update map

Author: cch123

map.md

@@ -492,65 +492,64 @@ func hashGrow(t *maptype, h *hmap) {
 ```
 
 ```go
-// 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.
+// makeBucketArray 为 map buckets 初始化底层数组
+// 1<<b 是需要分配的最小数量 buckets
+// dirtyalloc 要么是 nil，要么就是之前由 makeBucketArray 使用同样的 t 和 b 参数
+// 分配的数组
+// 如果 dirtyalloc 是 nil，那么就会分配一个新数组，否则会清空掉原来的 dirtyalloc
+// 然后重用这部分内存作为新的底层数组
 func makeBucketArray(t *maptype, b uint8, dirtyalloc unsafe.Pointer) (buckets unsafe.Pointer, nextOverflow *bmap) {
-	base := bucketShift(b)
-	nbuckets := base
-	// For small b, overflow buckets are unlikely.
-	// Avoid the overhead of the calculation.
-	if b >= 4 {
-		// Add on the estimated number of overflow buckets
-		// required to insert the median number of elements
-		// used with this value of b.
-		nbuckets += bucketShift(b - 4)
-		sz := t.bucket.size * nbuckets
-		up := roundupsize(sz)
-		if up != sz {
-			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.kind&kindNoPointers == 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.
-		nextOverflow = (*bmap)(add(buckets, base*uintptr(t.bucketsize)))
-		last := (*bmap)(add(buckets, (nbuckets-1)*uintptr(t.bucketsize)))
-		last.setoverflow(t, (*bmap)(buckets))
-	}
-	return buckets, nextOverflow
+    // base = 1 << b
+    base := bucketShift(b)
+    nbuckets := base
+    // 对于比较小的 b 来说，不太可能有 overflow buckets
+    // 这里省掉一些计算消耗
+    if b >= 4 {
+        // Add on the estimated number of overflow buckets
+        // required to insert the median number of elements
+        // used with this value of b.
+        nbuckets += bucketShift(b - 4)
+        sz := t.bucket.size * nbuckets
+        up := roundupsize(sz)
+        if up != sz {
+            nbuckets = up / t.bucket.size
+        }
+    }
+
+    if dirtyalloc == nil {
+        buckets = newarray(t.bucket, int(nbuckets))
+    } else {
+        // dirtyalloc 之前就是用上面的 newarray(t.bucket, int(nbuckets))
+        // 生成的，所以是非空
+        buckets = dirtyalloc
+        size := t.bucket.size * nbuckets
+        if t.bucket.kind&kindNoPointers == 0 {
+            memclrHasPointers(buckets, size)
+        } else {
+            memclrNoHeapPointers(buckets, size)
+        }
+    }
+
+    if base != nbuckets {
+        // 我们预分配了一些 overflow buckets
+        // 为了让追踪这些 overflow buckets 的成本最低
+        // 我们这里约定，如果预分配的 overflow bucket 的 overflow 指针是 nil
+        // 那么 there are more available by bumping the pointer.
+        // 我们需要一个安全的非空指针来作为 last overflow bucket，直接用 buckets 就行了
+        nextOverflow = (*bmap)(add(buckets, base*uintptr(t.bucketsize)))
+        last := (*bmap)(add(buckets, (nbuckets-1)*uintptr(t.bucketsize)))
+        last.setoverflow(t, (*bmap)(buckets))
+    }
+    return buckets, nextOverflow
 }
 ```
 
 ```go
 func growWork(t *maptype, h *hmap, bucket uintptr) {
-    // make sure we evacuate the oldbucket corresponding
-    // to the bucket we're about to use
+    // 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())
 
-    // evacuate one more oldbucket to make progress on growing
+    // 如果还在 growing 状态，再多移动一个 oldbucket
     if h.growing() {
         evacuate(t, h, h.nevacuate)
     }