1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
14 "cmd/compile/internal/base"
15 "cmd/compile/internal/ir"
16 "cmd/compile/internal/objw"
17 "cmd/compile/internal/reflectdata"
18 "cmd/compile/internal/ssagen"
19 "cmd/compile/internal/typecheck"
20 "cmd/compile/internal/types"
25 // walkSwitch walks a switch statement.
26 func walkSwitch(sw *ir.SwitchStmt) {
27 // Guard against double walk, see #25776.
29 return // Was fatal, but eliminating every possible source of double-walking is hard
33 if sw.Tag != nil && sw.Tag.Op() == ir.OTYPESW {
40 // walkSwitchExpr generates an AST implementing sw. sw is an
42 func walkSwitchExpr(sw *ir.SwitchStmt) {
48 // convert switch {...} to switch true {...}
50 cond = ir.NewBool(base.Pos, true)
51 cond = typecheck.Expr(cond)
52 cond = typecheck.DefaultLit(cond, nil)
55 // Given "switch string(byteslice)",
56 // with all cases being side-effect free,
57 // use a zero-cost alias of the byte slice.
58 // Do this before calling walkExpr on cond,
59 // because walkExpr will lower the string
60 // conversion into a runtime call.
61 // See issue 24937 for more discussion.
62 if cond.Op() == ir.OBYTES2STR && allCaseExprsAreSideEffectFree(sw) {
63 cond := cond.(*ir.ConvExpr)
64 cond.SetOp(ir.OBYTES2STRTMP)
67 cond = walkExpr(cond, sw.PtrInit())
68 if cond.Op() != ir.OLITERAL && cond.Op() != ir.ONIL {
69 cond = copyExpr(cond, cond.Type(), &sw.Compiled)
79 var defaultGoto ir.Node
81 for _, ncase := range sw.Cases {
82 label := typecheck.AutoLabel(".s")
83 jmp := ir.NewBranchStmt(ncase.Pos(), ir.OGOTO, label)
85 // Process case dispatch.
86 if len(ncase.List) == 0 {
87 if defaultGoto != nil {
88 base.Fatalf("duplicate default case not detected during typechecking")
93 for i, n1 := range ncase.List {
95 if i < len(ncase.RTypes) {
96 rtype = ncase.RTypes[i]
98 s.Add(ncase.Pos(), n1, rtype, jmp)
102 body.Append(ir.NewLabelStmt(ncase.Pos(), label))
103 body.Append(ncase.Body...)
104 if fall, pos := endsInFallthrough(ncase.Body); !fall {
105 br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)
112 if defaultGoto == nil {
113 br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)
114 br.SetPos(br.Pos().WithNotStmt())
119 sw.Compiled.Append(defaultGoto)
120 sw.Compiled.Append(body.Take()...)
121 walkStmtList(sw.Compiled)
124 // An exprSwitch walks an expression switch.
125 type exprSwitch struct {
127 exprname ir.Node // value being switched on
133 type exprClause struct {
136 rtype ir.Node // *runtime._type for OEQ node
140 func (s *exprSwitch) Add(pos src.XPos, expr, rtype, jmp ir.Node) {
141 c := exprClause{pos: pos, lo: expr, hi: expr, rtype: rtype, jmp: jmp}
142 if types.IsOrdered[s.exprname.Type().Kind()] && expr.Op() == ir.OLITERAL {
143 s.clauses = append(s.clauses, c)
148 s.clauses = append(s.clauses, c)
152 func (s *exprSwitch) Emit(out *ir.Nodes) {
154 out.Append(s.done.Take()...)
157 func (s *exprSwitch) flush() {
164 // Caution: If len(cc) == 1, then cc[0] might not an OLITERAL.
165 // The code below is structured to implicitly handle this case
166 // (e.g., sort.Slice doesn't need to invoke the less function
167 // when there's only a single slice element).
169 if s.exprname.Type().IsString() && len(cc) >= 2 {
170 // Sort strings by length and then by value. It is
171 // much cheaper to compare lengths than values, and
172 // all we need here is consistency. We respect this
174 sort.Slice(cc, func(i, j int) bool {
175 si := ir.StringVal(cc[i].lo)
176 sj := ir.StringVal(cc[j].lo)
177 if len(si) != len(sj) {
178 return len(si) < len(sj)
183 // runLen returns the string length associated with a
184 // particular run of exprClauses.
185 runLen := func(run []exprClause) int64 { return int64(len(ir.StringVal(run[0].lo))) }
187 // Collapse runs of consecutive strings with the same length.
188 var runs [][]exprClause
190 for i := 1; i < len(cc); i++ {
191 if runLen(cc[start:]) != runLen(cc[i:]) {
192 runs = append(runs, cc[start:i])
196 runs = append(runs, cc[start:])
198 // We have strings of more than one length. Generate an
199 // outer switch which switches on the length of the string
200 // and an inner switch in each case which resolves all the
201 // strings of the same length. The code looks something like this:
205 // ... search among length 5 strings ...
208 // ... search among length 8 strings ...
210 // ... other lengths ...
215 // ... other lengths ...
219 outerLabel := typecheck.AutoLabel(".s")
220 endLabel := typecheck.AutoLabel(".s")
222 // Jump around all the individual switches for each length.
223 s.done.Append(ir.NewBranchStmt(s.pos, ir.OGOTO, outerLabel))
226 outer.exprname = ir.NewUnaryExpr(s.pos, ir.OLEN, s.exprname)
227 outer.exprname.SetType(types.Types[types.TINT])
229 for _, run := range runs {
230 // Target label to jump to when we match this length.
231 label := typecheck.AutoLabel(".s")
233 // Search within this run of same-length strings.
235 s.done.Append(ir.NewLabelStmt(pos, label))
236 stringSearch(s.exprname, run, &s.done)
237 s.done.Append(ir.NewBranchStmt(pos, ir.OGOTO, endLabel))
239 // Add length case to outer switch.
240 cas := ir.NewInt(pos, runLen(run))
241 jmp := ir.NewBranchStmt(pos, ir.OGOTO, label)
242 outer.Add(pos, cas, nil, jmp)
244 s.done.Append(ir.NewLabelStmt(s.pos, outerLabel))
246 s.done.Append(ir.NewLabelStmt(s.pos, endLabel))
250 sort.Slice(cc, func(i, j int) bool {
251 return constant.Compare(cc[i].lo.Val(), token.LSS, cc[j].lo.Val())
254 // Merge consecutive integer cases.
255 if s.exprname.Type().IsInteger() {
256 consecutive := func(last, next constant.Value) bool {
257 delta := constant.BinaryOp(next, token.SUB, last)
258 return constant.Compare(delta, token.EQL, constant.MakeInt64(1))
262 for _, c := range cc[1:] {
263 last := &merged[len(merged)-1]
264 if last.jmp == c.jmp && consecutive(last.hi.Val(), c.lo.Val()) {
267 merged = append(merged, c)
273 s.search(cc, &s.done)
276 func (s *exprSwitch) search(cc []exprClause, out *ir.Nodes) {
277 if s.tryJumpTable(cc, out) {
280 binarySearch(len(cc), out,
281 func(i int) ir.Node {
282 return ir.NewBinaryExpr(base.Pos, ir.OLE, s.exprname, cc[i-1].hi)
284 func(i int, nif *ir.IfStmt) {
286 nif.Cond = c.test(s.exprname)
287 nif.Body = []ir.Node{c.jmp}
292 // Try to implement the clauses with a jump table. Returns true if successful.
293 func (s *exprSwitch) tryJumpTable(cc []exprClause, out *ir.Nodes) bool {
294 const minCases = 8 // have at least minCases cases in the switch
295 const minDensity = 4 // use at least 1 out of every minDensity entries
297 if base.Flag.N != 0 || !ssagen.Arch.LinkArch.CanJumpTable || base.Ctxt.Retpoline {
300 if len(cc) < minCases {
301 return false // not enough cases for it to be worth it
303 if cc[0].lo.Val().Kind() != constant.Int {
304 return false // e.g. float
306 if s.exprname.Type().Size() > int64(types.PtrSize) {
307 return false // 64-bit switches on 32-bit archs
309 min := cc[0].lo.Val()
310 max := cc[len(cc)-1].hi.Val()
311 width := constant.BinaryOp(constant.BinaryOp(max, token.SUB, min), token.ADD, constant.MakeInt64(1))
312 limit := constant.MakeInt64(int64(len(cc)) * minDensity)
313 if constant.Compare(width, token.GTR, limit) {
314 // We disable jump tables if we use less than a minimum fraction of the entries.
315 // i.e. for switch x {case 0: case 1000: case 2000:} we don't want to use a jump table.
318 jt := ir.NewJumpTableStmt(base.Pos, s.exprname)
319 for _, c := range cc {
320 jmp := c.jmp.(*ir.BranchStmt)
321 if jmp.Op() != ir.OGOTO || jmp.Label == nil {
322 panic("bad switch case body")
324 for i := c.lo.Val(); constant.Compare(i, token.LEQ, c.hi.Val()); i = constant.BinaryOp(i, token.ADD, constant.MakeInt64(1)) {
325 jt.Cases = append(jt.Cases, i)
326 jt.Targets = append(jt.Targets, jmp.Label)
333 func (c *exprClause) test(exprname ir.Node) ir.Node {
336 low := ir.NewBinaryExpr(c.pos, ir.OGE, exprname, c.lo)
337 high := ir.NewBinaryExpr(c.pos, ir.OLE, exprname, c.hi)
338 return ir.NewLogicalExpr(c.pos, ir.OANDAND, low, high)
341 // Optimize "switch true { ...}" and "switch false { ... }".
342 if ir.IsConst(exprname, constant.Bool) && !c.lo.Type().IsInterface() {
343 if ir.BoolVal(exprname) {
346 return ir.NewUnaryExpr(c.pos, ir.ONOT, c.lo)
350 n := ir.NewBinaryExpr(c.pos, ir.OEQ, exprname, c.lo)
355 func allCaseExprsAreSideEffectFree(sw *ir.SwitchStmt) bool {
356 // In theory, we could be more aggressive, allowing any
357 // side-effect-free expressions in cases, but it's a bit
358 // tricky because some of that information is unavailable due
359 // to the introduction of temporaries during order.
360 // Restricting to constants is simple and probably powerful
363 for _, ncase := range sw.Cases {
364 for _, v := range ncase.List {
365 if v.Op() != ir.OLITERAL {
373 // endsInFallthrough reports whether stmts ends with a "fallthrough" statement.
374 func endsInFallthrough(stmts []ir.Node) (bool, src.XPos) {
376 return false, src.NoXPos
379 return stmts[i].Op() == ir.OFALL, stmts[i].Pos()
382 // walkSwitchType generates an AST that implements sw, where sw is a
384 func walkSwitchType(sw *ir.SwitchStmt) {
386 s.srcName = sw.Tag.(*ir.TypeSwitchGuard).X
387 s.srcName = walkExpr(s.srcName, sw.PtrInit())
388 s.srcName = copyExpr(s.srcName, s.srcName.Type(), &sw.Compiled)
389 s.okName = typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TBOOL])
390 s.itabName = typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TUINT8].PtrTo())
392 // Get interface descriptor word.
393 // For empty interfaces this will be the type.
394 // For non-empty interfaces this will be the itab.
395 srcItab := ir.NewUnaryExpr(base.Pos, ir.OITAB, s.srcName)
396 srcData := ir.NewUnaryExpr(base.Pos, ir.OIDATA, s.srcName)
397 srcData.SetType(types.Types[types.TUINT8].PtrTo())
398 srcData.SetTypecheck(1)
400 // For empty interfaces, do:
401 // if e._type == nil {
402 // do nil case if it exists, otherwise default
405 // Use a similar strategy for non-empty interfaces.
406 ifNil := ir.NewIfStmt(base.Pos, nil, nil, nil)
407 ifNil.Cond = ir.NewBinaryExpr(base.Pos, ir.OEQ, srcItab, typecheck.NodNil())
408 base.Pos = base.Pos.WithNotStmt() // disable statement marks after the first check.
409 ifNil.Cond = typecheck.Expr(ifNil.Cond)
410 ifNil.Cond = typecheck.DefaultLit(ifNil.Cond, nil)
411 // ifNil.Nbody assigned later.
412 sw.Compiled.Append(ifNil)
414 // Load hash from type or itab.
415 dotHash := typeHashFieldOf(base.Pos, srcItab)
416 s.hashName = copyExpr(dotHash, dotHash.Type(), &sw.Compiled)
418 // Make a label for each case body.
419 labels := make([]*types.Sym, len(sw.Cases))
420 for i := range sw.Cases {
421 labels[i] = typecheck.AutoLabel(".s")
424 // "jump" to execute if no case matches.
425 br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)
427 // Assemble a list of all the types we're looking for.
428 // This pass flattens the case lists, as well as handles
429 // some unusual cases, like default and nil cases.
430 type oneCase struct {
432 jmp ir.Node // jump to body of selected case
434 // The case we're matching. Normally the type we're looking for
435 // is typ.Type(), but when typ is ODYNAMICTYPE the actual type
436 // we're looking for is not a compile-time constant (typ.Type()
437 // will be its shape).
441 var defaultGoto, nilGoto ir.Node
442 for i, ncase := range sw.Cases {
443 jmp := ir.NewBranchStmt(ncase.Pos(), ir.OGOTO, labels[i])
444 if len(ncase.List) == 0 { // default:
445 if defaultGoto != nil {
446 base.Fatalf("duplicate default case not detected during typechecking")
450 for _, n1 := range ncase.List {
451 if ir.IsNil(n1) { // case nil:
453 base.Fatalf("duplicate nil case not detected during typechecking")
458 if n1.Op() == ir.ODYNAMICTYPE {
459 // Convert dynamic to static, if the dynamic is actually static.
460 // TODO: why isn't this OTYPE to begin with?
461 dt := n1.(*ir.DynamicType)
462 if dt.RType != nil && dt.RType.Op() == ir.OADDR {
463 addr := dt.RType.(*ir.AddrExpr)
464 if addr.X.Op() == ir.OLINKSYMOFFSET {
465 n1 = ir.TypeNode(n1.Type())
468 if dt.ITab != nil && dt.ITab.Op() == ir.OADDR {
469 addr := dt.ITab.(*ir.AddrExpr)
470 if addr.X.Op() == ir.OLINKSYMOFFSET {
471 n1 = ir.TypeNode(n1.Type())
475 cases = append(cases, oneCase{
482 if defaultGoto == nil {
486 nilGoto = defaultGoto
488 ifNil.Body = []ir.Node{nilGoto}
490 // Now go through the list of cases, processing groups as we find them.
491 var concreteCases []oneCase
492 var interfaceCases []oneCase
494 // Process all the concrete types first. Because we handle shadowing
495 // below, it is correct to do all the concrete types before all of
496 // the interface types.
497 // The concrete cases can all be handled without a runtime call.
498 if len(concreteCases) > 0 {
499 var clauses []typeClause
500 for _, c := range concreteCases {
501 as := ir.NewAssignListStmt(c.pos, ir.OAS2,
502 []ir.Node{ir.BlankNode, s.okName}, // _, ok =
503 []ir.Node{ir.NewTypeAssertExpr(c.pos, s.srcName, c.typ.Type())}) // iface.(type)
504 nif := ir.NewIfStmt(c.pos, s.okName, []ir.Node{c.jmp}, nil)
505 clauses = append(clauses, typeClause{
506 hash: types.TypeHash(c.typ.Type()),
507 body: []ir.Node{typecheck.Stmt(as), typecheck.Stmt(nif)},
510 s.flush(clauses, &sw.Compiled)
511 concreteCases = concreteCases[:0]
514 // The "any" case, if it exists, must be the last interface case, because
515 // it would shadow all subsequent cases. Strip it off here so the runtime
516 // call only needs to handle non-empty interfaces.
518 if len(interfaceCases) > 0 && interfaceCases[len(interfaceCases)-1].typ.Type().IsEmptyInterface() {
519 anyGoto = interfaceCases[len(interfaceCases)-1].jmp
520 interfaceCases = interfaceCases[:len(interfaceCases)-1]
523 // Next, process all the interface types with a single call to the runtime.
524 if len(interfaceCases) > 0 {
526 // Build an internal/abi.InterfaceSwitch descriptor to pass to the runtime.
527 lsym := types.LocalPkg.Lookup(fmt.Sprintf(".interfaceSwitch.%d", interfaceSwitchGen)).LinksymABI(obj.ABI0)
530 off = objw.Uintptr(lsym, off, uint64(len(interfaceCases)))
531 for _, c := range interfaceCases {
532 off = objw.SymPtr(lsym, off, reflectdata.TypeSym(c.typ.Type()).Linksym(), 0)
534 // Note: it has pointers, just not ones the GC cares about.
535 objw.Global(lsym, int32(off), obj.LOCAL|obj.NOPTR)
537 // Call runtime to do switch
538 // case, itab = runtime.interfaceSwitch(&descriptor, typeof(arg))
540 if s.srcName.Type().IsEmptyInterface() {
541 typeArg = ir.NewConvExpr(base.Pos, ir.OCONVNOP, types.Types[types.TUINT8].PtrTo(), srcItab)
543 typeArg = itabType(srcItab)
545 caseVar := typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TINT])
546 isw := ir.NewInterfaceSwitchStmt(base.Pos, caseVar, s.itabName, typeArg, lsym)
547 sw.Compiled.Append(isw)
549 // Switch on the result of the call.
550 var newCases []*ir.CaseClause
551 for i, c := range interfaceCases {
552 newCases = append(newCases, &ir.CaseClause{
553 List: []ir.Node{ir.NewInt(base.Pos, int64(i))},
554 Body: []ir.Node{c.jmp},
557 // TODO: add len(newCases) case, mark switch as bounded
558 sw2 := ir.NewSwitchStmt(base.Pos, caseVar, newCases)
559 sw.Compiled.Append(typecheck.Stmt(sw2))
560 interfaceCases = interfaceCases[:0]
564 // We've already handled the nil case, so everything
565 // that reaches here matches the "any" case.
566 sw.Compiled.Append(anyGoto)
570 for _, c := range cases {
571 if c.typ.Op() == ir.ODYNAMICTYPE {
572 flush() // process all previous cases
573 dt := c.typ.(*ir.DynamicType)
574 dot := ir.NewDynamicTypeAssertExpr(c.pos, ir.ODYNAMICDOTTYPE, s.srcName, dt.RType)
576 dot.SetType(c.typ.Type())
579 as := ir.NewAssignListStmt(c.pos, ir.OAS2, nil, nil)
580 as.Lhs = []ir.Node{ir.BlankNode, s.okName} // _, ok =
581 as.Rhs = []ir.Node{dot}
584 nif := ir.NewIfStmt(c.pos, s.okName, []ir.Node{c.jmp}, nil)
585 sw.Compiled.Append(as, nif)
589 // Check for shadowing (a case that will never fire because
590 // a previous case would have always fired first). This check
591 // allows us to reorder concrete and interface cases.
592 // (TODO: these should be vet failures, maybe?)
593 for _, ic := range interfaceCases {
594 // An interface type case will shadow all
595 // subsequent types that implement that interface.
596 if typecheck.Implements(c.typ.Type(), ic.typ.Type()) {
599 // Note that we don't need to worry about:
600 // 1. Two concrete types shadowing each other. That's
601 // disallowed by the spec.
602 // 2. A concrete type shadowing an interface type.
603 // That can never happen, as interface types can
604 // be satisfied by an infinite set of concrete types.
605 // The correctness of this step also depends on handling
606 // the dynamic type cases separately, as we do above.
609 if c.typ.Type().IsInterface() {
610 interfaceCases = append(interfaceCases, c)
612 concreteCases = append(concreteCases, c)
617 sw.Compiled.Append(defaultGoto) // if none of the cases matched
619 // Now generate all the case bodies
620 for i, ncase := range sw.Cases {
621 sw.Compiled.Append(ir.NewLabelStmt(ncase.Pos(), labels[i]))
622 if caseVar := ncase.Var; caseVar != nil {
624 if len(ncase.List) == 1 {
625 // single type. We have to downcast the input value to the target type.
626 if ncase.List[0].Op() == ir.OTYPE { // single compile-time known type
627 t := ncase.List[0].Type()
629 // This case is an interface. Build case value from input interface.
630 // The data word will always be the same, but the itab/type changes.
631 if t.IsEmptyInterface() {
633 if s.srcName.Type().IsEmptyInterface() {
634 // E->E, nothing to do, type is already correct.
637 // I->E, load type out of itab
638 typ = itabType(srcItab)
639 typ.SetPos(ncase.Pos())
641 val = ir.NewBinaryExpr(ncase.Pos(), ir.OMAKEFACE, typ, srcData)
643 // The itab we need was returned by a runtime.interfaceSwitch call.
644 val = ir.NewBinaryExpr(ncase.Pos(), ir.OMAKEFACE, s.itabName, srcData)
647 // This case is a concrete type, just read its value out of the interface.
648 val = ifaceData(ncase.Pos(), s.srcName, t)
650 } else if ncase.List[0].Op() == ir.ODYNAMICTYPE { // single runtime known type
651 dt := ncase.List[0].(*ir.DynamicType)
652 x := ir.NewDynamicTypeAssertExpr(ncase.Pos(), ir.ODYNAMICDOTTYPE, val, dt.RType)
655 } else if ir.IsNil(ncase.List[0]) {
657 base.Fatalf("unhandled type switch case %v", ncase.List[0])
659 val.SetType(caseVar.Type())
663 ir.NewDecl(ncase.Pos(), ir.ODCL, caseVar),
664 ir.NewAssignStmt(ncase.Pos(), caseVar, val),
667 sw.Compiled.Append(l...)
669 sw.Compiled.Append(ncase.Body...)
670 sw.Compiled.Append(br)
673 walkStmtList(sw.Compiled)
678 var interfaceSwitchGen int
680 // typeHashFieldOf returns an expression to select the type hash field
681 // from an interface's descriptor word (whether a *runtime._type or
682 // *runtime.itab pointer).
683 func typeHashFieldOf(pos src.XPos, itab *ir.UnaryExpr) *ir.SelectorExpr {
684 if itab.Op() != ir.OITAB {
685 base.Fatalf("expected OITAB, got %v", itab.Op())
687 var hashField *types.Field
688 if itab.X.Type().IsEmptyInterface() {
689 // runtime._type's hash field
690 if rtypeHashField == nil {
691 rtypeHashField = runtimeField("hash", int64(2*types.PtrSize), types.Types[types.TUINT32])
693 hashField = rtypeHashField
695 // runtime.itab's hash field
696 if itabHashField == nil {
697 itabHashField = runtimeField("hash", int64(2*types.PtrSize), types.Types[types.TUINT32])
699 hashField = itabHashField
701 return boundedDotPtr(pos, itab, hashField)
704 var rtypeHashField, itabHashField *types.Field
706 // A typeSwitch walks a type switch.
707 type typeSwitch struct {
708 // Temporary variables (i.e., ONAMEs) used by type switch dispatch logic:
709 srcName ir.Node // value being type-switched on
710 hashName ir.Node // type hash of the value being type-switched on
711 okName ir.Node // boolean used for comma-ok type assertions
712 itabName ir.Node // itab value to use for first word of non-empty interface
715 type typeClause struct {
720 func (s *typeSwitch) flush(cc []typeClause, compiled *ir.Nodes) {
725 sort.Slice(cc, func(i, j int) bool { return cc[i].hash < cc[j].hash })
727 // Combine adjacent cases with the same hash.
729 for _, c := range cc[1:] {
730 last := &merged[len(merged)-1]
731 if last.hash == c.hash {
732 last.body.Append(c.body.Take()...)
734 merged = append(merged, c)
739 if s.tryJumpTable(cc, compiled) {
742 binarySearch(len(cc), compiled,
743 func(i int) ir.Node {
744 return ir.NewBinaryExpr(base.Pos, ir.OLE, s.hashName, ir.NewInt(base.Pos, int64(cc[i-1].hash)))
746 func(i int, nif *ir.IfStmt) {
747 // TODO(mdempsky): Omit hash equality check if
748 // there's only one type.
750 nif.Cond = ir.NewBinaryExpr(base.Pos, ir.OEQ, s.hashName, ir.NewInt(base.Pos, int64(c.hash)))
751 nif.Body.Append(c.body.Take()...)
756 // Try to implement the clauses with a jump table. Returns true if successful.
757 func (s *typeSwitch) tryJumpTable(cc []typeClause, out *ir.Nodes) bool {
758 const minCases = 5 // have at least minCases cases in the switch
759 if base.Flag.N != 0 || !ssagen.Arch.LinkArch.CanJumpTable || base.Ctxt.Retpoline {
762 if len(cc) < minCases {
763 return false // not enough cases for it to be worth it
765 hashes := make([]uint32, len(cc))
766 // b = # of bits to use. Start with the minimum number of
767 // bits possible, but try a few larger sizes if needed.
768 b0 := bits.Len(uint(len(cc) - 1))
769 for b := b0; b < b0+3; b++ {
771 for i := 0; i <= 32-b; i++ { // starting bit position
772 // Compute the hash we'd get from all the cases,
773 // selecting b bits starting at bit i.
775 for _, c := range cc {
776 h := c.hash >> i & (1<<b - 1)
777 hashes = append(hashes, h)
779 // Order by increasing hash.
780 sort.Slice(hashes, func(j, k int) bool {
781 return hashes[j] < hashes[k]
783 for j := 1; j < len(hashes); j++ {
784 if hashes[j] == hashes[j-1] {
785 // There is a duplicate hash; try a different b/i pair.
790 // All hashes are distinct. Use these values of b and i.
793 h = ir.NewBinaryExpr(base.Pos, ir.ORSH, h, ir.NewInt(base.Pos, int64(i)))
795 h = ir.NewBinaryExpr(base.Pos, ir.OAND, h, ir.NewInt(base.Pos, int64(1<<b-1)))
796 h = typecheck.Expr(h)
799 jt := ir.NewJumpTableStmt(base.Pos, h)
800 jt.Cases = make([]constant.Value, 1<<b)
801 jt.Targets = make([]*types.Sym, 1<<b)
804 // Start with all hashes going to the didn't-match target.
805 noMatch := typecheck.AutoLabel(".s")
806 for j := 0; j < 1<<b; j++ {
807 jt.Cases[j] = constant.MakeInt64(int64(j))
808 jt.Targets[j] = noMatch
810 // This statement is not reachable, but it will make it obvious that we don't
811 // fall through to the first case.
812 out.Append(ir.NewBranchStmt(base.Pos, ir.OGOTO, noMatch))
814 // Emit each of the actual cases.
815 for _, c := range cc {
816 h := c.hash >> i & (1<<b - 1)
817 label := typecheck.AutoLabel(".s")
818 jt.Targets[h] = label
819 out.Append(ir.NewLabelStmt(base.Pos, label))
820 out.Append(c.body...)
821 // We reach here if the hash matches but the type equality test fails.
822 out.Append(ir.NewBranchStmt(base.Pos, ir.OGOTO, noMatch))
824 // Emit point to go to if type doesn't match any case.
825 out.Append(ir.NewLabelStmt(base.Pos, noMatch))
829 // Couldn't find a perfect hash. Fall back to binary search.
833 // binarySearch constructs a binary search tree for handling n cases,
834 // and appends it to out. It's used for efficiently implementing
835 // switch statements.
837 // less(i) should return a boolean expression. If it evaluates true,
838 // then cases before i will be tested; otherwise, cases i and later.
840 // leaf(i, nif) should setup nif (an OIF node) to test case i. In
841 // particular, it should set nif.Cond and nif.Body.
842 func binarySearch(n int, out *ir.Nodes, less func(i int) ir.Node, leaf func(i int, nif *ir.IfStmt)) {
843 const binarySearchMin = 4 // minimum number of cases for binary search
845 var do func(lo, hi int, out *ir.Nodes)
846 do = func(lo, hi int, out *ir.Nodes) {
848 if n < binarySearchMin {
849 for i := lo; i < hi; i++ {
850 nif := ir.NewIfStmt(base.Pos, nil, nil, nil)
852 base.Pos = base.Pos.WithNotStmt()
853 nif.Cond = typecheck.Expr(nif.Cond)
854 nif.Cond = typecheck.DefaultLit(nif.Cond, nil)
862 nif := ir.NewIfStmt(base.Pos, nil, nil, nil)
863 nif.Cond = less(half)
864 base.Pos = base.Pos.WithNotStmt()
865 nif.Cond = typecheck.Expr(nif.Cond)
866 nif.Cond = typecheck.DefaultLit(nif.Cond, nil)
867 do(lo, half, &nif.Body)
868 do(half, hi, &nif.Else)
875 func stringSearch(expr ir.Node, cc []exprClause, out *ir.Nodes) {
877 // Short list, just do brute force equality checks.
878 for _, c := range cc {
879 nif := ir.NewIfStmt(base.Pos.WithNotStmt(), typecheck.DefaultLit(typecheck.Expr(c.test(expr)), nil), []ir.Node{c.jmp}, nil)
886 // The strategy here is to find a simple test to divide the set of possible strings
887 // that might match expr approximately in half.
888 // The test we're going to use is to do an ordered comparison of a single byte
889 // of expr to a constant. We will pick the index of that byte and the value we're
890 // comparing against to make the split as even as possible.
891 // if expr[3] <= 'd' { ... search strings with expr[3] at 'd' or lower ... }
892 // else { ... search strings with expr[3] at 'e' or higher ... }
894 // To add complication, we will do the ordered comparison in the signed domain.
895 // The reason for this is to prevent CSE from merging the load used for the
896 // ordered comparison with the load used for the later equality check.
897 // if expr[3] <= 'd' { ... if expr[0] == 'f' && expr[1] == 'o' && expr[2] == 'o' && expr[3] == 'd' { ... } }
898 // If we did both expr[3] loads in the unsigned domain, they would be CSEd, and that
899 // would in turn defeat the combining of expr[0]...expr[3] into a single 4-byte load.
901 // By using signed loads for the ordered comparison and unsigned loads for the
902 // equality comparison, they don't get CSEd and the equality comparisons will be
903 // done using wider loads.
905 n := len(ir.StringVal(cc[0].lo)) // Length of the constant strings.
906 bestScore := int64(0) // measure of how good the split is.
907 bestIdx := 0 // split using expr[bestIdx]
908 bestByte := int8(0) // compare expr[bestIdx] against bestByte
909 for idx := 0; idx < n; idx++ {
910 for b := int8(-128); b < 127; b++ {
912 for _, c := range cc {
913 s := ir.StringVal(c.lo)
914 if int8(s[idx]) <= b {
918 score := int64(le) * int64(len(cc)-le)
919 if score > bestScore {
927 // The split must be at least 1:n-1 because we have at least 2 distinct strings; they
928 // have to be different somewhere.
929 // TODO: what if the best split is still pretty bad?
931 base.Fatalf("unable to split string set")
934 // Convert expr to a []int8
935 slice := ir.NewConvExpr(base.Pos, ir.OSTR2BYTESTMP, types.NewSlice(types.Types[types.TINT8]), expr)
936 slice.SetTypecheck(1) // legacy typechecker doesn't handle this op
938 // Load the byte we're splitting on.
939 load := ir.NewIndexExpr(base.Pos, slice, ir.NewInt(base.Pos, int64(bestIdx)))
940 // Compare with the value we're splitting on.
941 cmp := ir.Node(ir.NewBinaryExpr(base.Pos, ir.OLE, load, ir.NewInt(base.Pos, int64(bestByte))))
942 cmp = typecheck.DefaultLit(typecheck.Expr(cmp), nil)
943 nif := ir.NewIfStmt(base.Pos, cmp, nil, nil)
947 for _, c := range cc {
948 s := ir.StringVal(c.lo)
949 if int8(s[bestIdx]) <= bestByte {
955 stringSearch(expr, le, &nif.Body)
956 stringSearch(expr, gt, &nif.Else)
959 // TODO: if expr[bestIdx] has enough different possible values, use a jump table.