foreword
The Go language does not have an encapsulation like ArrayList, but the official natively provides slices, the bottom layer is array storage, and can be automatically expanded. Compared with the default 10 and expanded 5 of ArrayList, the logic of slices is different. The default capacity of slice is 0.
demo
go version number
huahua@Mac-mini ~ % go version
go version go1.18.1 darwin/amd64
func main() {
var s0 []int
fmt.Println(s0, len(s0), cap(s0))
s1 := []int{1, 2, 3}
fmt.Println(s1, len(s1), cap(s1))
s1 = append(s1, 1, 2, 3, 4)
fmt.Println(s1, len(s1), cap(s1))
}The result is as follows
It can be seen that the default capacity of go slice is 0, and the capacity can be inferred based on the actual data length.
When expanding, the default expansion is 2 times the length (premise <256). If it exceeds 2 times the current capacity, then the expansion capacity is an even value of the actual length (if the length is an odd number, then add 1 ). If the capacity exceeds 256, the actual execution 1.25x expansion until the capacity is sufficient for storage.
If it exceeds twice the current capacity, then the expansion capacity is an even value of the actual length
This may be related to memory allocation, you can see the source code analysis below
Source code analysis
go/src/runtime/slice.go
func growslice(et *_type, old slice, cap int) slice {
//...省略...
newcap := old.cap //旧容量
doublecap := newcap + newcap //2倍扩容
if cap > doublecap {
newcap = cap //如果超过，就用实际容量
} else {
const threshold = 256
if old.cap < threshold {
newcap = doublecap //小于256，双倍扩容
} else {
// Check 0 < newcap to detect overflow
// and prevent an infinite loop.
for 0 < newcap && newcap < cap {
// Transition from growing 2x for small slices
// to growing 1.25x for large slices. This formula
// gives a smooth-ish transition between the two.
newcap += (newcap + 3*threshold) / 4 //上面的注释很明显 1.25X，知道容量足够
}
// Set newcap to the requested cap when
// the newcap calculation overflowed.
if newcap <= 0 {
newcap = cap
}
}
}
//...省略...
memmove(p, old.array, lenmem)
return slice{p, old.len, newcap}
}This is the principle of capacity expansion, so why allocate even capacity when the required capacity is odd? It is related to memory allocation.
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 goarch.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 == goarch.PtrSize:
lenmem = uintptr(old.len) * goarch.PtrSize
newlenmem = uintptr(cap) * goarch.PtrSize
capmem = roundupsize(uintptr(newcap) * goarch.PtrSize)
overflow = uintptr(newcap) > maxAlloc/goarch.PtrSize
newcap = int(capmem / goarch.PtrSize)
case isPowerOfTwo(et.size):
var shift uintptr
if goarch.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)
}Related to roundupsize, optimization is everywhere, which involves the concept of memory page and the logic of memory allocation
func roundupsize(size uintptr) uintptr {
if size < _MaxSmallSize {
if size <= smallSizeMax-8 {
return uintptr(class_to_size[size_to_class8[divRoundUp(size, smallSizeDiv)]])
} else {
return uintptr(class_to_size[size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]])
}
}
if size+_PageSize < size {
return size
}
return alignUp(size, _PageSize)
}The author deducted this part of the code by himself.
package runtime_demo
func Calc(cap int) int {
capmem := roundupsize(uintptr(cap))
return int(capmem)
}
const (
_MaxSmallSize = 32768
smallSizeDiv = 8
smallSizeMax = 1024
largeSizeDiv = 128
_NumSizeClasses = 68
_PageShift = 13
_PageSize = 1 << _PageShift
)
var class_to_size = [_NumSizeClasses]uint16{0, 8, 16, 24, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256, 288, 320, 352, 384, 416, 448, 480, 512, 576, 640, 704, 768, 896, 1024, 1152, 1280, 1408, 1536, 1792, 2048, 2304, 2688, 3072, 3200, 3456, 4096, 4864, 5376, 6144, 6528, 6784, 6912, 8192, 9472, 9728, 10240, 10880, 12288, 13568, 14336, 16384, 18432, 19072, 20480, 21760, 24576, 27264, 28672, 32768}
var class_to_allocnpages = [_NumSizeClasses]uint8{0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 1, 3, 2, 3, 1, 3, 2, 3, 4, 5, 6, 1, 7, 6, 5, 4, 3, 5, 7, 2, 9, 7, 5, 8, 3, 10, 7, 4}
var class_to_divmagic = [_NumSizeClasses]uint32{0, ^uint32(0)/8 + 1, ^uint32(0)/16 + 1, ^uint32(0)/24 + 1, ^uint32(0)/32 + 1, ^uint32(0)/48 + 1, ^uint32(0)/64 + 1, ^uint32(0)/80 + 1, ^uint32(0)/96 + 1, ^uint32(0)/112 + 1, ^uint32(0)/128 + 1, ^uint32(0)/144 + 1, ^uint32(0)/160 + 1, ^uint32(0)/176 + 1, ^uint32(0)/192 + 1, ^uint32(0)/208 + 1, ^uint32(0)/224 + 1, ^uint32(0)/240 + 1, ^uint32(0)/256 + 1, ^uint32(0)/288 + 1, ^uint32(0)/320 + 1, ^uint32(0)/352 + 1, ^uint32(0)/384 + 1, ^uint32(0)/416 + 1, ^uint32(0)/448 + 1, ^uint32(0)/480 + 1, ^uint32(0)/512 + 1, ^uint32(0)/576 + 1, ^uint32(0)/640 + 1, ^uint32(0)/704 + 1, ^uint32(0)/768 + 1, ^uint32(0)/896 + 1, ^uint32(0)/1024 + 1, ^uint32(0)/1152 + 1, ^uint32(0)/1280 + 1, ^uint32(0)/1408 + 1, ^uint32(0)/1536 + 1, ^uint32(0)/1792 + 1, ^uint32(0)/2048 + 1, ^uint32(0)/2304 + 1, ^uint32(0)/2688 + 1, ^uint32(0)/3072 + 1, ^uint32(0)/3200 + 1, ^uint32(0)/3456 + 1, ^uint32(0)/4096 + 1, ^uint32(0)/4864 + 1, ^uint32(0)/5376 + 1, ^uint32(0)/6144 + 1, ^uint32(0)/6528 + 1, ^uint32(0)/6784 + 1, ^uint32(0)/6912 + 1, ^uint32(0)/8192 + 1, ^uint32(0)/9472 + 1, ^uint32(0)/9728 + 1, ^uint32(0)/10240 + 1, ^uint32(0)/10880 + 1, ^uint32(0)/12288 + 1, ^uint32(0)/13568 + 1, ^uint32(0)/14336 + 1, ^uint32(0)/16384 + 1, ^uint32(0)/18432 + 1, ^uint32(0)/19072 + 1, ^uint32(0)/20480 + 1, ^uint32(0)/21760 + 1, ^uint32(0)/24576 + 1, ^uint32(0)/27264 + 1, ^uint32(0)/28672 + 1, ^uint32(0)/32768 + 1}
var size_to_class8 = [smallSizeMax/smallSizeDiv + 1]uint8{0, 1, 2, 3, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 23, 23, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 28, 29, 29, 29, 29, 29, 29, 29, 29, 30, 30, 30, 30, 30, 30, 30, 30, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32}
var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv + 1]uint8{32, 33, 34, 35, 36, 37, 37, 38, 38, 39, 39, 40, 40, 40, 41, 41, 41, 42, 43, 43, 44, 44, 44, 44, 44, 45, 45, 45, 45, 45, 45, 46, 46, 46, 46, 47, 47, 47, 47, 47, 47, 48, 48, 48, 49, 49, 50, 51, 51, 51, 51, 51, 51, 51, 51, 51, 51, 52, 52, 52, 52, 52, 52, 52, 52, 52, 52, 53, 53, 54, 54, 54, 54, 55, 55, 55, 55, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 57, 57, 57, 57, 57, 57, 57, 57, 57, 57, 58, 58, 58, 58, 58, 58, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 61, 61, 61, 61, 61, 62, 62, 62, 62, 62, 62, 62, 62, 62, 62, 62, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67}
func roundupsize(size uintptr) uintptr {
if size < _MaxSmallSize {
if size <= smallSizeMax-8 {
return uintptr(class_to_size[size_to_class8[divRoundUp(size, smallSizeDiv)]])
} else {
return uintptr(class_to_size[size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]])
}
}
if size+_PageSize < size {
return size
}
return alignUp(size, _PageSize)
}
func alignUp(n, a uintptr) uintptr {
return (n + a - 1) &^ (a - 1)
}
func divRoundUp(n, a uintptr) uintptr {
// a is generally a power of two. This will get inlined and
// the compiler will optimize the division.
return (n + a - 1) / a
}
Slice Expansion Principle Summary
Arrays are inherently non-expandable. The expansion of an array is actually to create a new array, allocate new memory, and then execute a copy of the array. Therefore, a slice actually needs to return the new memory address of the array, and the pointer points to the new memory address.
When expanding:
If the current required capacity is less than the current capacity, there is no need to expand; otherwise, double the capacity
The current required capacity is twice the current capacity, directly expand to the current capacity
If the required capacity is less than 256, it is directly expanded by 2 times
If the required capacity is greater than 256, expand the capacity by 1.25x until the capacity is sufficient
The actual expanded capacity will be optimized according to the memory allocation page, and the odd-numbered capacity will be allocated to the even-numbered capacity