前言
select作为Go chan通信的重要监听工具，有着很广泛的使用场景。select的使用主要是搭配通信case使用，表面上看，只是简单的select及case搭配，实际上根据case的数量及类型，在编译时select会进行优化处理，根据不同的情况调用不同的底层逻辑。
select的编译处理
select编译时的核心处理逻辑如下:
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func walkselectcases(cases *Nodes) []*Node { ncas := cases.Len() sellineno := lineno // optimization: zero-case select // 针对没有case的select优化 if ncas == 0 { return []*Node{mkcall("block", nil, nil)} } // optimization: one-case select: single op. // 针对1个case（单个操作）select的优化 if ncas == 1 { cas := cases.First() setlineno(cas) l := cas.Ninit.Slice() if cas.Left != nil { // not default: 非default case n := cas.Left // 获取case表达式 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 { // 左侧有0或1个接收者 if n.Left == nil { // 没有接收者 n = n.Right // 只需保留右侧 } else { // n.Op = OAS // 只有一个接收者，更新Op为OAS } break } if n.Left == nil { // 检查是否表达式或赋值 nblank = typecheck(nblank, ctxExpr|ctxAssign) n.Left = nblank } n.Op = OAS2 // OSELRECV2多个接收者 n.List.Prepend(n.Left) // 将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内的处理 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} |
select编译时会根据case的数量进行优化：
1.没有case
直接调用block
2.1个case
(1)default case，直接执行body
(2) send/recv case (block为true)，按照单独执行的结果确认，可能会发生block
(3) send调用对应的chansend1
(4) recv调用对应的chanrecv1/chanrecv2
3.2个case且包含一个default case
(1) send/recv case (block为false)，按照单独执行的结果确认case是否ok，!ok则执行default case，不会发生block
(2) send调用对应的selectnbsend
(3) recv调用对应的selectnbrecv/selectnbrecv2
4.一般的case
selectgo
总结
最后，以一张图进行简单总结。
以上就是go select编译期的优化处理逻辑使用场景分析的详细内容，更多关于go select编译的资料请关注服务器之家其它相关文章！
原文链接：https://blog.csdn.net/xz_studying/article/details/118280886
