// go/src/cmd/compile/internal/syntax/parser.go
// TypeSpec = identifier [ TypeParams ] [ "=" ] Type .
func (p *parser) typeDecl(group *Group) Decl {
    ...
    if p.tok == _Lbrack {
        // d.Name "[" ...
        // array/slice type or type parameter list
        pos := p.pos()
        p.next()
        switch p.tok {
        ...
        case _Rbrack:
            // d.Name "[" "]" ...
            p.next()
            d.Type = p.sliceType(pos)
        ...
        }
    } 
    ...
}
func (p *parser) sliceType(pos Pos) Expr {
    t := new(SliceType)
    t.pos = pos
    t.Elem = p.type_()
    return t
}
// go/src/cmd/compile/internal/syntax/nodes.go
type (
    ...
  // []Elem
    SliceType struct {
        Elem Expr
        expr
    }
  ...
)

// go/src/cmd/compile/internal/types/type.go
type Slice struct {
    Elem *Type // element type
}
func NewSlice(elem *Type) *Type {
    if t := elem.cache.slice; t != nil {
        if t.Elem() != elem {
            base.Fatalf("elem mismatch")
        }
        if elem.HasTParam() != t.HasTParam() || elem.HasShape() != t.HasShape() {
            base.Fatalf("Incorrect HasTParam/HasShape flag for cached slice type")
        }
        return t
    }
    t := newType(TSLICE)
    t.extra = Slice{Elem: elem}
    elem.cache.slice = t
    if elem.HasTParam() {
        t.SetHasTParam(true)
    }
    if elem.HasShape() {
        t.SetHasShape(true)
    }
    return t
}

litSlic := []int{1,2,3,4}  // 字面量初始化
makeSlic := make([]int,0)  // make初始化

// go/src/cmd/compile/internal/walk/complit.go
// walkCompLit walks a composite literal node:
// OARRAYLIT, OSLICELIT, OMAPLIT, OSTRUCTLIT (all CompLitExpr), or OPTRLIT (AddrExpr).
func walkCompLit(n ir.Node, init *ir.Nodes) ir.Node {
    if isStaticCompositeLiteral(n) && !ssagen.TypeOK(n.Type()) {
        n := n.(*ir.CompLitExpr) // not OPTRLIT
        // n can be directly represented in the read-only data section.
        // Make direct reference to the static data. See issue 12841.
        vstat := readonlystaticname(n.Type())
        fixedlit(inInitFunction, initKindStatic, n, vstat, init)
        return typecheck.Expr(vstat)
    }
    var_ := typecheck.Temp(n.Type())
    anylit(n, var_, init)
    return var_
}

// go/src/cmd/compile/internal/typecheck/expr.go
func tcCompLit(n *ir.CompLitExpr) (res ir.Node) {
    ...
    t := n.Type()
    base.AssertfAt(t != nil, n.Pos(), "missing type in composite literal")

    switch t.Kind() {
    ...
    case types.TSLICE:
        length := typecheckarraylit(t.Elem(), -1, n.List, "slice literal")
        n.SetOp(ir.OSLICELIT)
        n.Len = length
    ...
  }
    return n
}

// go/src/cmd/compile/internal/walk/complit.go
func anylit(n ir.Node, var_ ir.Node, init *ir.Nodes) {
    t := n.Type()
    switch n.Op() {
  ...
  case ir.OSLICELIT:
        n := n.(*ir.CompLitExpr)
        slicelit(inInitFunction, n, var_, init)
  ...
  }
}
func slicelit(ctxt initContext, n *ir.CompLitExpr, var_ ir.Node, init *ir.Nodes) {
    // make an array type corresponding the number of elements we have
    t := types.NewArray(n.Type().Elem(), n.Len)
    types.CalcSize(t)

    if ctxt == inNonInitFunction {
        // put everything into static array
        vstat := staticinit.StaticName(t)

        fixedlit(ctxt, initKindStatic, n, vstat, init)
        fixedlit(ctxt, initKindDynamic, n, vstat, init)

        // copy static to slice
        var_ = typecheck.AssignExpr(var_)
        name, offset, ok := staticinit.StaticLoc(var_)
        if !ok || name.Class != ir.PEXTERN {
            base.Fatalf("slicelit: %v", var_)
        }
        staticdata.InitSlice(name, offset, vstat.Linksym(), t.NumElem())
        return
    }

    // recipe for var = []t{...}
    // 1. make a static array
    //  var vstat [...]t
    // 2. assign (data statements) the constant part
    //  vstat = constpart{}
    // 3. make an auto pointer to array and allocate heap to it
    //  var vauto *[...]t = new([...]t)
    // 4. copy the static array to the auto array
    //  *vauto = vstat
    // 5. for each dynamic part assign to the array
    //  vauto[i] = dynamic part
    // 6. assign slice of allocated heap to var
    //  var = vauto[:]
    //
    // an optimization is done if there is no constant part
    //  3. var vauto *[...]t = new([...]t)
    //  5. vauto[i] = dynamic part
    //  6. var = vauto[:]

    // if the literal contains constants,
    // make static initialized array (1),(2)
    var vstat ir.Node

    mode := getdyn(n, true)
    if mode&initConst != 0 && !isSmallSliceLit(n) {
        if ctxt == inInitFunction {
            vstat = readonlystaticname(t)
        } else {
            vstat = staticinit.StaticName(t)
        }
        fixedlit(ctxt, initKindStatic, n, vstat, init)
    }

    // make new auto *array (3 declare)
    vauto := typecheck.Temp(types.NewPtr(t))

    // set auto to point at new temp or heap (3 assign)
    var a ir.Node
    if x := n.Prealloc; x != nil {
        // temp allocated during order.go for dddarg
        if !types.Identical(t, x.Type()) {
            panic("dotdotdot base type does not match order's assigned type")
        }
        a = initStackTemp(init, x, vstat)
    } else if n.Esc() == ir.EscNone {
        a = initStackTemp(init, typecheck.Temp(t), vstat)
    } else {
        a = ir.NewUnaryExpr(base.Pos, ir.ONEW, ir.TypeNode(t))
    }
    appendWalkStmt(init, ir.NewAssignStmt(base.Pos, vauto, a))

    if vstat != nil && n.Prealloc == nil && n.Esc() != ir.EscNone {
        // If we allocated on the heap with ONEW, copy the static to the
        // heap (4). We skip this for stack temporaries, because
        // initStackTemp already handled the copy.
        a = ir.NewStarExpr(base.Pos, vauto)
        appendWalkStmt(init, ir.NewAssignStmt(base.Pos, a, vstat))
    }

    // put dynamics into array (5)
    var index int64
    for _, value := range n.List {
        if value.Op() == ir.OKEY {
            kv := value.(*ir.KeyExpr)
            index = typecheck.IndexConst(kv.Key)
            if index < 0 {
                base.Fatalf("slicelit: invalid index %v", kv.Key)
            }
            value = kv.Value
        }
        a := ir.NewIndexExpr(base.Pos, vauto, ir.NewInt(index))
        a.SetBounded(true)
        index++
        // TODO need to check bounds?
        switch value.Op() {
        case ir.OSLICELIT:
            break
        case ir.OARRAYLIT, ir.OSTRUCTLIT:
            value := value.(*ir.CompLitExpr)
            k := initKindDynamic
            if vstat == nil {
                // Generate both static and dynamic initializations.
                // See issue #31987.
                k = initKindLocalCode
            }
            fixedlit(ctxt, k, value, a, init)
            continue
        }
        if vstat != nil && ir.IsConstNode(value) { // already set by copy from static value
            continue
        }
        // build list of vauto[c] = expr
        ir.SetPos(value)
        as := ir.NewAssignStmt(base.Pos, a, value)
        appendWalkStmt(init, orderStmtInPlace(typecheck.Stmt(as), map[string][]*ir.Name{}))
    }
    // make slice out of heap (6)
    a = ir.NewAssignStmt(base.Pos, var_, ir.NewSliceExpr(base.Pos, ir.OSLICE, vauto, nil, nil, nil))
    appendWalkStmt(init, orderStmtInPlace(typecheck.Stmt(a), map[string][]*ir.Name{}))
}

// go/src/cmd/compile/internal/walk/expr.go
func walkExpr1(n ir.Node, init *ir.Nodes) ir.Node {
    switch n.Op() {
    ...
    case ir.OMAKESLICE:
        n := n.(*ir.MakeExpr)
        return walkMakeSlice(n, init)
    ...
}

// go/src/cmd/compile/internal/walk/builtin.go
func walkMakeSlice(n *ir.MakeExpr, init *ir.Nodes) ir.Node {
    l := n.Len
    r := n.Cap
    if r == nil {
        r = safeExpr(l, init)
        l = r
    }
    ...
    if n.Esc() == ir.EscNone {
        if why := escape.HeapAllocReason(n); why != "" {
            base.Fatalf("%v has EscNone, but %v", n, why)
        }
        // var arr [r]T
        // n = arr[:l]
        i := typecheck.IndexConst(r)
        if i < 0 {
            base.Fatalf("walkExpr: invalid index %v", r)
        }
        ...
        t = types.NewArray(t.Elem(), i) // [r]T
        var_ := typecheck.Temp(t)
        appendWalkStmt(init, ir.NewAssignStmt(base.Pos, var_, nil))  // zero temp
        r := ir.NewSliceExpr(base.Pos, ir.OSLICE, var_, nil, l, nil) // arr[:l]
        // The conv is necessary in case n.Type is named.
        return walkExpr(typecheck.Expr(typecheck.Conv(r, n.Type())), init)
    }
    // n escapes; set up a call to makeslice.
    // When len and cap can fit into int, use makeslice instead of
    // makeslice64, which is faster and shorter on 32 bit platforms.
    len, cap := l, r
    fnname := "makeslice64"
    argtype := types.Types[types.TINT64]
    // Type checking guarantees that TIDEAL len/cap are positive and fit in an int.
    // The case of len or cap overflow when converting TUINT or TUINTPTR to TINT
    // will be handled by the negative range checks in makeslice during runtime.
    if (len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size()) &&
        (cap.Type().IsKind(types.TIDEAL) || cap.Type().Size() <= types.Types[types.TUINT].Size()) {
        fnname = "makeslice"
        argtype = types.Types[types.TINT]
    }
    fn := typecheck.LookupRuntime(fnname)
    ptr := mkcall1(fn, types.Types[types.TUNSAFEPTR], init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(len, argtype), typecheck.Conv(cap, argtype))
    ptr.MarkNonNil()
    len = typecheck.Conv(len, types.Types[types.TINT])
    cap = typecheck.Conv(cap, types.Types[types.TINT])
    sh := ir.NewSliceHeaderExpr(base.Pos, t, ptr, len, cap)
    return walkExpr(typecheck.Expr(sh), init)
}

p := new(T)          
p := &T{}           
s := make([]T, n)    
s := []byte("...") 

// go/src/cmd/compile/internal/ir/cfg.go
var (
    // maximum size variable which we will allocate on the stack.
    // This limit is for explicit variable declarations like "var x T" or "x := ...".
    // Note: the flag smallframes can update this value.
    MaxStackVarSize = int64(10 * 1024 * 1024)
    // maximum size of implicit variables that we will allocate on the stack.
    //   p := new(T)          allocating T on the stack
    //   p := &T{}            allocating T on the stack
    //   s := make([]T, n)    allocating [n]T on the stack
    //   s := []byte("...")   allocating [n]byte on the stack
    // Note: the flag smallframes can update this value.
    MaxImplicitStackVarSize = int64(64 * 1024)
    // MaxSmallArraySize is the maximum size of an array which is considered small.
    // Small arrays will be initialized directly with a sequence of constant stores.
    // Large arrays will be initialized by copying from a static temp.
    // 256 bytes was chosen to minimize generated code + statictmp size.
    MaxSmallArraySize = int64(256)
)

// go/src/runtime/slice.go
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)
}

// go/src/runtime/slice.go
type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}
// 或者
// go/src/reflect/value.go
// SliceHeader is the runtime representation of a slice.
type SliceHeader struct {
    Data uintptr
    Len  int
    Cap  int
}

slic := []int{1, 2, 3, 4, 5}
slic1 := slic[:2]
fmt.Printf("slic1: %v\n", slic1)
slic[0] = 0
fmt.Printf("slic: %v\n", slic)
fmt.Printf("slic1: %v\n", slic1)
// slic1: [1 2]
// slic: [0 2 3 4 5]
// slic1: [0 2]

slic := []int{1, 2, 3, 4, 5}
slic1 := slic[:2]
slic2 := slic[2:]
fmt.Printf("len(slic): %v\n", len(slic))
fmt.Printf("cap(slic): %v\n", cap(slic))
fmt.Printf("len(slic1): %v\n", len(slic1))
fmt.Printf("cap(slic1): %v\n", cap(slic1))
fmt.Printf("len(slic2): %v\n", len(slic2))
fmt.Printf("cap(slic2): %v\n", cap(slic2))
// len(slic): 5
// cap(slic): 5

// len(slic1): 2
// cap(slic1): 5

// len(slic2): 3
// cap(slic2): 3

slic := []int{1, 2, 3, 4, 5}
s := slic  // 等價于 s := slic[:]

slic := []int{1, 2, 3, 4, 5}
var slic1 []int
copy(slic1, slic)
fmt.Printf("slic: %p\n", slic)
fmt.Printf("slic1: %p\n", slic1)
// slic: 0xc0000aa030
// slic1: 0x0

// go/src/runtime/slice.go
func slicecopy(toPtr unsafe.Pointer, toLen int, fromPtr unsafe.Pointer, fromLen int, width uintptr) int {
    if fromLen == 0 || toLen == 0 {
        return 0
    }

    n := fromLen
    if toLen < n {
        n = toLen
    }
    ...
    if size == 1 { // common case worth about 2x to do here
        // TODO: is this still worth it with new memmove impl?
        *(*byte)(toPtr) = *(*byte)(fromPtr) // known to be a byte pointer
    } else {
        memmove(toPtr, fromPtr, size)
    }
    return n
}

func growslice(et *_type, old slice, cap int) slice {
    ...
    newcap := old.cap
    doublecap := newcap + newcap
    if cap > doublecap {
        newcap = cap
    } else {
        const threshold = 256
        if old.cap < threshold {
            newcap = doublecap
        } 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
            }
            // Set newcap to the requested cap when
            // the newcap calculation overflowed.
            if newcap <= 0 {
                newcap = cap
            }
        }
    }
    ...
    memmove(p, old.array, lenmem)
    return slice{p, old.len, newcap}
}

var slic []int
oldCap := cap(slic)
for i := 0; i < 2048; i++ {
  slic = append(slic, i)
  newCap := cap(slic)
  grow := float32(newCap) / float32(oldCap)
  if newCap != oldCap {
    fmt.Printf("len(slic):%v cap(slic):%v grow:%v %p\n", len(slic), cap(slic), grow, slic)
  }
  oldCap = newCap
}
// len(slic):1     cap(slic):1     grow:+Inf       0xc0000140c0
// len(slic):2     cap(slic):2     grow:2          0xc0000140e0
// len(slic):3     cap(slic):4     grow:2          0xc000020100
// len(slic):5     cap(slic):8     grow:2          0xc00001e340
// len(slic):9     cap(slic):16    grow:2          0xc000026080
// len(slic):17    cap(slic):32    grow:2          0xc00007e000
// len(slic):33    cap(slic):64    grow:2          0xc000100000
// len(slic):65    cap(slic):128   grow:2          0xc000102000
// len(slic):129   cap(slic):256   grow:2          0xc000104000
// len(slic):257   cap(slic):512   grow:2          0xc000106000
// len(slic):513   cap(slic):1024  grow:2          0xc000108000
// len(slic):1025  cap(slic):1280  grow:1.25       0xc00010a000
// len(slic):1281  cap(slic):1696  grow:1.325      0xc000114000
// len(slic):1697  cap(slic):2304  grow:1.3584906  0xc00011e000

type Slice struct {
  Elem *Type // element type
}

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