slice.go - 理解Go的切片容器

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阅读Go源码，理解内置切片（slice）容器的数据结构与算法原理。

slice.go - 理解Go的切片容器

Slice的实现位于go.go，总共仅318行。

本文以目前Go源码最新的1.17.2版本为例。

数据结构

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type slice struct {
	array unsafe.Pointer
	len   int
	cap   int
}

slice的数据结构并不复杂，本质上是对array的一层封装，类似Java中的ArrayList。

slice底层数据由array存储，由len标记当前实际存储的元素数量，cap标记当前array指针指向的内存对象的元素容量。

算法

构造（makeslice）

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func makeslice(et *_type, len, cap int) unsafe.Pointer {
	mem, overflow := math.MulUintptr(et.size, uintptr(cap))
	if overflow || mem > maxAlloc || len < 0 || len > cap {
		// NOTE: Produce a 'len out of range' error instead of a
		// 'cap out of range' error when someone does make([]T, bignumber).
		// 'cap out of range' is true too, but since the cap is only being
		// supplied implicitly, saying len is clearer.
		// See golang.org/issue/4085.
		mem, overflow := math.MulUintptr(et.size, uintptr(len))
		if overflow || mem > maxAlloc || len < 0 {
			panicmakeslicelen()
		}
		panicmakeslicecap()
	}

	return mallocgc(mem, et, true)
}

构造过程输入et，即ElementType的缩写，用于记录slice中存储的元素类型、

首先，通过math.MulUintptr函数实现带溢出检测的uintptr类型乘法。

https://pkg.go.dev/runtime/internal/math#MulUintptr

https://cs.opensource.google/go/go/+/go1.17.2:src/runtime/internal/math/math.go;l=13

math.MulUintptr函数的实现挺巧妙的，此处暂不深究

随后，根据计算出的内存长度，通过mallocgc函数（位于go.go中，基于TCMalloc机制实现）分配相应的内存对象。

扩容（growslice）

slice能够在append时自动扩容。

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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.cap < 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.ptrdata == 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 lenmem > 0 && 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-et.size+et.ptrdata)
		}
	}
	memmove(p, old.array, lenmem)

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

在扩容时，如果新容量已经超过现有容量的两倍，则以更大的新容量为准。

如果指定的新容量不足两倍，则分两种情况：

  1. 如果现有容量较小（<1024），那就直接容量翻倍2x（直接成倍增长的策略有助于避免频繁扩容，而容量较小时，即使有空间冗余浪费，也是比较少的）；
  2. 如果现有容量不小了（>=1024），此时翻倍式扩容可能会浪费较多的内存，因此以1.25x渐进式增长至不低于目标容量，既满足目标容量，又避免浪费内存。

runtime: make slice growth formula a bit smoother

不过值得注意的是，这样的扩容算法未必是最优的，仍然存在改进的研究空间。从master分支上最新commit中可以看到，新的commit正在尝试更平滑的扩容函数（及参数）。高的增长倍率，一方面有助于避免频繁扩容（避免分配内存时潜在的系统调用代价），另一方面也更容易造成内存冗余。

此后，计算新slice的array所需的内存容量capmen和相应的元素容量newcap。（该计算过程针对元素尺寸做了优化）

最后，通过mallocgc函数申请capmem尺寸的内存对象，并且用memmove函数将原slice数据拷贝到新slice的内存（指针p）中。