了
        growWork_faststr(t, h, bucket)
    }
    // 取得对应bucket的内存地址
    b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
    // 取hash的高8位
    top := tophash(hash)

    // 实际插入的bucket,虽然上面计算出了b,但可能b已经满了,需要插入到b的overflow bucket,或者map需要扩容了
    var insertb *bmap
    // 插入到bucket中的哪个位置
    var inserti uintptr
    // bucket中key的地址
    var insertk unsafe.Pointer

bucketloop:
    for {
        // bucketCnt = 8
        for i := uintptr(0); i < bucketCnt; i++ {
            if b.tophash[i] != top {
                if isEmpty(b.tophash[i]) && insertb == nil { // 在b中找到位置i可以存放赋值的KV
                    insertb = b
                    inserti = i
                    // 为何这里不执行break bucketloop?因为有可能K已经存在,需要找到它的位置
                }
                // 如果余下的位置都是空的,则不需要再往下找了
                if b.tophash[i] == emptyRest {
                    break bucketloop
                }
                continue
            }
            // tophash 相同,还需要仔细比较实际的K是否一样
            k := (*stringStruct)(add(unsafe.Pointer(b), dataOffset+i*2*sys.PtrSize))
            if k.len != key.len {
                continue
            }
            if k.str != key.str && !memequal(k.str, key.str, uintptr(key.len)) {
                continue
            }
            // K已经在map中了
            // already have a mapping for key. Update it.
            inserti = i
            insertb = b
            goto done
        }
        ovf := b.overflow(t)
        if ovf == nil {
            break
        }
        b = ovf
    }

    // K不在map中,需要判断是否进行扩容或者增加overflow bucket
    // Did not find mapping for key. Allocate new cell & add entry.

    // If we hit the max load factor or we have too many overflow buckets,
    // and we're not already in the middle of growing, start growing.
    if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
        // 如果map没有扩容,并且负载因子超过阈值或者有太多overflow bucket,则进行扩容
        hashGrow(t, h)
        // 跳转回again
        goto again // Growing the table invalidates everything, so try again
    }

    // 如果还是没找到空闲的位置存放新的KV,则需要存储到overflow bucket中
    if insertb == nil {
        // all current buckets are full, allocate a new one.
        insertb = h.newoverflow(t, b)
        inserti = 0 // not necessary, but avoids needlessly spilling inserti
    }
    insertb.tophash[inserti&(bucketCnt-1)] = top // mask inserti to avoid bounds checks

    // 插入K
    insertk = add(unsafe.Pointer(insertb), dataOffset+inserti*2*sys.PtrSize)
    // store new key at insert position
    *((*stringStruct)(insertk)) = *key
    h.count++

done:
    // 获取V的地址
    elem := add(unsafe.Pointer(insertb), dataOffset+bucketCnt*2*sys.PtrSize+inserti*uintptr(t.elemsize))
    if h.flags&hashWriting == 0 {
        throw("concurrent map writes")
    }
    // 清除写标记位
    h.flags &^= hashWriting
    // 返回V的地址,实际赋值是由编译器生成的汇编代码进行赋值的
    return elem
}

当出现 key 冲突时,key 会存储到 overflow bucket 中,以上面的图为例,假设超过8个 key 都 hash 到了索引0的位置:

h.mapextra.nextOverflow 指向下一个可用作 overflow bucket 的空闲 bucket。

访问

func mapaccess2_faststr(t *maptype, h *hmap, ky string) (unsafe.Pointer, bool) {
    if raceenabled && h != nil {
        callerpc := getcallerpc()
        racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapaccess2_faststr))
    }
    // 返回零值,已经false,表示key不存在
    if h == nil || h.count == 0 {
        return unsafe.Pointer(&zeroVal[0]), false
    }
    if h.flags&hashWriting != 0 {
        throw("concurrent map read and map write")
    }
    key := stringStructOf(&ky)
    if h.B == 0 {
        // 只有1个bucket
        // One-bucket table.
        b := (*bmap)(h.buckets)
        if key.len < 32 {
            // key比较短,直接进行比较
            // short key, doing lots of comparisons is ok
            for i, kptr := uintptr(0), b.keys(); i < bucketCnt; i, kptr = i+1, add(kptr, 2*sys.PtrSize) {
                k := (*stringStruct)(kptr)
                if k.len != key.len || isEmpty(b.tophash[i]) {
                    // 后面已经没有KV了,不用再找下去了
                    if b.tophash[i] == emptyRest {
                        break
                    }
                    continue
                }
                // 找到key
                if k.str == key.str || memequal(k.str, key.str, uintptr(key.len)) {
                    return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*sys.PtrSize+i*uintptr(t.elemsize)), true
                }
            }
            // 未找到,返回零值
            return unsafe.Pointer(&zeroVal[0]), false
        }
        // key 比较长
        // long key, try not to do more comparisons than necessary
        keymaybe := uintptr(bucketCnt)
        for i, kptr := uintptr(0), b.keys(); i < bucketCnt; i, kptr = i+1, add(kptr, 2*sys.PtrSize) {
            k := (*stringStruct)(kptr)
            if k.len != key.len || isEmpty(b.tophash[i]) {
                // 后面已经没有KV了,不用再找下去了
                if b.tophash[i] == emptyRest {
                    break
                }
                continue
            }
            // 找到了,内存地址一样
            if k.str == key.str {
                return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*sys.PtrSize+i*uintptr(t.elemsize)), true
            }
            // 检查头4字节
            // check first 4 bytes
            if *((*[4]byte)(key.str)) != *((*[4]byte)(k.str)) {
                continue
            }
            // 检查尾4字节
            // check last 4 bytes
            if *((*[4]byte)(add(key.str, uintptr(key.len)-4))) != *((*[4]byte)(add(k.str, uintptr(key.len)-4))) {
                continue
            }
            // 走到这里,说明有至少2个key有可能匹配
            if keymaybe != bucketCnt {
                // Two keys are potential matches. Use hash to distinguish them.
                goto dohash
            }
            keymaybe = i
        }
        // 有1个key可能匹配
        if keymaybe != bucketCnt {
            k := (*stringStruct)(add(unsafe.Pointer(b), dataOffset+keymaybe*2*sys.PtrSize))
            if memequal(k.str, key.str, uintptr(key.len)) {
                return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*sys.PtrSize+keymaybe*uintptr(t.elemsize)), true
            }
        }
        return unsafe.Pointer(&zeroVal[0]), false
    }
dohash:
    hash := t.key.alg.hash(noescape(unsafe.Pointer(&ky)), uintptr(h.hash0))
    m := bucketMask(h.B)
    b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
    // 判断是否正在扩容
    if c := h.oldbuckets; c != nil {
        if !h.sameSizeGrow() {
            // There used to be half as many buckets; mask down one more power of two.
            // 如果不是相同大小的扩容,则需要缩小一倍,因为此时 len(h.buckets) = 2*len(h.oldbuckets)
            m >>= 1
        }
        oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
        // 判断对应的bucket是否已经从h.oldbuckets搬到h.buckets
        if !evacuated(oldb) {
            // 还没有搬
            b = oldb
        }
    }
    top := tophash(hash)
    // 在b已经b的overflow bucket中查找
    for ; b != nil; b = b.overflow(t) {
        for i, kptr := uintptr(0), b.keys(); i < bucketCnt; i, kptr = i+1, add(kptr, 2*sys.PtrSize) {
            k := (*stringStruct)(kptr)
            if k.len != key.len || b.tophash[i] != top {
                continue
            }
            if k.str == key.str || memequal(k.str, key.str, uintptr(key.len)) {
                return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*sys.PtrSize+i*uintptr(t.elemsize)), true
            }
        }
    }
    return unsafe.Pointer(&zeroVal[0]), false
}

删除

func mapdelete_faststr(t *maptype, h *hmap, ky string) {
    if raceenabled && h != nil {
        callerpc := getcallerpc()
        racewritepc(unsafe.Pointer(h), callerpc, funcPC(mapdelete_faststr))
    }
    if h == nil || h.count == 0 {
        return
    }
    if h.flags&hashWriting != 0 {
        throw("concurrent map writes")
    }

    key := stringStructOf(&ky)
    hash := t.key.alg.hash(noescape(unsafe.Pointer(&ky)), uintptr(h.hash0))

    // Set hashWriting after calling alg.hash for consistency with mapdelete
    h.flags ^= hashWriting

    bucket := hash & bucketMask(h.B)
    // 如果正在扩容,确保bucket已经从h.oldbuckets搬到h.buckets
    if h.growing() {
        growWork_faststr(t, h, bucket)
    }
    b := (*bmap)(add(h.buckets, bucket*uintptr(t.bucketsize)))
    bOrig := b
    top := tophash(hash)
search:
    // 在b,已经b的overflow bucket中查找
    for ; b != nil; b = b.overflow(t) {
        for i, kptr := uintptr(0), b.keys(); i < bucketCnt; i, kptr = i+1, add(kptr, 2*sys.PtrSize) {
            k := (*stringStruct)(kptr)
            if k.len != key.len || b.tophash[i] != top {
                continue
            }
            if k.str != key.str && !memequal(k.str, key.str, uintptr(key.len)) {
                continue
            }
            // 找到了
            // Clear key's pointer.
            k.str = nil
            e := add(unsafe.Pointer(b), dataOffset+bucketCnt*2*sys.PtrSize+i*uintptr(t.elemsize))
            // 与GC相关
            if t.elem.ptrdata != 0 {
                memclrHasPointers(e, t.elem.size)
            } else {
                memclrNoHeapPointers(e, t.elem.size)
            }
            // 标记当前单元是空闲的
            b.tophash[i] = emptyOne
            // If the bucket now ends in a bunch of emptyOne states,
            // change those to emptyRest states.
            // 判断>i的单元是否都是空闲的
            if i == bucketCnt-1 {
                if b.overflow(t) != nil && b.overflow(t).tophash[0] != emptyRest {
                    goto notLast
                }
            } else {
                if b.tophash[i+1] != emptyRest {
                    goto notLast
                }
            }
            // >i的单元都是空闲的,那么将当前单元,以及<i的emptyOne单元都标记为emptyRest
            // emptyRest的作用就是在查找的时候,遇到emptyRest就不用再往下找了,加速查找的过程
            for {
                b.tophash[i] = emptyRest
                if i == 0 {
                    if b == bOrig {
                        break // beginning of initial bucket, we're done.
                    }
                    // Find previous bucket, continue at its last entry.
                    c := b
                    for b = bOrig; b.overflow(t) != c; b = b.overflow(t) {
                    }
                    i = bucketCnt - 1
                } else {
                    i--
                }
                if b.tophash[i] != emptyOne {
                    break
                }
            }
        notLast:
            h.count--
            break search
        }
    }

    if h.flags&hashWriting == 0 {
        throw("concurrent map writes")
    }
    h.flags &^= hashWriting
}

下图演示了在一个有一个 overflow bucket 的 bucket 中删除 KV,bmap.tophash 标记位变化的过程:

扩容

两种情况下会进行扩容:

  1. overLoadFactor(h.count+1, h.B) 装载因子过大时,扩容一倍
  2. tooManyOverflowBuckets(h.noverflow, h.B)) 当使用的 overflow bucket 过多时,实际上没有扩容,重新分配了一样大的空间,主要是为了回收空闲的 overflow bucket

启动扩容:

func hashGrow(t *maptype, h *hmap) {
    // If we've hit the load factor, get bigger.
    // Otherwise, there are too many overflow buckets,
    // so keep the same number of buckets and "grow" laterally.
    bigger := uint8(1)
    if !overLoadFactor(h.count+1, h.B) {
        // 如果装载因子没有超过阈值,那么按相同大小的空间“扩容”
        bigger = 0
        h.flags |= sameSizeGrow
    }
    oldbuckets := h.buckets
    // 分配新空间
    newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)

    // 清除 iterator,oldIterator 的标记位
    flags := h.flags &^ (iterator | oldIterator)
    if h.flags&iterator != 0 {
        flags |= oldIterator
    }
    // commit the grow (atomic wrt gc)
    h.B += bigger
    h.flags = flags
    h.oldbuckets = oldbuckets
    h.buckets = newbuckets
    h.nevacuate = 0 // 统计搬了多少个bucket
    h.noverflow = 0

    if h.extra != nil && h.extra.overflow != nil {
        // Promote current overflow buckets to the old generation.
        if h.extra.oldoverflow != nil {
            throw("oldoverflow is not nil")
        }
        h.extra.oldoverflow = h.extra.overflow
        h.extra.overflow = nil
    }
    if nextOverflow != nil {
        if h.extra == nil {
            h.extra = new(mapextra)
        }
        h.extra.nextOverflow = nextOverflow
    }

    // the actual copying of the hash table data is done incrementally
    // by growWork() and evacuate().
}

实际的搬迁bucket:

// 插入和删除的时候,发现正在扩容的话,会调用该方法
func growWork_faststr(t *maptype, h *hmap, bucket uintptr) {
    // make sure we evacuate the oldbucket corresponding
    // to the bucket we're about to use
    evacuate_faststr(t, h, bucket&h.oldbucketmask())

    // evacuate one more oldbucket to make progress on growing
    if h.growing() {
        evacuate_faststr(t, h, h.nevacuate)
    }
}

func evacuate_faststr(t *maptype, h *hmap, oldbucket uintptr) {
    b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
    newbit := h.noldbuckets()
    // 判断该bucket是否已经搬迁了
    if !evacuated(b) {
        // TODO: reuse overflow buckets instead of using new ones, if there
        // is no iterator using the old buckets.  (If !oldIterator.)

        // xy contains the x and y (low and high) evacuation destinations.
        // xy 指向新空间的高低区间的起点
        var xy [2]evacDst
        x := &xy[0]
        x.b = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
        x.k = add(unsafe.Pointer(x.b), dataOffset)
        x.e = add(x.k, bucketCnt*2*sys.PtrSize)

        // 如果是扩容一倍,才会用到 y
        if !h.sameSizeGrow() {
            // Only calculate y pointers if we're growing bigger.
            // Otherwise GC can see bad pointers.
            y := &xy[1]
            y.b = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
            y.k = add(unsafe.Pointer(y.b), dataOffset)
            y.e = add(y.k, bucketCnt*2*sys.PtrSize)
        }

        // 将当前 bucket 以及其 overflow bucket 进行搬迁
        for ; b != nil; b = b.overflow(t) {
            k := add(unsafe.Pointer(b), dataOffset)
            e := add(k, bucketCnt*2*sys.PtrSize)
            for i := 0; i < bucketCnt; i, k, e = i+1, add(k, 2*sys.PtrSize), add(e, uintptr(t.elemsize)) {
                top := b.tophash[i]
                // 这里是不是可以判断到 emptyRest 就停止循环了?
                if isEmpty(top) {
                    b.tophash[i] = evacuatedEmpty
                    continue
                }
                if top < minTopHash {
                    throw("bad map state")
                }
                var useY uint8
                if !h.sameSizeGrow() {
                    // Compute hash to make our evacuation decision (whether we need
                    // to send this key/elem to bucket x or bucket y).
                    hash := t.key.alg.hash(k, uintptr(h.hash0))
                    if hash&newbit != 0 { // 新的位置位于高区间
                        useY = 1
                    }
                }

                b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY, enforced in makemap
                dst := &xy[useY]                 // evacuation destination

                if dst.i == bucketCnt { // 是否要放到 overflow bucket 中
                    dst.b = h.newoverflow(t, dst.b)
                    dst.i = 0
                    dst.k = add(unsafe.Pointer(dst.b), dataOffset)
                    dst.e = add(dst.k, bucketCnt*2*sys.PtrSize)
                }
                dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check

                // Copy key.
                *(*string)(dst.k) = *(*string)(k)

                typedmemmove(t.elem, dst.e, e)
                dst.i++
                // These updates might push these pointers past the end of the
                // key or elem arrays.  That's ok, as we have the overflow pointer
                // at the end of the bucket to protect against pointing past the
                // end of the bucket.
                dst.k = add(dst.k, 2*sys.PtrSize)
                dst.e = add(dst.e, uintptr(t.elemsize))
            }
        }
        // Unlink the overflow buckets & clear key/elem to help GC.
        if h.flags&oldIterator == 0 && t.bucket.ptrdata != 0 {
            b := add(h.oldbuckets, oldbucket*uintptr(t.bucketsize))
            // Preserve b.tophash because the evacuation
            // state is maintained there.
            ptr := add(b, dataOffset)
            n := uintptr(t.bucketsize) - dataOffset
            memclrHasPointers(ptr, n)
        }
    }

    // 统计搬迁的进度,如果数据都搬迁完了,则结束扩容
    if oldbucket == h.nevacuate {
        advanceEvacuationMark(h, t, newbit)
    }
}

关于 map 元素无法取址问题

如果我们尝试对 map 的元素取址,会遇到 cannot take the address of m["a"] 错误。

因为 map 在扩容后m["a"]的地址是会发生改变的。

关于 map 的类型 value 是 struct 或数组类型无法直接修改 value 的某个字段/元素的问题

cannot assign to struct field m["a"].i in mapcannot assign to m["a"][0] 错误是在编译阶段就报错的。

如果 key "a" 存在的话,从实现上来讲,是可以做到对 m["a"].im["a"][0] 进行赋值的。 如果 key "a" 不存在的话,就需要考虑是否抛出 runtime error(返回零值使赋值能成功不太合适,因为需要把零值的key "a" 插入到 map 中,但又感觉又不符合代码的语意)。

关于这个问题在 Go 的代码仓库有个 issue:proposal: spec: cannot assign to a field of a map element directly: m[“foo”].f = x #3117

如果 key 的类型是 struct 或 指针

对于不同类型的 key 会调用相应的runtime.mapassign*runtime.mapaccess*函数,计算 key 的哈希算法也不一样。

比如type Key struct{a int}会使用与 key 类型为 int 相同的runtime.mapassign_fast64runtime.mapaccess1_fast64函数,type Key struct{a string}会使用与 key 类型为 string 相同的runtime.mapassign_faststrruntime.mapaccess1_faststr函数。但是type Key struct{a int; b string}则使用的是runtime.mapassignruntime.mapaccess1

遗留问题

  1. map 并发写的检测是通过判断 h.flags 是否有标记位 hashWriting 这种方式是否不够严谨?
  2. 相同大小容量的“扩容”,我判断出来的是为了解决过多空闲 overflow bucket 的问题,如果是真的要解决这个问题,是否可以在删除key的时候做回收?