Golang-map的操作

// 不指定map长度
ageMp := make(map[string]int)
// 指定map长度
ageMp := make(map[string]int, 8)
// ageMp 为 nil，不能向其添加元素，会直接panic
var ageMp map[string]int

// makemap implements Go map creation for make(map[k]v, hint).
// If the compiler has determined that the map or the first bucket
// can be created on the stack, h and/or bucket may be non-nil.
// If h != nil, the map can be created directly in h.
// If h.buckets != nil, bucket pointed to can be used as the first bucket.
// 翻译：makemap实现了make(map[k]v, hint)这种形式语法的Go map创建。
// 如果编译器已经确定该map或者第一个bucket能够在这个栈上创建，那么h和(或)bucket可能为非nil。
// 如果h不是nil，那么该map能直接创建在这个h上。
// 如果h.buckets不是nil, 则被指向的bucket能被用来做第一个bucket
func makemap(t *maptype, hint int, h *hmap) *hmap {
  // 计算指定的大小所需要的内容是否超出出系统允许的最大分配大小
    mem, overflow := math.MulUintptr(uintptr(hint), t.bucket.size)
    if overflow || mem > maxAlloc {
        hint = 0
    }

    // 初始化hmap，并指定随机种子
    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.
      // 通过overLoadFactor(hint, B)函数找到一个能装下指定map大小个元素个数的最小B，要满足 装载因子*2^B < hint
    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.
      // 开始对hash table进行初始化。如果B==0则buckets 进行懒初始化操作（赋值的时候才进行初始化），如果B值特别大，则初始化需要一段时间，主要通过 makeBucketArray() 函数实现
    if h.B != 0 {
        var nextOverflow *bmap
        h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
        if nextOverflow != nil {
            h.extra = new(mapextra)
      // 溢出桶地址赋值
            h.extra.nextOverflow = nextOverflow
        }
    }

    return h
}

// makeBucketArray initializes a backing array for map buckets.
// 1<<b is the minimum number of buckets to allocate.
// dirtyalloc should either be nil or a bucket array previously
// allocated by makeBucketArray with the same t and b parameters.
// If dirtyalloc is nil a new backing array will be alloced and
// otherwise dirtyalloc will be cleared and reused as backing array.
// 翻译：makeBucketArray为map buckets初始化一个备用数组
// 2^b是该buckets长度的最小值
// dirtyalloc之前应该是nil或者bucket数组
// 如果dirtyalloc 为nil，将分配一个新的后备数组，否则将清除dirtyalloc 并作为后备数组重用。
func makeBucketArray(t *maptype, b uint8, dirtyalloc unsafe.Pointer) (buckets unsafe.Pointer, nextOverflow *bmap) {
    base := bucketShift(b) // base = 2^b
    nbuckets := base
    // For small b, overflow buckets are unlikely.
    // Avoid the overhead of the calculation.
    // 对于小的b，不太可能会需要溢出桶，可以避免这部分计算开销
    // 对于b>4（桶的数量> 2^4）的话，则需要创建2^(b-4)个溢出桶。
    if b >= 4 {
        // 加上溢出桶的数量
        nbuckets += bucketShift(b - 4)
    // 总大小
        sz := t.bucket.size * nbuckets
    // 针对所需要的内存大小，mallocgc将分配的内存块大小
        up := roundupsize(sz)
        if up != sz {
      // 如果总大小和mallocgc将分配的内存块大小不同，以mallocgc分配的为准，计算nbuckets
            nbuckets = up / t.bucket.size
        }
    }

    if dirtyalloc == nil {
        buckets = newarray(t.bucket, int(nbuckets))
    } else {
        // dirtyalloc was previously generated by
        // the above newarray(t.bucket, int(nbuckets))
        // but may not be empty.
        buckets = dirtyalloc
        size := t.bucket.size * nbuckets
        if t.bucket.ptrdata != 0 {
            memclrHasPointers(buckets, size)
        } else {
            memclrNoHeapPointers(buckets, size)
        }
    }

    if base != nbuckets {
        // We preallocated some overflow buckets.
        // To keep the overhead of tracking these overflow buckets to a minimum,
        // we use the convention that if a preallocated overflow bucket's overflow
        // pointer is nil, then there are more available by bumping the pointer.
        // We need a safe non-nil pointer for the last overflow bucket; just use buckets.
        // 我们提前分配了一些溢出桶，为了使得追踪溢出桶的开销最小，我们这样约定:
        // 如果溢出桶的overflow指针为nil，那么代表还有空间在出现哈希碰撞时使用，
        // 溢出桶的最后一个桶的overflow需要指向一个安全的非空指针，这里指向了buckets的第一个桶
        // 具体的用处在map写入时，需要创建溢出桶时会用到，具体在newoverflow函数中
        nextOverflow = (*bmap)(add(buckets, base*uintptr(t.bucketsize))) // 指向第一个溢出桶
        last := (*bmap)(add(buckets, (nbuckets-1)*uintptr(t.bucketsize))) // 指向最后一个溢出桶
        last.setoverflow(t, (*bmap)(buckets)) // 溢出桶的最后一个桶的overflow指向了buckets的第一个桶
    }
    return buckets, nextOverflow
}

v := ageMap["yicixin"]
v, ok := ageMap["yicixin"]

// mapaccess1 returns a pointer to h[key].  Never returns nil, instead
// it will return a reference to the zero object for the elem type if
// the key is not in the map.
// NOTE: The returned pointer may keep the whole map live, so don't
// hold onto it for very long.
// mapaccess1返回指向h[key]的指针，永远不会返回nil，如果key不存在，就会返回元素对应类型的空值。
// 注意：返回的指针可能会使得整个map处于活动状态，所以不要长时间持有它
func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
    if raceenabled && h != nil {
        callerpc := getcallerpc()
        pc := funcPC(mapaccess1)
        racereadpc(unsafe.Pointer(h), callerpc, pc)
        raceReadObjectPC(t.key, key, callerpc, pc)
    }
    if msanenabled && h != nil {
        msanread(key, t.key.size)
    }
      // 判断h是否为nil或者h.count值是否为0，如果h为nil则表示未初始化，则可能panic，如果h.count=0,则表示map为空，则直接返回一个zero值。
    if h == nil || h.count == 0 {
        if t.hashMightPanic() {
            t.hasher(key, 0) // see issue 23734
        }
        return unsafe.Pointer(&zeroVal[0])
    }
      // 考虑是否处于并发读写状态，否则产生panic
    if h.flags&hashWriting != 0 {
        throw("concurrent map read and map write")
    }
      // 根据键key计算hash值
    hash := t.hasher(key, uintptr(h.hash0))
      // 计算低B位的掩码bucketMask(h.B)，比如 B=5，那 m 就是31，低五位二进制是全1
    m := bucketMask(h.B)
      // 计算当前bucket的地址
    b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
      // 根据h.oldbuckets判断是否处于扩容中，如果不是nil则表示当前map正处于扩容状态中
    if c := h.oldbuckets; c != nil {
        if !h.sameSizeGrow() {
            // There used to be half as many buckets; mask down one more power of two.
            m >>= 1
        }
        oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
        if !evacuated(oldb) {
            b = oldb
        }
    }
      // 计算tophash，即高八位
    top := tophash(hash)
    // 真正开始查找key,外层for是循环bucket及溢出桶overflow，内层for是循环桶内的8个slot
bucketloop:
    for ; b != nil; b = b.overflow(t) {
        for i := uintptr(0); i < bucketCnt; i++ {
            if b.tophash[i] != top {
                if b.tophash[i] == emptyRest {
                    break bucketloop
                }
                continue
            }
            k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
            if t.indirectkey() {
                k = *((*unsafe.Pointer)(k))
            }
            if t.key.equal(key, k) {
                e := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
                if t.indirectelem() {
                    e = *((*unsafe.Pointer)(e))
                }
                return e
            }
        }
    }
    return unsafe.Pointer(&zeroVal[0])
}

k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))

e := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))

dataOffset = unsafe.Offsetof(struct {
        b bmap
        v int64
    }{}.v)

    emptyRest      = 0 // 当前cell为空, 并且它后面的所有cell也为空, 包括溢出桶overflow
    emptyOne       = 1 // 当前cell为空
    evacuatedX     = 2 // key/elem is valid.  Entry has been evacuated to first half of larger table.
    evacuatedY     = 3 // 扩容相关，第二部分迁移完毕。
    evacuatedEmpty = 4 // 当前cell为空，且迁移完成。
    minTopHash     = 5 // tophash最小值，如果在调用 tophash(hash)时，计算出的值小于此值，则会加上此值

    // tophash calculates the tophash value for hash.
    func tophash(hash uintptr) uint8 {
        top := uint8(hash >> (sys.PtrSize*8 - 8))
        if top < minTopHash {
        top += minTopHash
        }
        return top
    }

mapassign_faststr(t *maptype, h *hmap, s string) unsafe.Pointer
mapassign_fast64(t *maptype, h *hmap, key uint64) unsafe.Pointer
mapassign_fast32(t *maptype, h *hmap, key uint32) unsafe.Pointer

// Like mapaccess, but allocates a slot for the key if it is not present in the map.
func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
    // 空map写入会触发panic
    if h == nil {
        panic(plainError("assignment to entry in nil map"))
    }
      // raceenabled-是否启用数据竞争检测
    if raceenabled {
        callerpc := getcallerpc()
        pc := funcPC(mapassign)
        racewritepc(unsafe.Pointer(h), callerpc, pc)
        raceReadObjectPC(t.key, key, callerpc, pc)
    }
    if msanenabled {
        msanread(key, t.key.size)
    }
      // 如果并发写入会触发panic
    if h.flags&hashWriting != 0 {
        throw("concurrent map writes")
    }
      // 计算hash值
    hash := t.hasher(key, uintptr(h.hash0))

    // Set hashWriting after calling t.hasher, since t.hasher may panic,
    // in which case we have not actually done a write.
      // 设置写入标志位
    h.flags ^= hashWriting

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

again:
      // 定位bucket
    bucket := hash & bucketMask(h.B)
    if h.growing() {
        growWork(t, h, bucket)
    }
    b := (*bmap)(add(h.buckets, bucket*uintptr(t.bucketsize)))
    top := tophash(hash)

    var inserti *uint8 // tophash的位置
    var insertk unsafe.Pointer // key的地址
    var elem unsafe.Pointer // val的地址
bucketloop:
    for {
        for i := uintptr(0); i < bucketCnt; i++ {
            if b.tophash[i] != top {
                if isEmpty(b.tophash[i]) && inserti == nil {
                    inserti = &b.tophash[i]
                    insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
                    elem = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
                }
                // 当前cell为空, 并且它后面的所有cell也为空, 包括溢出桶overflow, 可以退出循环了
                if b.tophash[i] == emptyRest {
                    break bucketloop
                }
                continue
            }
              // b.tophash[i] == top
            k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
            if t.indirectkey() {
                k = *((*unsafe.Pointer)(k))
            }
            if !t.key.equal(key, k) {
                continue
            }
            // already have a mapping for key. Update it.
            if t.needkeyupdate() {
                typedmemmove(t.key, k, key)
            }
            elem = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
            goto done
        }
        // 如果还有溢出桶，继续查询溢出桶
        ovf := b.overflow(t)
        if ovf == nil {
            break
        }
        b = ovf
    }

    // 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.
      // 如果负载因子到了最大或者有太多的溢出桶，并且不在扩容中，开始扩容
    if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
        hashGrow(t, h)
        goto again // Growing the table invalidates everything, so try again
    }

    if inserti == nil {
        // The current bucket and all the overflow buckets connected to it are full, allocate a new one.
        // 当前bucket和所有的溢出桶都满了，创建新的溢出桶
        newb := h.newoverflow(t, b)
        inserti = &newb.tophash[0]
        insertk = add(unsafe.Pointer(newb), dataOffset)
        elem = add(insertk, bucketCnt*uintptr(t.keysize))
    }

    // store new key/elem at insert position
    if t.indirectkey() {
        kmem := newobject(t.key)
        *(*unsafe.Pointer)(insertk) = kmem
        insertk = kmem
    }
    if t.indirectelem() {
        vmem := newobject(t.elem)
        *(*unsafe.Pointer)(elem) = vmem
    }
      // 写入key
    typedmemmove(t.key, insertk, key)
    *inserti = top
      // 哈希元素数量+1
    h.count++

done:
    if h.flags&hashWriting == 0 {
        throw("concurrent map writes")
    }
    h.flags &^= hashWriting
    if t.indirectelem() {
        elem = *((*unsafe.Pointer)(elem))
    }
    return elem
}

    var a = make(map[int]int, 7)
    for i := 0; i < 1000; i++ {
        a[i] = 99999
    }

    0x003f 00063 (m.go:9)    MOVQ    DX, (SP) // 第一个参数
    0x0043 00067 (m.go:9)    MOVQ    AX, 8(SP) // 第二个参数
    0x0048 00072 (m.go:9)    MOVQ    CX, 16(SP) // 第三个参数
    0x004d 00077 (m.go:9)    PCDATA    $0, $1 // GC 相关
    0x004d 00077 (m.go:9)    CALL    runtime.mapassign_fast64(SB) // 调用函数
    0x0052 00082 (m.go:9)    MOVQ    24(SP), AX // 返回值，即 value 应该存放的内存地址
    0x0057 00087 (m.go:9)    MOVQ    $99999, (AX) // 把 99999 放入该地址中

    if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
        hashGrow(t, h)
        goto again // Growing the table invalidates everything, so try again
    }

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)
  // 如果不是负载因子超出上限，那么进行的是一次等量扩容，bucket的数量不变
    if !overLoadFactor(h.count+1, h.B) {
        bigger = 0
    // sameSizeGrow，即为等量扩容
        h.flags |= sameSizeGrow
    }
    oldbuckets := h.buckets
  // 等量扩容bigger为0，bucket数不变。否则bigger为1，bucket数翻倍
    newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)

    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
    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().
  // 哈希表数据的实际复制是由 growWork() 和 evacuate() 逐步完成的
}

func growWork(t *maptype, h *hmap, bucket uintptr) {
    // 确保我们移动的 oldbucket 对应的是我们马上就要用到的那一个
    evacuate(t, h, bucket&h.oldbucketmask())

    // 如果还在 growing 状态，再多移动一个 oldbucket
    if h.growing() {
        evacuate(t, h, h.nevacuate)
    }
}

func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
    b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
    newbit := h.noldbuckets()
    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.
        // evacDst用于保存分配上下文
        // xy 包含的是移动的目标
        // x 表示新 bucket 数组的前(low)半部分
        // y 表示新 bucket 数组的后(high)半部分
        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*uintptr(t.keysize))

    // 如果是等量扩容，新旧bucket是一对一，不需要两个evacDst，而当哈希表的容量翻倍时，每个旧桶的元素会都分流到新创建的两个桶中
        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*uintptr(t.keysize))
        }

        for ; b != nil; b = b.overflow(t) {
            k := add(unsafe.Pointer(b), dataOffset)
            e := add(k, bucketCnt*uintptr(t.keysize))
            for i := 0; i < bucketCnt; i, k, e = i+1, add(k, uintptr(t.keysize)), add(e, uintptr(t.elemsize)) {
                top := b.tophash[i]
                if isEmpty(top) {
                    b.tophash[i] = evacuatedEmpty
                    continue
                }
                if top < minTopHash {
                    throw("bad map state")
                }
                k2 := k
                if t.indirectkey() {
                    k2 = *((*unsafe.Pointer)(k2))
                }
                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.hasher(k2, uintptr(h.hash0))
                    if h.flags&iterator != 0 && !t.reflexivekey() && !t.key.equal(k2, k2) {
                        // If key != key (NaNs), then the hash could be (and probably
                        // will be) entirely different from the old hash. Moreover,
                        // it isn't reproducible. Reproducibility is required in the
                        // presence of iterators, as our evacuation decision must
                        // match whatever decision the iterator made.
                        // Fortunately, we have the freedom to send these keys either
                        // way. Also, tophash is meaningless for these kinds of keys.
                        // We let the low bit of tophash drive the evacuation decision.
                        // We recompute a new random tophash for the next level so
                        // these keys will get evenly distributed across all buckets
                        // after multiple grows.
                        useY = top & 1
                        top = tophash(hash)
                    } else {
                        if hash&newbit != 0 {
                            useY = 1
                        }
                    }
                }

                if evacuatedX+1 != evacuatedY || evacuatedX^1 != evacuatedY {
                    throw("bad evacuatedN")
                }

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

                if dst.i == bucketCnt {
                    dst.b = h.newoverflow(t, dst.b)
                    dst.i = 0
                    dst.k = add(unsafe.Pointer(dst.b), dataOffset)
                    dst.e = add(dst.k, bucketCnt*uintptr(t.keysize))
                }
                dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
                if t.indirectkey() {
                    *(*unsafe.Pointer)(dst.k) = k2 // copy pointer
                } else {
                    typedmemmove(t.key, dst.k, k) // copy elem
                }
                if t.indirectelem() {
                    *(*unsafe.Pointer)(dst.e) = *(*unsafe.Pointer)(e)
                } else {
                    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, uintptr(t.keysize))
                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)
        }
    }

    if oldbucket == h.nevacuate {
        advanceEvacuationMark(h, t, newbit)
    }
}