func walkselectcases(cases *Nodes) []*Node {
ncas := cases.Len()
sellineno := lineno
// optimization: zero-case select
// 針對沒有case的select優(yōu)化
if ncas == 0 {
return []*Node{mkcall("block", nil, nil)}
}
// optimization: one-case select: single op.
// 針對1個case（單個操作）select的優(yōu)化
if ncas == 1 {
cas := cases.First()
setlineno(cas)
l := cas.Ninit.Slice()
if cas.Left != nil { // not default: 非default case
n := cas.Left // 獲取case表達(dá)式
l = append(l, n.Ninit.Slice()...)
n.Ninit.Set(nil)
switch n.Op {
default:
Fatalf("select %v", n.Op)
case OSEND: // Left - Right
// already ok
// n中已包含left/right
case OSELRECV, OSELRECV2: // OSELRECV(Left = -Right.Left) OSELRECV2(List = -Right.Left)
if n.Op == OSELRECV || n.List.Len() == 0 { // 左側(cè)有0或1個接收者
if n.Left == nil { // 沒有接收者
n = n.Right // 只需保留右側(cè)
} else { //
n.Op = OAS // 只有一個接收者，更新Op為OAS
}
break
}
if n.Left == nil { // 檢查是否表達(dá)式或賦值
nblank = typecheck(nblank, ctxExpr|ctxAssign)
n.Left = nblank
}
n.Op = OAS2 // OSELRECV2多個接收者
n.List.Prepend(n.Left) // 將left放在前面
n.Rlist.Set1(n.Right)
n.Right = nil
n.Left = nil
n.SetTypecheck(0)
n = typecheck(n, ctxStmt)
}
l = append(l, n)
}
l = append(l, cas.Nbody.Slice()...) // case內(nèi)的處理
l = append(l, nod(OBREAK, nil, nil)) // 添加break
return l
}
// convert case value arguments to addresses.
// this rewrite is used by both the general code and the next optimization.
var dflt *Node
for _, cas := range cases.Slice() {
setlineno(cas)
n := cas.Left
if n == nil {
dflt = cas
continue
}
switch n.Op {
case OSEND:
n.Right = nod(OADDR, n.Right, nil)
n.Right = typecheck(n.Right, ctxExpr)
case OSELRECV, OSELRECV2:
if n.Op == OSELRECV2 n.List.Len() == 0 {
n.Op = OSELRECV
}
if n.Left != nil {
n.Left = nod(OADDR, n.Left, nil)
n.Left = typecheck(n.Left, ctxExpr)
}
}
}
// optimization: two-case select but one is default: single non-blocking op.
if ncas == 2 dflt != nil {
cas := cases.First()
if cas == dflt {
cas = cases.Second()
}
n := cas.Left
setlineno(n)
r := nod(OIF, nil, nil)
r.Ninit.Set(cas.Ninit.Slice())
switch n.Op {
default:
Fatalf("select %v", n.Op)
case OSEND:
// if selectnbsend(c, v) { body } else { default body }
ch := n.Left
r.Left = mkcall1(chanfn("selectnbsend", 2, ch.Type), types.Types[TBOOL], r.Ninit, ch, n.Right)
case OSELRECV:
// if selectnbrecv(v, c) { body } else { default body }
ch := n.Right.Left
elem := n.Left
if elem == nil {
elem = nodnil()
}
r.Left = mkcall1(chanfn("selectnbrecv", 2, ch.Type), types.Types[TBOOL], r.Ninit, elem, ch)
case OSELRECV2:
// if selectnbrecv2(v, received, c) { body } else { default body }
ch := n.Right.Left
elem := n.Left
if elem == nil {
elem = nodnil()
}
receivedp := nod(OADDR, n.List.First(), nil)
receivedp = typecheck(receivedp, ctxExpr)
r.Left = mkcall1(chanfn("selectnbrecv2", 2, ch.Type), types.Types[TBOOL], r.Ninit, elem, receivedp, ch)
}
r.Left = typecheck(r.Left, ctxExpr)
r.Nbody.Set(cas.Nbody.Slice())
r.Rlist.Set(append(dflt.Ninit.Slice(), dflt.Nbody.Slice()...))
return []*Node{r, nod(OBREAK, nil, nil)}
}
if dflt != nil {
ncas--
}
casorder := make([]*Node, ncas)
nsends, nrecvs := 0, 0
var init []*Node
// generate sel-struct
lineno = sellineno
selv := temp(types.NewArray(scasetype(), int64(ncas)))
r := nod(OAS, selv, nil)
r = typecheck(r, ctxStmt)
init = append(init, r)
// No initialization for order; runtime.selectgo is responsible for that.
order := temp(types.NewArray(types.Types[TUINT16], 2*int64(ncas)))
var pc0, pcs *Node
if flag_race {
pcs = temp(types.NewArray(types.Types[TUINTPTR], int64(ncas)))
pc0 = typecheck(nod(OADDR, nod(OINDEX, pcs, nodintconst(0)), nil), ctxExpr)
} else {
pc0 = nodnil()
}
// register cases
for _, cas := range cases.Slice() {
setlineno(cas)
init = append(init, cas.Ninit.Slice()...)
cas.Ninit.Set(nil)
n := cas.Left
if n == nil { // default:
continue
}
var i int
var c, elem *Node
switch n.Op {
default:
Fatalf("select %v", n.Op)
case OSEND:
i = nsends
nsends++
c = n.Left
elem = n.Right
case OSELRECV, OSELRECV2:
nrecvs++
i = ncas - nrecvs
c = n.Right.Left
elem = n.Left
}
casorder[i] = cas
setField := func(f string, val *Node) {
r := nod(OAS, nodSym(ODOT, nod(OINDEX, selv, nodintconst(int64(i))), lookup(f)), val)
r = typecheck(r, ctxStmt)
init = append(init, r)
}
c = convnop(c, types.Types[TUNSAFEPTR])
setField("c", c)
if elem != nil {
elem = convnop(elem, types.Types[TUNSAFEPTR])
setField("elem", elem)
}
// TODO(mdempsky): There should be a cleaner way to
// handle this.
if flag_race {
r = mkcall("selectsetpc", nil, nil, nod(OADDR, nod(OINDEX, pcs, nodintconst(int64(i))), nil))
init = append(init, r)
}
}
if nsends+nrecvs != ncas {
Fatalf("walkselectcases: miscount: %v + %v != %v", nsends, nrecvs, ncas)
}
// run the select
lineno = sellineno
chosen := temp(types.Types[TINT])
recvOK := temp(types.Types[TBOOL])
r = nod(OAS2, nil, nil)
r.List.Set2(chosen, recvOK)
fn := syslook("selectgo")
r.Rlist.Set1(mkcall1(fn, fn.Type.Results(), nil, bytePtrToIndex(selv, 0), bytePtrToIndex(order, 0), pc0, nodintconst(int64(nsends)), nodintconst(int64(nrecvs)), nodbool(dflt == nil)))
r = typecheck(r, ctxStmt)
init = append(init, r)
// selv and order are no longer alive after selectgo.
init = append(init, nod(OVARKILL, selv, nil))
init = append(init, nod(OVARKILL, order, nil))
if flag_race {
init = append(init, nod(OVARKILL, pcs, nil))
}
// dispatch cases
dispatch := func(cond, cas *Node) {
cond = typecheck(cond, ctxExpr)
cond = defaultlit(cond, nil)
r := nod(OIF, cond, nil)
if n := cas.Left; n != nil n.Op == OSELRECV2 {
x := nod(OAS, n.List.First(), recvOK)
x = typecheck(x, ctxStmt)
r.Nbody.Append(x)
}
r.Nbody.AppendNodes(cas.Nbody)
r.Nbody.Append(nod(OBREAK, nil, nil))
init = append(init, r)
}
if dflt != nil {
setlineno(dflt)
dispatch(nod(OLT, chosen, nodintconst(0)), dflt)
}
for i, cas := range casorder {
setlineno(cas)
dispatch(nod(OEQ, chosen, nodintconst(int64(i))), cas)
}
return init
}
以上就是go select編譯期的優(yōu)化處理邏輯使用場景分析的詳細(xì)內(nèi)容，更多關(guān)于go select編譯的資料請關(guān)注腳本之家其它相關(guān)文章！
