依据大多数资料,slice的扩容机制是当切片的容量小于1024时,进行双倍扩容;当大于1024时,进行1.25倍扩容,见如下代码:

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var sli = []int{}

sli = append(sli, 666)

fmt.Println(cap(sli))

sli = append(sli, 777)

fmt.Println(cap(sli))

sli = append(sli, 888)

fmt.Println(sli)

fmt.Println(cap(sli))

sli = append(sli, 999)

fmt.Println(cap(sli))

sli = append(sli, 1000)

fmt.Println(cap(sli))

输出为:

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1
2
[666 777 888]
4
4
8

又见如下代码:

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var sli2 = []int{}

for i := 0; i < 10; i++ {
	sli2 = append(sli2, i)
}

fmt.Println(sli2)

fmt.Println(len(sli2))

fmt.Println(cap(sli2))

输出为:

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3
[0 1 2 3 4 5 6 7 8 9]
10
16

用更”准确”的话描述,是当cap<1024时,cap的值一定是2的n次方,且cap>=len;每当发生append使len增加,如果导致len>cap,此时cap会先于append操作进行double

即有

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var sli3 = []int{}

for i := 0; i < 512; i++ {
	sli3 = append(sli3, i)
}

fmt.Println(len(sli3))

fmt.Println(cap(sli3))

sli3 = append(sli3, 123)

fmt.Println(len(sli3))

fmt.Println(cap(sli3))

结果为:

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4
512
512
513
1024

对于

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var sli4 = []int{}

for i := 0; i < 1024; i++ {
	sli4 = append(sli4, i)
}

fmt.Println(len(sli4))

fmt.Println(cap(sli4))

sli4 = append(sli4, 123)

fmt.Println(len(sli4))

fmt.Println(cap(sli4))

结果为:

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1024
1024
1025
1280

1024*1.25 = 1280

看似无懈可击的结果,然鹅,果真确凿如此吗?

且看切片扩容的源码,即$GOROOT/src/runtime/slice.go中的growslice方法,

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// growslice handles slice growth during append.
// It is passed the slice element type, the old slice, and the desired new minimum capacity,
// and it returns a new slice with at least that capacity, with the old data
// copied into it.
// The new slice's length is set to the old slice's length,
// NOT to the new requested capacity.
// This is for codegen convenience. The old slice's length is used immediately
// to calculate where to write new values during an append.
// TODO: When the old backend is gone, reconsider this decision.
// The SSA backend might prefer the new length or to return only ptr/cap and save stack space.
func growslice(et *_type, old slice, cap int) slice {
	if raceenabled {
		callerpc := getcallerpc()
		racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice))
	}
	if msanenabled {
		msanread(old.array, uintptr(old.len*int(et.size)))
	}

	if cap < old.cap {
		panic(errorString("growslice: cap out of range"))
	}

	if et.size == 0 {
		// append should not create a slice with nil pointer but non-zero len.
		// We assume that append doesn't need to preserve old.array in this case.
		return slice{unsafe.Pointer(&zerobase), old.len, cap}
	}

	newcap := old.cap
	doublecap := newcap + newcap
	if cap > doublecap {
		newcap = cap
	} else {
		if old.len < 1024 {
			newcap = doublecap
		} else {
			// Check 0 < newcap to detect overflow
			// and prevent an infinite loop.
			for 0 < newcap && newcap < cap {
				newcap += newcap / 4
			}
			// Set newcap to the requested cap when
			// the newcap calculation overflowed.
			if newcap <= 0 {
				newcap = cap
			}
		}
	}

	var overflow bool
	var lenmem, newlenmem, capmem uintptr
	// Specialize for common values of et.size.
	// For 1 we don't need any division/multiplication.
	// For sys.PtrSize, compiler will optimize division/multiplication into a shift by a constant.
	// For powers of 2, use a variable shift.
	switch {
	case et.size == 1:
		lenmem = uintptr(old.len)
		newlenmem = uintptr(cap)
		capmem = roundupsize(uintptr(newcap))
		overflow = uintptr(newcap) > maxAlloc
		newcap = int(capmem)
	case et.size == sys.PtrSize:
		lenmem = uintptr(old.len) * sys.PtrSize
		newlenmem = uintptr(cap) * sys.PtrSize
		capmem = roundupsize(uintptr(newcap) * sys.PtrSize)
		overflow = uintptr(newcap) > maxAlloc/sys.PtrSize
		newcap = int(capmem / sys.PtrSize)
	case isPowerOfTwo(et.size):
		var shift uintptr
		if sys.PtrSize == 8 {
			// Mask shift for better code generation.
			shift = uintptr(sys.Ctz64(uint64(et.size))) & 63
		} else {
			shift = uintptr(sys.Ctz32(uint32(et.size))) & 31
		}
		lenmem = uintptr(old.len) << shift
		newlenmem = uintptr(cap) << shift
		capmem = roundupsize(uintptr(newcap) << shift)
		overflow = uintptr(newcap) > (maxAlloc >> shift)
		newcap = int(capmem >> shift)
	default:
		lenmem = uintptr(old.len) * et.size
		newlenmem = uintptr(cap) * et.size
		capmem, overflow = math.MulUintptr(et.size, uintptr(newcap))
		capmem = roundupsize(capmem)
		newcap = int(capmem / et.size)
	}

	// The check of overflow in addition to capmem > maxAlloc is needed
	// to prevent an overflow which can be used to trigger a segfault
	// on 32bit architectures with this example program:
	//
	// type T [1<<27 + 1]int64
	//
	// var d T
	// var s []T
	//
	// func main() {
	//   s = append(s, d, d, d, d)
	//   print(len(s), "\n")
	// }
	if overflow || capmem > maxAlloc {
		panic(errorString("growslice: cap out of range"))
	}

	var p unsafe.Pointer
	if et.kind&kindNoPointers != 0 {
		p = mallocgc(capmem, nil, false)
		// The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length).
		// Only clear the part that will not be overwritten.
		memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
	} else {
		// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
		p = mallocgc(capmem, et, true)
		if writeBarrier.enabled {
			// Only shade the pointers in old.array since we know the destination slice p
			// only contains nil pointers because it has been cleared during alloc.
			bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem)
		}
	}
	memmove(p, old.array, lenmem)

	return slice{p, old.len, newcap}
}

上面的操作是每次append一个元素，考虑另一种情形，一次性append很多元素，会发生什么呢？

当同时append进多个元素时,如下:

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package main

import "fmt"

func main() {
	a := []byte{1, 0}
	a = append(a, 1, 1, 1)
	fmt.Println("cap of a is ", cap(a))

	b := []int{23, 51}
	b = append(b, 4, 5, 6)
	fmt.Println("cap of b is ", cap(b))

	c := []int32{1, 23}
	c = append(c, 2, 5, 6)
	fmt.Println("cap of c is ", cap(c))

	type D struct {
		age  byte
		name string
	}
	d := []D{
		{1, "123"},
		{2, "234"},
	}

	d = append(d, D{4, "456"}, D{5, "567"}, D{6, "678"})
	fmt.Println("cap of d is ", cap(d))

}

结果为:

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cap of a is  8
cap of b is  6
cap of c is  8
cap of d is  5

匪夷所思…

在此使用gdb调试工具进行调试