github.com/jd-ly/tools@v0.5.7/go/ssa/builder.go (about)

     1  // Copyright 2013 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.
     4  
     5  package ssa
     6  
     7  // This file implements the BUILD phase of SSA construction.
     8  //
     9  // SSA construction has two phases, CREATE and BUILD.  In the CREATE phase
    10  // (create.go), all packages are constructed and type-checked and
    11  // definitions of all package members are created, method-sets are
    12  // computed, and wrapper methods are synthesized.
    13  // ssa.Packages are created in arbitrary order.
    14  //
    15  // In the BUILD phase (builder.go), the builder traverses the AST of
    16  // each Go source function and generates SSA instructions for the
    17  // function body.  Initializer expressions for package-level variables
    18  // are emitted to the package's init() function in the order specified
    19  // by go/types.Info.InitOrder, then code for each function in the
    20  // package is generated in lexical order.
    21  // The BUILD phases for distinct packages are independent and are
    22  // executed in parallel.
    23  //
    24  // TODO(adonovan): indeed, building functions is now embarrassingly parallel.
    25  // Audit for concurrency then benchmark using more goroutines.
    26  //
    27  // The builder's and Program's indices (maps) are populated and
    28  // mutated during the CREATE phase, but during the BUILD phase they
    29  // remain constant.  The sole exception is Prog.methodSets and its
    30  // related maps, which are protected by a dedicated mutex.
    31  
    32  import (
    33  	"fmt"
    34  	"go/ast"
    35  	"go/constant"
    36  	"go/token"
    37  	"go/types"
    38  	"os"
    39  	"sync"
    40  )
    41  
    42  type opaqueType struct {
    43  	types.Type
    44  	name string
    45  }
    46  
    47  func (t *opaqueType) String() string { return t.name }
    48  
    49  var (
    50  	varOk    = newVar("ok", tBool)
    51  	varIndex = newVar("index", tInt)
    52  
    53  	// Type constants.
    54  	tBool       = types.Typ[types.Bool]
    55  	tByte       = types.Typ[types.Byte]
    56  	tInt        = types.Typ[types.Int]
    57  	tInvalid    = types.Typ[types.Invalid]
    58  	tString     = types.Typ[types.String]
    59  	tUntypedNil = types.Typ[types.UntypedNil]
    60  	tRangeIter  = &opaqueType{nil, "iter"} // the type of all "range" iterators
    61  	tEface      = types.NewInterface(nil, nil).Complete()
    62  
    63  	// SSA Value constants.
    64  	vZero = intConst(0)
    65  	vOne  = intConst(1)
    66  	vTrue = NewConst(constant.MakeBool(true), tBool)
    67  )
    68  
    69  // builder holds state associated with the package currently being built.
    70  // Its methods contain all the logic for AST-to-SSA conversion.
    71  type builder struct{}
    72  
    73  // cond emits to fn code to evaluate boolean condition e and jump
    74  // to t or f depending on its value, performing various simplifications.
    75  //
    76  // Postcondition: fn.currentBlock is nil.
    77  //
    78  func (b *builder) cond(fn *Function, e ast.Expr, t, f *BasicBlock) {
    79  	switch e := e.(type) {
    80  	case *ast.ParenExpr:
    81  		b.cond(fn, e.X, t, f)
    82  		return
    83  
    84  	case *ast.BinaryExpr:
    85  		switch e.Op {
    86  		case token.LAND:
    87  			ltrue := fn.newBasicBlock("cond.true")
    88  			b.cond(fn, e.X, ltrue, f)
    89  			fn.currentBlock = ltrue
    90  			b.cond(fn, e.Y, t, f)
    91  			return
    92  
    93  		case token.LOR:
    94  			lfalse := fn.newBasicBlock("cond.false")
    95  			b.cond(fn, e.X, t, lfalse)
    96  			fn.currentBlock = lfalse
    97  			b.cond(fn, e.Y, t, f)
    98  			return
    99  		}
   100  
   101  	case *ast.UnaryExpr:
   102  		if e.Op == token.NOT {
   103  			b.cond(fn, e.X, f, t)
   104  			return
   105  		}
   106  	}
   107  
   108  	// A traditional compiler would simplify "if false" (etc) here
   109  	// but we do not, for better fidelity to the source code.
   110  	//
   111  	// The value of a constant condition may be platform-specific,
   112  	// and may cause blocks that are reachable in some configuration
   113  	// to be hidden from subsequent analyses such as bug-finding tools.
   114  	emitIf(fn, b.expr(fn, e), t, f)
   115  }
   116  
   117  // logicalBinop emits code to fn to evaluate e, a &&- or
   118  // ||-expression whose reified boolean value is wanted.
   119  // The value is returned.
   120  //
   121  func (b *builder) logicalBinop(fn *Function, e *ast.BinaryExpr) Value {
   122  	rhs := fn.newBasicBlock("binop.rhs")
   123  	done := fn.newBasicBlock("binop.done")
   124  
   125  	// T(e) = T(e.X) = T(e.Y) after untyped constants have been
   126  	// eliminated.
   127  	// TODO(adonovan): not true; MyBool==MyBool yields UntypedBool.
   128  	t := fn.Pkg.typeOf(e)
   129  
   130  	var short Value // value of the short-circuit path
   131  	switch e.Op {
   132  	case token.LAND:
   133  		b.cond(fn, e.X, rhs, done)
   134  		short = NewConst(constant.MakeBool(false), t)
   135  
   136  	case token.LOR:
   137  		b.cond(fn, e.X, done, rhs)
   138  		short = NewConst(constant.MakeBool(true), t)
   139  	}
   140  
   141  	// Is rhs unreachable?
   142  	if rhs.Preds == nil {
   143  		// Simplify false&&y to false, true||y to true.
   144  		fn.currentBlock = done
   145  		return short
   146  	}
   147  
   148  	// Is done unreachable?
   149  	if done.Preds == nil {
   150  		// Simplify true&&y (or false||y) to y.
   151  		fn.currentBlock = rhs
   152  		return b.expr(fn, e.Y)
   153  	}
   154  
   155  	// All edges from e.X to done carry the short-circuit value.
   156  	var edges []Value
   157  	for range done.Preds {
   158  		edges = append(edges, short)
   159  	}
   160  
   161  	// The edge from e.Y to done carries the value of e.Y.
   162  	fn.currentBlock = rhs
   163  	edges = append(edges, b.expr(fn, e.Y))
   164  	emitJump(fn, done)
   165  	fn.currentBlock = done
   166  
   167  	phi := &Phi{Edges: edges, Comment: e.Op.String()}
   168  	phi.pos = e.OpPos
   169  	phi.typ = t
   170  	return done.emit(phi)
   171  }
   172  
   173  // exprN lowers a multi-result expression e to SSA form, emitting code
   174  // to fn and returning a single Value whose type is a *types.Tuple.
   175  // The caller must access the components via Extract.
   176  //
   177  // Multi-result expressions include CallExprs in a multi-value
   178  // assignment or return statement, and "value,ok" uses of
   179  // TypeAssertExpr, IndexExpr (when X is a map), and UnaryExpr (when Op
   180  // is token.ARROW).
   181  //
   182  func (b *builder) exprN(fn *Function, e ast.Expr) Value {
   183  	typ := fn.Pkg.typeOf(e).(*types.Tuple)
   184  	switch e := e.(type) {
   185  	case *ast.ParenExpr:
   186  		return b.exprN(fn, e.X)
   187  
   188  	case *ast.CallExpr:
   189  		// Currently, no built-in function nor type conversion
   190  		// has multiple results, so we can avoid some of the
   191  		// cases for single-valued CallExpr.
   192  		var c Call
   193  		b.setCall(fn, e, &c.Call)
   194  		c.typ = typ
   195  		return fn.emit(&c)
   196  
   197  	case *ast.IndexExpr:
   198  		mapt := fn.Pkg.typeOf(e.X).Underlying().(*types.Map)
   199  		lookup := &Lookup{
   200  			X:       b.expr(fn, e.X),
   201  			Index:   emitConv(fn, b.expr(fn, e.Index), mapt.Key()),
   202  			CommaOk: true,
   203  		}
   204  		lookup.setType(typ)
   205  		lookup.setPos(e.Lbrack)
   206  		return fn.emit(lookup)
   207  
   208  	case *ast.TypeAssertExpr:
   209  		return emitTypeTest(fn, b.expr(fn, e.X), typ.At(0).Type(), e.Lparen)
   210  
   211  	case *ast.UnaryExpr: // must be receive <-
   212  		unop := &UnOp{
   213  			Op:      token.ARROW,
   214  			X:       b.expr(fn, e.X),
   215  			CommaOk: true,
   216  		}
   217  		unop.setType(typ)
   218  		unop.setPos(e.OpPos)
   219  		return fn.emit(unop)
   220  	}
   221  	panic(fmt.Sprintf("exprN(%T) in %s", e, fn))
   222  }
   223  
   224  // builtin emits to fn SSA instructions to implement a call to the
   225  // built-in function obj with the specified arguments
   226  // and return type.  It returns the value defined by the result.
   227  //
   228  // The result is nil if no special handling was required; in this case
   229  // the caller should treat this like an ordinary library function
   230  // call.
   231  //
   232  func (b *builder) builtin(fn *Function, obj *types.Builtin, args []ast.Expr, typ types.Type, pos token.Pos) Value {
   233  	switch obj.Name() {
   234  	case "make":
   235  		switch typ.Underlying().(type) {
   236  		case *types.Slice:
   237  			n := b.expr(fn, args[1])
   238  			m := n
   239  			if len(args) == 3 {
   240  				m = b.expr(fn, args[2])
   241  			}
   242  			if m, ok := m.(*Const); ok {
   243  				// treat make([]T, n, m) as new([m]T)[:n]
   244  				cap := m.Int64()
   245  				at := types.NewArray(typ.Underlying().(*types.Slice).Elem(), cap)
   246  				alloc := emitNew(fn, at, pos)
   247  				alloc.Comment = "makeslice"
   248  				v := &Slice{
   249  					X:    alloc,
   250  					High: n,
   251  				}
   252  				v.setPos(pos)
   253  				v.setType(typ)
   254  				return fn.emit(v)
   255  			}
   256  			v := &MakeSlice{
   257  				Len: n,
   258  				Cap: m,
   259  			}
   260  			v.setPos(pos)
   261  			v.setType(typ)
   262  			return fn.emit(v)
   263  
   264  		case *types.Map:
   265  			var res Value
   266  			if len(args) == 2 {
   267  				res = b.expr(fn, args[1])
   268  			}
   269  			v := &MakeMap{Reserve: res}
   270  			v.setPos(pos)
   271  			v.setType(typ)
   272  			return fn.emit(v)
   273  
   274  		case *types.Chan:
   275  			var sz Value = vZero
   276  			if len(args) == 2 {
   277  				sz = b.expr(fn, args[1])
   278  			}
   279  			v := &MakeChan{Size: sz}
   280  			v.setPos(pos)
   281  			v.setType(typ)
   282  			return fn.emit(v)
   283  		}
   284  
   285  	case "new":
   286  		alloc := emitNew(fn, deref(typ), pos)
   287  		alloc.Comment = "new"
   288  		return alloc
   289  
   290  	case "len", "cap":
   291  		// Special case: len or cap of an array or *array is
   292  		// based on the type, not the value which may be nil.
   293  		// We must still evaluate the value, though.  (If it
   294  		// was side-effect free, the whole call would have
   295  		// been constant-folded.)
   296  		t := deref(fn.Pkg.typeOf(args[0])).Underlying()
   297  		if at, ok := t.(*types.Array); ok {
   298  			b.expr(fn, args[0]) // for effects only
   299  			return intConst(at.Len())
   300  		}
   301  		// Otherwise treat as normal.
   302  
   303  	case "panic":
   304  		fn.emit(&Panic{
   305  			X:   emitConv(fn, b.expr(fn, args[0]), tEface),
   306  			pos: pos,
   307  		})
   308  		fn.currentBlock = fn.newBasicBlock("unreachable")
   309  		return vTrue // any non-nil Value will do
   310  	}
   311  	return nil // treat all others as a regular function call
   312  }
   313  
   314  // addr lowers a single-result addressable expression e to SSA form,
   315  // emitting code to fn and returning the location (an lvalue) defined
   316  // by the expression.
   317  //
   318  // If escaping is true, addr marks the base variable of the
   319  // addressable expression e as being a potentially escaping pointer
   320  // value.  For example, in this code:
   321  //
   322  //   a := A{
   323  //     b: [1]B{B{c: 1}}
   324  //   }
   325  //   return &a.b[0].c
   326  //
   327  // the application of & causes a.b[0].c to have its address taken,
   328  // which means that ultimately the local variable a must be
   329  // heap-allocated.  This is a simple but very conservative escape
   330  // analysis.
   331  //
   332  // Operations forming potentially escaping pointers include:
   333  // - &x, including when implicit in method call or composite literals.
   334  // - a[:] iff a is an array (not *array)
   335  // - references to variables in lexically enclosing functions.
   336  //
   337  func (b *builder) addr(fn *Function, e ast.Expr, escaping bool) lvalue {
   338  	switch e := e.(type) {
   339  	case *ast.Ident:
   340  		if isBlankIdent(e) {
   341  			return blank{}
   342  		}
   343  		obj := fn.Pkg.objectOf(e)
   344  		v := fn.Prog.packageLevelValue(obj) // var (address)
   345  		if v == nil {
   346  			v = fn.lookup(obj, escaping)
   347  		}
   348  		return &address{addr: v, pos: e.Pos(), expr: e}
   349  
   350  	case *ast.CompositeLit:
   351  		t := deref(fn.Pkg.typeOf(e))
   352  		var v *Alloc
   353  		if escaping {
   354  			v = emitNew(fn, t, e.Lbrace)
   355  		} else {
   356  			v = fn.addLocal(t, e.Lbrace)
   357  		}
   358  		v.Comment = "complit"
   359  		var sb storebuf
   360  		b.compLit(fn, v, e, true, &sb)
   361  		sb.emit(fn)
   362  		return &address{addr: v, pos: e.Lbrace, expr: e}
   363  
   364  	case *ast.ParenExpr:
   365  		return b.addr(fn, e.X, escaping)
   366  
   367  	case *ast.SelectorExpr:
   368  		sel, ok := fn.Pkg.info.Selections[e]
   369  		if !ok {
   370  			// qualified identifier
   371  			return b.addr(fn, e.Sel, escaping)
   372  		}
   373  		if sel.Kind() != types.FieldVal {
   374  			panic(sel)
   375  		}
   376  		wantAddr := true
   377  		v := b.receiver(fn, e.X, wantAddr, escaping, sel)
   378  		last := len(sel.Index()) - 1
   379  		return &address{
   380  			addr: emitFieldSelection(fn, v, sel.Index()[last], true, e.Sel),
   381  			pos:  e.Sel.Pos(),
   382  			expr: e.Sel,
   383  		}
   384  
   385  	case *ast.IndexExpr:
   386  		var x Value
   387  		var et types.Type
   388  		switch t := fn.Pkg.typeOf(e.X).Underlying().(type) {
   389  		case *types.Array:
   390  			x = b.addr(fn, e.X, escaping).address(fn)
   391  			et = types.NewPointer(t.Elem())
   392  		case *types.Pointer: // *array
   393  			x = b.expr(fn, e.X)
   394  			et = types.NewPointer(t.Elem().Underlying().(*types.Array).Elem())
   395  		case *types.Slice:
   396  			x = b.expr(fn, e.X)
   397  			et = types.NewPointer(t.Elem())
   398  		case *types.Map:
   399  			return &element{
   400  				m:   b.expr(fn, e.X),
   401  				k:   emitConv(fn, b.expr(fn, e.Index), t.Key()),
   402  				t:   t.Elem(),
   403  				pos: e.Lbrack,
   404  			}
   405  		default:
   406  			panic("unexpected container type in IndexExpr: " + t.String())
   407  		}
   408  		v := &IndexAddr{
   409  			X:     x,
   410  			Index: emitConv(fn, b.expr(fn, e.Index), tInt),
   411  		}
   412  		v.setPos(e.Lbrack)
   413  		v.setType(et)
   414  		return &address{addr: fn.emit(v), pos: e.Lbrack, expr: e}
   415  
   416  	case *ast.StarExpr:
   417  		return &address{addr: b.expr(fn, e.X), pos: e.Star, expr: e}
   418  	}
   419  
   420  	panic(fmt.Sprintf("unexpected address expression: %T", e))
   421  }
   422  
   423  type store struct {
   424  	lhs lvalue
   425  	rhs Value
   426  }
   427  
   428  type storebuf struct{ stores []store }
   429  
   430  func (sb *storebuf) store(lhs lvalue, rhs Value) {
   431  	sb.stores = append(sb.stores, store{lhs, rhs})
   432  }
   433  
   434  func (sb *storebuf) emit(fn *Function) {
   435  	for _, s := range sb.stores {
   436  		s.lhs.store(fn, s.rhs)
   437  	}
   438  }
   439  
   440  // assign emits to fn code to initialize the lvalue loc with the value
   441  // of expression e.  If isZero is true, assign assumes that loc holds
   442  // the zero value for its type.
   443  //
   444  // This is equivalent to loc.store(fn, b.expr(fn, e)), but may generate
   445  // better code in some cases, e.g., for composite literals in an
   446  // addressable location.
   447  //
   448  // If sb is not nil, assign generates code to evaluate expression e, but
   449  // not to update loc.  Instead, the necessary stores are appended to the
   450  // storebuf sb so that they can be executed later.  This allows correct
   451  // in-place update of existing variables when the RHS is a composite
   452  // literal that may reference parts of the LHS.
   453  //
   454  func (b *builder) assign(fn *Function, loc lvalue, e ast.Expr, isZero bool, sb *storebuf) {
   455  	// Can we initialize it in place?
   456  	if e, ok := unparen(e).(*ast.CompositeLit); ok {
   457  		// A CompositeLit never evaluates to a pointer,
   458  		// so if the type of the location is a pointer,
   459  		// an &-operation is implied.
   460  		if _, ok := loc.(blank); !ok { // avoid calling blank.typ()
   461  			if isPointer(loc.typ()) {
   462  				ptr := b.addr(fn, e, true).address(fn)
   463  				// copy address
   464  				if sb != nil {
   465  					sb.store(loc, ptr)
   466  				} else {
   467  					loc.store(fn, ptr)
   468  				}
   469  				return
   470  			}
   471  		}
   472  
   473  		if _, ok := loc.(*address); ok {
   474  			if isInterface(loc.typ()) {
   475  				// e.g. var x interface{} = T{...}
   476  				// Can't in-place initialize an interface value.
   477  				// Fall back to copying.
   478  			} else {
   479  				// x = T{...} or x := T{...}
   480  				addr := loc.address(fn)
   481  				if sb != nil {
   482  					b.compLit(fn, addr, e, isZero, sb)
   483  				} else {
   484  					var sb storebuf
   485  					b.compLit(fn, addr, e, isZero, &sb)
   486  					sb.emit(fn)
   487  				}
   488  
   489  				// Subtle: emit debug ref for aggregate types only;
   490  				// slice and map are handled by store ops in compLit.
   491  				switch loc.typ().Underlying().(type) {
   492  				case *types.Struct, *types.Array:
   493  					emitDebugRef(fn, e, addr, true)
   494  				}
   495  
   496  				return
   497  			}
   498  		}
   499  	}
   500  
   501  	// simple case: just copy
   502  	rhs := b.expr(fn, e)
   503  	if sb != nil {
   504  		sb.store(loc, rhs)
   505  	} else {
   506  		loc.store(fn, rhs)
   507  	}
   508  }
   509  
   510  // expr lowers a single-result expression e to SSA form, emitting code
   511  // to fn and returning the Value defined by the expression.
   512  //
   513  func (b *builder) expr(fn *Function, e ast.Expr) Value {
   514  	e = unparen(e)
   515  
   516  	tv := fn.Pkg.info.Types[e]
   517  
   518  	// Is expression a constant?
   519  	if tv.Value != nil {
   520  		return NewConst(tv.Value, tv.Type)
   521  	}
   522  
   523  	var v Value
   524  	if tv.Addressable() {
   525  		// Prefer pointer arithmetic ({Index,Field}Addr) followed
   526  		// by Load over subelement extraction (e.g. Index, Field),
   527  		// to avoid large copies.
   528  		v = b.addr(fn, e, false).load(fn)
   529  	} else {
   530  		v = b.expr0(fn, e, tv)
   531  	}
   532  	if fn.debugInfo() {
   533  		emitDebugRef(fn, e, v, false)
   534  	}
   535  	return v
   536  }
   537  
   538  func (b *builder) expr0(fn *Function, e ast.Expr, tv types.TypeAndValue) Value {
   539  	switch e := e.(type) {
   540  	case *ast.BasicLit:
   541  		panic("non-constant BasicLit") // unreachable
   542  
   543  	case *ast.FuncLit:
   544  		fn2 := &Function{
   545  			name:      fmt.Sprintf("%s$%d", fn.Name(), 1+len(fn.AnonFuncs)),
   546  			Signature: fn.Pkg.typeOf(e.Type).Underlying().(*types.Signature),
   547  			pos:       e.Type.Func,
   548  			parent:    fn,
   549  			Pkg:       fn.Pkg,
   550  			Prog:      fn.Prog,
   551  			syntax:    e,
   552  		}
   553  		fn.AnonFuncs = append(fn.AnonFuncs, fn2)
   554  		b.buildFunction(fn2)
   555  		if fn2.FreeVars == nil {
   556  			return fn2
   557  		}
   558  		v := &MakeClosure{Fn: fn2}
   559  		v.setType(tv.Type)
   560  		for _, fv := range fn2.FreeVars {
   561  			v.Bindings = append(v.Bindings, fv.outer)
   562  			fv.outer = nil
   563  		}
   564  		return fn.emit(v)
   565  
   566  	case *ast.TypeAssertExpr: // single-result form only
   567  		return emitTypeAssert(fn, b.expr(fn, e.X), tv.Type, e.Lparen)
   568  
   569  	case *ast.CallExpr:
   570  		if fn.Pkg.info.Types[e.Fun].IsType() {
   571  			// Explicit type conversion, e.g. string(x) or big.Int(x)
   572  			x := b.expr(fn, e.Args[0])
   573  			y := emitConv(fn, x, tv.Type)
   574  			if y != x {
   575  				switch y := y.(type) {
   576  				case *Convert:
   577  					y.pos = e.Lparen
   578  				case *ChangeType:
   579  					y.pos = e.Lparen
   580  				case *MakeInterface:
   581  					y.pos = e.Lparen
   582  				}
   583  			}
   584  			return y
   585  		}
   586  		// Call to "intrinsic" built-ins, e.g. new, make, panic.
   587  		if id, ok := unparen(e.Fun).(*ast.Ident); ok {
   588  			if obj, ok := fn.Pkg.info.Uses[id].(*types.Builtin); ok {
   589  				if v := b.builtin(fn, obj, e.Args, tv.Type, e.Lparen); v != nil {
   590  					return v
   591  				}
   592  			}
   593  		}
   594  		// Regular function call.
   595  		var v Call
   596  		b.setCall(fn, e, &v.Call)
   597  		v.setType(tv.Type)
   598  		return fn.emit(&v)
   599  
   600  	case *ast.UnaryExpr:
   601  		switch e.Op {
   602  		case token.AND: // &X --- potentially escaping.
   603  			addr := b.addr(fn, e.X, true)
   604  			if _, ok := unparen(e.X).(*ast.StarExpr); ok {
   605  				// &*p must panic if p is nil (http://golang.org/s/go12nil).
   606  				// For simplicity, we'll just (suboptimally) rely
   607  				// on the side effects of a load.
   608  				// TODO(adonovan): emit dedicated nilcheck.
   609  				addr.load(fn)
   610  			}
   611  			return addr.address(fn)
   612  		case token.ADD:
   613  			return b.expr(fn, e.X)
   614  		case token.NOT, token.ARROW, token.SUB, token.XOR: // ! <- - ^
   615  			v := &UnOp{
   616  				Op: e.Op,
   617  				X:  b.expr(fn, e.X),
   618  			}
   619  			v.setPos(e.OpPos)
   620  			v.setType(tv.Type)
   621  			return fn.emit(v)
   622  		default:
   623  			panic(e.Op)
   624  		}
   625  
   626  	case *ast.BinaryExpr:
   627  		switch e.Op {
   628  		case token.LAND, token.LOR:
   629  			return b.logicalBinop(fn, e)
   630  		case token.SHL, token.SHR:
   631  			fallthrough
   632  		case token.ADD, token.SUB, token.MUL, token.QUO, token.REM, token.AND, token.OR, token.XOR, token.AND_NOT:
   633  			return emitArith(fn, e.Op, b.expr(fn, e.X), b.expr(fn, e.Y), tv.Type, e.OpPos)
   634  
   635  		case token.EQL, token.NEQ, token.GTR, token.LSS, token.LEQ, token.GEQ:
   636  			cmp := emitCompare(fn, e.Op, b.expr(fn, e.X), b.expr(fn, e.Y), e.OpPos)
   637  			// The type of x==y may be UntypedBool.
   638  			return emitConv(fn, cmp, types.Default(tv.Type))
   639  		default:
   640  			panic("illegal op in BinaryExpr: " + e.Op.String())
   641  		}
   642  
   643  	case *ast.SliceExpr:
   644  		var low, high, max Value
   645  		var x Value
   646  		switch fn.Pkg.typeOf(e.X).Underlying().(type) {
   647  		case *types.Array:
   648  			// Potentially escaping.
   649  			x = b.addr(fn, e.X, true).address(fn)
   650  		case *types.Basic, *types.Slice, *types.Pointer: // *array
   651  			x = b.expr(fn, e.X)
   652  		default:
   653  			panic("unreachable")
   654  		}
   655  		if e.High != nil {
   656  			high = b.expr(fn, e.High)
   657  		}
   658  		if e.Low != nil {
   659  			low = b.expr(fn, e.Low)
   660  		}
   661  		if e.Slice3 {
   662  			max = b.expr(fn, e.Max)
   663  		}
   664  		v := &Slice{
   665  			X:    x,
   666  			Low:  low,
   667  			High: high,
   668  			Max:  max,
   669  		}
   670  		v.setPos(e.Lbrack)
   671  		v.setType(tv.Type)
   672  		return fn.emit(v)
   673  
   674  	case *ast.Ident:
   675  		obj := fn.Pkg.info.Uses[e]
   676  		// Universal built-in or nil?
   677  		switch obj := obj.(type) {
   678  		case *types.Builtin:
   679  			return &Builtin{name: obj.Name(), sig: tv.Type.(*types.Signature)}
   680  		case *types.Nil:
   681  			return nilConst(tv.Type)
   682  		}
   683  		// Package-level func or var?
   684  		if v := fn.Prog.packageLevelValue(obj); v != nil {
   685  			if _, ok := obj.(*types.Var); ok {
   686  				return emitLoad(fn, v) // var (address)
   687  			}
   688  			return v // (func)
   689  		}
   690  		// Local var.
   691  		return emitLoad(fn, fn.lookup(obj, false)) // var (address)
   692  
   693  	case *ast.SelectorExpr:
   694  		sel, ok := fn.Pkg.info.Selections[e]
   695  		if !ok {
   696  			// qualified identifier
   697  			return b.expr(fn, e.Sel)
   698  		}
   699  		switch sel.Kind() {
   700  		case types.MethodExpr:
   701  			// (*T).f or T.f, the method f from the method-set of type T.
   702  			// The result is a "thunk".
   703  			return emitConv(fn, makeThunk(fn.Prog, sel), tv.Type)
   704  
   705  		case types.MethodVal:
   706  			// e.f where e is an expression and f is a method.
   707  			// The result is a "bound".
   708  			obj := sel.Obj().(*types.Func)
   709  			rt := recvType(obj)
   710  			wantAddr := isPointer(rt)
   711  			escaping := true
   712  			v := b.receiver(fn, e.X, wantAddr, escaping, sel)
   713  			if isInterface(rt) {
   714  				// If v has interface type I,
   715  				// we must emit a check that v is non-nil.
   716  				// We use: typeassert v.(I).
   717  				emitTypeAssert(fn, v, rt, token.NoPos)
   718  			}
   719  			c := &MakeClosure{
   720  				Fn:       makeBound(fn.Prog, obj),
   721  				Bindings: []Value{v},
   722  			}
   723  			c.setPos(e.Sel.Pos())
   724  			c.setType(tv.Type)
   725  			return fn.emit(c)
   726  
   727  		case types.FieldVal:
   728  			indices := sel.Index()
   729  			last := len(indices) - 1
   730  			v := b.expr(fn, e.X)
   731  			v = emitImplicitSelections(fn, v, indices[:last])
   732  			v = emitFieldSelection(fn, v, indices[last], false, e.Sel)
   733  			return v
   734  		}
   735  
   736  		panic("unexpected expression-relative selector")
   737  
   738  	case *ast.IndexExpr:
   739  		switch t := fn.Pkg.typeOf(e.X).Underlying().(type) {
   740  		case *types.Array:
   741  			// Non-addressable array (in a register).
   742  			v := &Index{
   743  				X:     b.expr(fn, e.X),
   744  				Index: emitConv(fn, b.expr(fn, e.Index), tInt),
   745  			}
   746  			v.setPos(e.Lbrack)
   747  			v.setType(t.Elem())
   748  			return fn.emit(v)
   749  
   750  		case *types.Map:
   751  			// Maps are not addressable.
   752  			mapt := fn.Pkg.typeOf(e.X).Underlying().(*types.Map)
   753  			v := &Lookup{
   754  				X:     b.expr(fn, e.X),
   755  				Index: emitConv(fn, b.expr(fn, e.Index), mapt.Key()),
   756  			}
   757  			v.setPos(e.Lbrack)
   758  			v.setType(mapt.Elem())
   759  			return fn.emit(v)
   760  
   761  		case *types.Basic: // => string
   762  			// Strings are not addressable.
   763  			v := &Lookup{
   764  				X:     b.expr(fn, e.X),
   765  				Index: b.expr(fn, e.Index),
   766  			}
   767  			v.setPos(e.Lbrack)
   768  			v.setType(tByte)
   769  			return fn.emit(v)
   770  
   771  		case *types.Slice, *types.Pointer: // *array
   772  			// Addressable slice/array; use IndexAddr and Load.
   773  			return b.addr(fn, e, false).load(fn)
   774  
   775  		default:
   776  			panic("unexpected container type in IndexExpr: " + t.String())
   777  		}
   778  
   779  	case *ast.CompositeLit, *ast.StarExpr:
   780  		// Addressable types (lvalues)
   781  		return b.addr(fn, e, false).load(fn)
   782  	}
   783  
   784  	panic(fmt.Sprintf("unexpected expr: %T", e))
   785  }
   786  
   787  // stmtList emits to fn code for all statements in list.
   788  func (b *builder) stmtList(fn *Function, list []ast.Stmt) {
   789  	for _, s := range list {
   790  		b.stmt(fn, s)
   791  	}
   792  }
   793  
   794  // receiver emits to fn code for expression e in the "receiver"
   795  // position of selection e.f (where f may be a field or a method) and
   796  // returns the effective receiver after applying the implicit field
   797  // selections of sel.
   798  //
   799  // wantAddr requests that the result is an an address.  If
   800  // !sel.Indirect(), this may require that e be built in addr() mode; it
   801  // must thus be addressable.
   802  //
   803  // escaping is defined as per builder.addr().
   804  //
   805  func (b *builder) receiver(fn *Function, e ast.Expr, wantAddr, escaping bool, sel *types.Selection) Value {
   806  	var v Value
   807  	if wantAddr && !sel.Indirect() && !isPointer(fn.Pkg.typeOf(e)) {
   808  		v = b.addr(fn, e, escaping).address(fn)
   809  	} else {
   810  		v = b.expr(fn, e)
   811  	}
   812  
   813  	last := len(sel.Index()) - 1
   814  	v = emitImplicitSelections(fn, v, sel.Index()[:last])
   815  	if !wantAddr && isPointer(v.Type()) {
   816  		v = emitLoad(fn, v)
   817  	}
   818  	return v
   819  }
   820  
   821  // setCallFunc populates the function parts of a CallCommon structure
   822  // (Func, Method, Recv, Args[0]) based on the kind of invocation
   823  // occurring in e.
   824  //
   825  func (b *builder) setCallFunc(fn *Function, e *ast.CallExpr, c *CallCommon) {
   826  	c.pos = e.Lparen
   827  
   828  	// Is this a method call?
   829  	if selector, ok := unparen(e.Fun).(*ast.SelectorExpr); ok {
   830  		sel, ok := fn.Pkg.info.Selections[selector]
   831  		if ok && sel.Kind() == types.MethodVal {
   832  			obj := sel.Obj().(*types.Func)
   833  			recv := recvType(obj)
   834  			wantAddr := isPointer(recv)
   835  			escaping := true
   836  			v := b.receiver(fn, selector.X, wantAddr, escaping, sel)
   837  			if isInterface(recv) {
   838  				// Invoke-mode call.
   839  				c.Value = v
   840  				c.Method = obj
   841  			} else {
   842  				// "Call"-mode call.
   843  				c.Value = fn.Prog.declaredFunc(obj)
   844  				c.Args = append(c.Args, v)
   845  			}
   846  			return
   847  		}
   848  
   849  		// sel.Kind()==MethodExpr indicates T.f() or (*T).f():
   850  		// a statically dispatched call to the method f in the
   851  		// method-set of T or *T.  T may be an interface.
   852  		//
   853  		// e.Fun would evaluate to a concrete method, interface
   854  		// wrapper function, or promotion wrapper.
   855  		//
   856  		// For now, we evaluate it in the usual way.
   857  		//
   858  		// TODO(adonovan): opt: inline expr() here, to make the
   859  		// call static and to avoid generation of wrappers.
   860  		// It's somewhat tricky as it may consume the first
   861  		// actual parameter if the call is "invoke" mode.
   862  		//
   863  		// Examples:
   864  		//  type T struct{}; func (T) f() {}   // "call" mode
   865  		//  type T interface { f() }           // "invoke" mode
   866  		//
   867  		//  type S struct{ T }
   868  		//
   869  		//  var s S
   870  		//  S.f(s)
   871  		//  (*S).f(&s)
   872  		//
   873  		// Suggested approach:
   874  		// - consume the first actual parameter expression
   875  		//   and build it with b.expr().
   876  		// - apply implicit field selections.
   877  		// - use MethodVal logic to populate fields of c.
   878  	}
   879  
   880  	// Evaluate the function operand in the usual way.
   881  	c.Value = b.expr(fn, e.Fun)
   882  }
   883  
   884  // emitCallArgs emits to f code for the actual parameters of call e to
   885  // a (possibly built-in) function of effective type sig.
   886  // The argument values are appended to args, which is then returned.
   887  //
   888  func (b *builder) emitCallArgs(fn *Function, sig *types.Signature, e *ast.CallExpr, args []Value) []Value {
   889  	// f(x, y, z...): pass slice z straight through.
   890  	if e.Ellipsis != 0 {
   891  		for i, arg := range e.Args {
   892  			v := emitConv(fn, b.expr(fn, arg), sig.Params().At(i).Type())
   893  			args = append(args, v)
   894  		}
   895  		return args
   896  	}
   897  
   898  	offset := len(args) // 1 if call has receiver, 0 otherwise
   899  
   900  	// Evaluate actual parameter expressions.
   901  	//
   902  	// If this is a chained call of the form f(g()) where g has
   903  	// multiple return values (MRV), they are flattened out into
   904  	// args; a suffix of them may end up in a varargs slice.
   905  	for _, arg := range e.Args {
   906  		v := b.expr(fn, arg)
   907  		if ttuple, ok := v.Type().(*types.Tuple); ok { // MRV chain
   908  			for i, n := 0, ttuple.Len(); i < n; i++ {
   909  				args = append(args, emitExtract(fn, v, i))
   910  			}
   911  		} else {
   912  			args = append(args, v)
   913  		}
   914  	}
   915  
   916  	// Actual->formal assignability conversions for normal parameters.
   917  	np := sig.Params().Len() // number of normal parameters
   918  	if sig.Variadic() {
   919  		np--
   920  	}
   921  	for i := 0; i < np; i++ {
   922  		args[offset+i] = emitConv(fn, args[offset+i], sig.Params().At(i).Type())
   923  	}
   924  
   925  	// Actual->formal assignability conversions for variadic parameter,
   926  	// and construction of slice.
   927  	if sig.Variadic() {
   928  		varargs := args[offset+np:]
   929  		st := sig.Params().At(np).Type().(*types.Slice)
   930  		vt := st.Elem()
   931  		if len(varargs) == 0 {
   932  			args = append(args, nilConst(st))
   933  		} else {
   934  			// Replace a suffix of args with a slice containing it.
   935  			at := types.NewArray(vt, int64(len(varargs)))
   936  			a := emitNew(fn, at, token.NoPos)
   937  			a.setPos(e.Rparen)
   938  			a.Comment = "varargs"
   939  			for i, arg := range varargs {
   940  				iaddr := &IndexAddr{
   941  					X:     a,
   942  					Index: intConst(int64(i)),
   943  				}
   944  				iaddr.setType(types.NewPointer(vt))
   945  				fn.emit(iaddr)
   946  				emitStore(fn, iaddr, arg, arg.Pos())
   947  			}
   948  			s := &Slice{X: a}
   949  			s.setType(st)
   950  			args[offset+np] = fn.emit(s)
   951  			args = args[:offset+np+1]
   952  		}
   953  	}
   954  	return args
   955  }
   956  
   957  // setCall emits to fn code to evaluate all the parameters of a function
   958  // call e, and populates *c with those values.
   959  //
   960  func (b *builder) setCall(fn *Function, e *ast.CallExpr, c *CallCommon) {
   961  	// First deal with the f(...) part and optional receiver.
   962  	b.setCallFunc(fn, e, c)
   963  
   964  	// Then append the other actual parameters.
   965  	sig, _ := fn.Pkg.typeOf(e.Fun).Underlying().(*types.Signature)
   966  	if sig == nil {
   967  		panic(fmt.Sprintf("no signature for call of %s", e.Fun))
   968  	}
   969  	c.Args = b.emitCallArgs(fn, sig, e, c.Args)
   970  }
   971  
   972  // assignOp emits to fn code to perform loc <op>= val.
   973  func (b *builder) assignOp(fn *Function, loc lvalue, val Value, op token.Token, pos token.Pos) {
   974  	oldv := loc.load(fn)
   975  	loc.store(fn, emitArith(fn, op, oldv, emitConv(fn, val, oldv.Type()), loc.typ(), pos))
   976  }
   977  
   978  // localValueSpec emits to fn code to define all of the vars in the
   979  // function-local ValueSpec, spec.
   980  //
   981  func (b *builder) localValueSpec(fn *Function, spec *ast.ValueSpec) {
   982  	switch {
   983  	case len(spec.Values) == len(spec.Names):
   984  		// e.g. var x, y = 0, 1
   985  		// 1:1 assignment
   986  		for i, id := range spec.Names {
   987  			if !isBlankIdent(id) {
   988  				fn.addLocalForIdent(id)
   989  			}
   990  			lval := b.addr(fn, id, false) // non-escaping
   991  			b.assign(fn, lval, spec.Values[i], true, nil)
   992  		}
   993  
   994  	case len(spec.Values) == 0:
   995  		// e.g. var x, y int
   996  		// Locals are implicitly zero-initialized.
   997  		for _, id := range spec.Names {
   998  			if !isBlankIdent(id) {
   999  				lhs := fn.addLocalForIdent(id)
  1000  				if fn.debugInfo() {
  1001  					emitDebugRef(fn, id, lhs, true)
  1002  				}
  1003  			}
  1004  		}
  1005  
  1006  	default:
  1007  		// e.g. var x, y = pos()
  1008  		tuple := b.exprN(fn, spec.Values[0])
  1009  		for i, id := range spec.Names {
  1010  			if !isBlankIdent(id) {
  1011  				fn.addLocalForIdent(id)
  1012  				lhs := b.addr(fn, id, false) // non-escaping
  1013  				lhs.store(fn, emitExtract(fn, tuple, i))
  1014  			}
  1015  		}
  1016  	}
  1017  }
  1018  
  1019  // assignStmt emits code to fn for a parallel assignment of rhss to lhss.
  1020  // isDef is true if this is a short variable declaration (:=).
  1021  //
  1022  // Note the similarity with localValueSpec.
  1023  //
  1024  func (b *builder) assignStmt(fn *Function, lhss, rhss []ast.Expr, isDef bool) {
  1025  	// Side effects of all LHSs and RHSs must occur in left-to-right order.
  1026  	lvals := make([]lvalue, len(lhss))
  1027  	isZero := make([]bool, len(lhss))
  1028  	for i, lhs := range lhss {
  1029  		var lval lvalue = blank{}
  1030  		if !isBlankIdent(lhs) {
  1031  			if isDef {
  1032  				if obj := fn.Pkg.info.Defs[lhs.(*ast.Ident)]; obj != nil {
  1033  					fn.addNamedLocal(obj)
  1034  					isZero[i] = true
  1035  				}
  1036  			}
  1037  			lval = b.addr(fn, lhs, false) // non-escaping
  1038  		}
  1039  		lvals[i] = lval
  1040  	}
  1041  	if len(lhss) == len(rhss) {
  1042  		// Simple assignment:   x     = f()        (!isDef)
  1043  		// Parallel assignment: x, y  = f(), g()   (!isDef)
  1044  		// or short var decl:   x, y := f(), g()   (isDef)
  1045  		//
  1046  		// In all cases, the RHSs may refer to the LHSs,
  1047  		// so we need a storebuf.
  1048  		var sb storebuf
  1049  		for i := range rhss {
  1050  			b.assign(fn, lvals[i], rhss[i], isZero[i], &sb)
  1051  		}
  1052  		sb.emit(fn)
  1053  	} else {
  1054  		// e.g. x, y = pos()
  1055  		tuple := b.exprN(fn, rhss[0])
  1056  		emitDebugRef(fn, rhss[0], tuple, false)
  1057  		for i, lval := range lvals {
  1058  			lval.store(fn, emitExtract(fn, tuple, i))
  1059  		}
  1060  	}
  1061  }
  1062  
  1063  // arrayLen returns the length of the array whose composite literal elements are elts.
  1064  func (b *builder) arrayLen(fn *Function, elts []ast.Expr) int64 {
  1065  	var max int64 = -1
  1066  	var i int64 = -1
  1067  	for _, e := range elts {
  1068  		if kv, ok := e.(*ast.KeyValueExpr); ok {
  1069  			i = b.expr(fn, kv.Key).(*Const).Int64()
  1070  		} else {
  1071  			i++
  1072  		}
  1073  		if i > max {
  1074  			max = i
  1075  		}
  1076  	}
  1077  	return max + 1
  1078  }
  1079  
  1080  // compLit emits to fn code to initialize a composite literal e at
  1081  // address addr with type typ.
  1082  //
  1083  // Nested composite literals are recursively initialized in place
  1084  // where possible. If isZero is true, compLit assumes that addr
  1085  // holds the zero value for typ.
  1086  //
  1087  // Because the elements of a composite literal may refer to the
  1088  // variables being updated, as in the second line below,
  1089  //	x := T{a: 1}
  1090  //	x = T{a: x.a}
  1091  // all the reads must occur before all the writes.  Thus all stores to
  1092  // loc are emitted to the storebuf sb for later execution.
  1093  //
  1094  // A CompositeLit may have pointer type only in the recursive (nested)
  1095  // case when the type name is implicit.  e.g. in []*T{{}}, the inner
  1096  // literal has type *T behaves like &T{}.
  1097  // In that case, addr must hold a T, not a *T.
  1098  //
  1099  func (b *builder) compLit(fn *Function, addr Value, e *ast.CompositeLit, isZero bool, sb *storebuf) {
  1100  	typ := deref(fn.Pkg.typeOf(e))
  1101  	switch t := typ.Underlying().(type) {
  1102  	case *types.Struct:
  1103  		if !isZero && len(e.Elts) != t.NumFields() {
  1104  			// memclear
  1105  			sb.store(&address{addr, e.Lbrace, nil},
  1106  				zeroValue(fn, deref(addr.Type())))
  1107  			isZero = true
  1108  		}
  1109  		for i, e := range e.Elts {
  1110  			fieldIndex := i
  1111  			pos := e.Pos()
  1112  			if kv, ok := e.(*ast.KeyValueExpr); ok {
  1113  				fname := kv.Key.(*ast.Ident).Name
  1114  				for i, n := 0, t.NumFields(); i < n; i++ {
  1115  					sf := t.Field(i)
  1116  					if sf.Name() == fname {
  1117  						fieldIndex = i
  1118  						pos = kv.Colon
  1119  						e = kv.Value
  1120  						break
  1121  					}
  1122  				}
  1123  			}
  1124  			sf := t.Field(fieldIndex)
  1125  			faddr := &FieldAddr{
  1126  				X:     addr,
  1127  				Field: fieldIndex,
  1128  			}
  1129  			faddr.setType(types.NewPointer(sf.Type()))
  1130  			fn.emit(faddr)
  1131  			b.assign(fn, &address{addr: faddr, pos: pos, expr: e}, e, isZero, sb)
  1132  		}
  1133  
  1134  	case *types.Array, *types.Slice:
  1135  		var at *types.Array
  1136  		var array Value
  1137  		switch t := t.(type) {
  1138  		case *types.Slice:
  1139  			at = types.NewArray(t.Elem(), b.arrayLen(fn, e.Elts))
  1140  			alloc := emitNew(fn, at, e.Lbrace)
  1141  			alloc.Comment = "slicelit"
  1142  			array = alloc
  1143  		case *types.Array:
  1144  			at = t
  1145  			array = addr
  1146  
  1147  			if !isZero && int64(len(e.Elts)) != at.Len() {
  1148  				// memclear
  1149  				sb.store(&address{array, e.Lbrace, nil},
  1150  					zeroValue(fn, deref(array.Type())))
  1151  			}
  1152  		}
  1153  
  1154  		var idx *Const
  1155  		for _, e := range e.Elts {
  1156  			pos := e.Pos()
  1157  			if kv, ok := e.(*ast.KeyValueExpr); ok {
  1158  				idx = b.expr(fn, kv.Key).(*Const)
  1159  				pos = kv.Colon
  1160  				e = kv.Value
  1161  			} else {
  1162  				var idxval int64
  1163  				if idx != nil {
  1164  					idxval = idx.Int64() + 1
  1165  				}
  1166  				idx = intConst(idxval)
  1167  			}
  1168  			iaddr := &IndexAddr{
  1169  				X:     array,
  1170  				Index: idx,
  1171  			}
  1172  			iaddr.setType(types.NewPointer(at.Elem()))
  1173  			fn.emit(iaddr)
  1174  			if t != at { // slice
  1175  				// backing array is unaliased => storebuf not needed.
  1176  				b.assign(fn, &address{addr: iaddr, pos: pos, expr: e}, e, true, nil)
  1177  			} else {
  1178  				b.assign(fn, &address{addr: iaddr, pos: pos, expr: e}, e, true, sb)
  1179  			}
  1180  		}
  1181  
  1182  		if t != at { // slice
  1183  			s := &Slice{X: array}
  1184  			s.setPos(e.Lbrace)
  1185  			s.setType(typ)
  1186  			sb.store(&address{addr: addr, pos: e.Lbrace, expr: e}, fn.emit(s))
  1187  		}
  1188  
  1189  	case *types.Map:
  1190  		m := &MakeMap{Reserve: intConst(int64(len(e.Elts)))}
  1191  		m.setPos(e.Lbrace)
  1192  		m.setType(typ)
  1193  		fn.emit(m)
  1194  		for _, e := range e.Elts {
  1195  			e := e.(*ast.KeyValueExpr)
  1196  
  1197  			// If a key expression in a map literal is itself a
  1198  			// composite literal, the type may be omitted.
  1199  			// For example:
  1200  			//	map[*struct{}]bool{{}: true}
  1201  			// An &-operation may be implied:
  1202  			//	map[*struct{}]bool{&struct{}{}: true}
  1203  			var key Value
  1204  			if _, ok := unparen(e.Key).(*ast.CompositeLit); ok && isPointer(t.Key()) {
  1205  				// A CompositeLit never evaluates to a pointer,
  1206  				// so if the type of the location is a pointer,
  1207  				// an &-operation is implied.
  1208  				key = b.addr(fn, e.Key, true).address(fn)
  1209  			} else {
  1210  				key = b.expr(fn, e.Key)
  1211  			}
  1212  
  1213  			loc := element{
  1214  				m:   m,
  1215  				k:   emitConv(fn, key, t.Key()),
  1216  				t:   t.Elem(),
  1217  				pos: e.Colon,
  1218  			}
  1219  
  1220  			// We call assign() only because it takes care
  1221  			// of any &-operation required in the recursive
  1222  			// case, e.g.,
  1223  			// map[int]*struct{}{0: {}} implies &struct{}{}.
  1224  			// In-place update is of course impossible,
  1225  			// and no storebuf is needed.
  1226  			b.assign(fn, &loc, e.Value, true, nil)
  1227  		}
  1228  		sb.store(&address{addr: addr, pos: e.Lbrace, expr: e}, m)
  1229  
  1230  	default:
  1231  		panic("unexpected CompositeLit type: " + t.String())
  1232  	}
  1233  }
  1234  
  1235  // switchStmt emits to fn code for the switch statement s, optionally
  1236  // labelled by label.
  1237  //
  1238  func (b *builder) switchStmt(fn *Function, s *ast.SwitchStmt, label *lblock) {
  1239  	// We treat SwitchStmt like a sequential if-else chain.
  1240  	// Multiway dispatch can be recovered later by ssautil.Switches()
  1241  	// to those cases that are free of side effects.
  1242  	if s.Init != nil {
  1243  		b.stmt(fn, s.Init)
  1244  	}
  1245  	var tag Value = vTrue
  1246  	if s.Tag != nil {
  1247  		tag = b.expr(fn, s.Tag)
  1248  	}
  1249  	done := fn.newBasicBlock("switch.done")
  1250  	if label != nil {
  1251  		label._break = done
  1252  	}
  1253  	// We pull the default case (if present) down to the end.
  1254  	// But each fallthrough label must point to the next
  1255  	// body block in source order, so we preallocate a
  1256  	// body block (fallthru) for the next case.
  1257  	// Unfortunately this makes for a confusing block order.
  1258  	var dfltBody *[]ast.Stmt
  1259  	var dfltFallthrough *BasicBlock
  1260  	var fallthru, dfltBlock *BasicBlock
  1261  	ncases := len(s.Body.List)
  1262  	for i, clause := range s.Body.List {
  1263  		body := fallthru
  1264  		if body == nil {
  1265  			body = fn.newBasicBlock("switch.body") // first case only
  1266  		}
  1267  
  1268  		// Preallocate body block for the next case.
  1269  		fallthru = done
  1270  		if i+1 < ncases {
  1271  			fallthru = fn.newBasicBlock("switch.body")
  1272  		}
  1273  
  1274  		cc := clause.(*ast.CaseClause)
  1275  		if cc.List == nil {
  1276  			// Default case.
  1277  			dfltBody = &cc.Body
  1278  			dfltFallthrough = fallthru
  1279  			dfltBlock = body
  1280  			continue
  1281  		}
  1282  
  1283  		var nextCond *BasicBlock
  1284  		for _, cond := range cc.List {
  1285  			nextCond = fn.newBasicBlock("switch.next")
  1286  			// TODO(adonovan): opt: when tag==vTrue, we'd
  1287  			// get better code if we use b.cond(cond)
  1288  			// instead of BinOp(EQL, tag, b.expr(cond))
  1289  			// followed by If.  Don't forget conversions
  1290  			// though.
  1291  			cond := emitCompare(fn, token.EQL, tag, b.expr(fn, cond), token.NoPos)
  1292  			emitIf(fn, cond, body, nextCond)
  1293  			fn.currentBlock = nextCond
  1294  		}
  1295  		fn.currentBlock = body
  1296  		fn.targets = &targets{
  1297  			tail:         fn.targets,
  1298  			_break:       done,
  1299  			_fallthrough: fallthru,
  1300  		}
  1301  		b.stmtList(fn, cc.Body)
  1302  		fn.targets = fn.targets.tail
  1303  		emitJump(fn, done)
  1304  		fn.currentBlock = nextCond
  1305  	}
  1306  	if dfltBlock != nil {
  1307  		emitJump(fn, dfltBlock)
  1308  		fn.currentBlock = dfltBlock
  1309  		fn.targets = &targets{
  1310  			tail:         fn.targets,
  1311  			_break:       done,
  1312  			_fallthrough: dfltFallthrough,
  1313  		}
  1314  		b.stmtList(fn, *dfltBody)
  1315  		fn.targets = fn.targets.tail
  1316  	}
  1317  	emitJump(fn, done)
  1318  	fn.currentBlock = done
  1319  }
  1320  
  1321  // typeSwitchStmt emits to fn code for the type switch statement s, optionally
  1322  // labelled by label.
  1323  //
  1324  func (b *builder) typeSwitchStmt(fn *Function, s *ast.TypeSwitchStmt, label *lblock) {
  1325  	// We treat TypeSwitchStmt like a sequential if-else chain.
  1326  	// Multiway dispatch can be recovered later by ssautil.Switches().
  1327  
  1328  	// Typeswitch lowering:
  1329  	//
  1330  	// var x X
  1331  	// switch y := x.(type) {
  1332  	// case T1, T2: S1                  // >1 	(y := x)
  1333  	// case nil:    SN                  // nil 	(y := x)
  1334  	// default:     SD                  // 0 types 	(y := x)
  1335  	// case T3:     S3                  // 1 type 	(y := x.(T3))
  1336  	// }
  1337  	//
  1338  	//      ...s.Init...
  1339  	// 	x := eval x
  1340  	// .caseT1:
  1341  	// 	t1, ok1 := typeswitch,ok x <T1>
  1342  	// 	if ok1 then goto S1 else goto .caseT2
  1343  	// .caseT2:
  1344  	// 	t2, ok2 := typeswitch,ok x <T2>
  1345  	// 	if ok2 then goto S1 else goto .caseNil
  1346  	// .S1:
  1347  	//      y := x
  1348  	// 	...S1...
  1349  	// 	goto done
  1350  	// .caseNil:
  1351  	// 	if t2, ok2 := typeswitch,ok x <T2>
  1352  	// 	if x == nil then goto SN else goto .caseT3
  1353  	// .SN:
  1354  	//      y := x
  1355  	// 	...SN...
  1356  	// 	goto done
  1357  	// .caseT3:
  1358  	// 	t3, ok3 := typeswitch,ok x <T3>
  1359  	// 	if ok3 then goto S3 else goto default
  1360  	// .S3:
  1361  	//      y := t3
  1362  	// 	...S3...
  1363  	// 	goto done
  1364  	// .default:
  1365  	//      y := x
  1366  	// 	...SD...
  1367  	// 	goto done
  1368  	// .done:
  1369  
  1370  	if s.Init != nil {
  1371  		b.stmt(fn, s.Init)
  1372  	}
  1373  
  1374  	var x Value
  1375  	switch ass := s.Assign.(type) {
  1376  	case *ast.ExprStmt: // x.(type)
  1377  		x = b.expr(fn, unparen(ass.X).(*ast.TypeAssertExpr).X)
  1378  	case *ast.AssignStmt: // y := x.(type)
  1379  		x = b.expr(fn, unparen(ass.Rhs[0]).(*ast.TypeAssertExpr).X)
  1380  	}
  1381  
  1382  	done := fn.newBasicBlock("typeswitch.done")
  1383  	if label != nil {
  1384  		label._break = done
  1385  	}
  1386  	var default_ *ast.CaseClause
  1387  	for _, clause := range s.Body.List {
  1388  		cc := clause.(*ast.CaseClause)
  1389  		if cc.List == nil {
  1390  			default_ = cc
  1391  			continue
  1392  		}
  1393  		body := fn.newBasicBlock("typeswitch.body")
  1394  		var next *BasicBlock
  1395  		var casetype types.Type
  1396  		var ti Value // ti, ok := typeassert,ok x <Ti>
  1397  		for _, cond := range cc.List {
  1398  			next = fn.newBasicBlock("typeswitch.next")
  1399  			casetype = fn.Pkg.typeOf(cond)
  1400  			var condv Value
  1401  			if casetype == tUntypedNil {
  1402  				condv = emitCompare(fn, token.EQL, x, nilConst(x.Type()), token.NoPos)
  1403  				ti = x
  1404  			} else {
  1405  				yok := emitTypeTest(fn, x, casetype, cc.Case)
  1406  				ti = emitExtract(fn, yok, 0)
  1407  				condv = emitExtract(fn, yok, 1)
  1408  			}
  1409  			emitIf(fn, condv, body, next)
  1410  			fn.currentBlock = next
  1411  		}
  1412  		if len(cc.List) != 1 {
  1413  			ti = x
  1414  		}
  1415  		fn.currentBlock = body
  1416  		b.typeCaseBody(fn, cc, ti, done)
  1417  		fn.currentBlock = next
  1418  	}
  1419  	if default_ != nil {
  1420  		b.typeCaseBody(fn, default_, x, done)
  1421  	} else {
  1422  		emitJump(fn, done)
  1423  	}
  1424  	fn.currentBlock = done
  1425  }
  1426  
  1427  func (b *builder) typeCaseBody(fn *Function, cc *ast.CaseClause, x Value, done *BasicBlock) {
  1428  	if obj := fn.Pkg.info.Implicits[cc]; obj != nil {
  1429  		// In a switch y := x.(type), each case clause
  1430  		// implicitly declares a distinct object y.
  1431  		// In a single-type case, y has that type.
  1432  		// In multi-type cases, 'case nil' and default,
  1433  		// y has the same type as the interface operand.
  1434  		emitStore(fn, fn.addNamedLocal(obj), x, obj.Pos())
  1435  	}
  1436  	fn.targets = &targets{
  1437  		tail:   fn.targets,
  1438  		_break: done,
  1439  	}
  1440  	b.stmtList(fn, cc.Body)
  1441  	fn.targets = fn.targets.tail
  1442  	emitJump(fn, done)
  1443  }
  1444  
  1445  // selectStmt emits to fn code for the select statement s, optionally
  1446  // labelled by label.
  1447  //
  1448  func (b *builder) selectStmt(fn *Function, s *ast.SelectStmt, label *lblock) {
  1449  	// A blocking select of a single case degenerates to a
  1450  	// simple send or receive.
  1451  	// TODO(adonovan): opt: is this optimization worth its weight?
  1452  	if len(s.Body.List) == 1 {
  1453  		clause := s.Body.List[0].(*ast.CommClause)
  1454  		if clause.Comm != nil {
  1455  			b.stmt(fn, clause.Comm)
  1456  			done := fn.newBasicBlock("select.done")
  1457  			if label != nil {
  1458  				label._break = done
  1459  			}
  1460  			fn.targets = &targets{
  1461  				tail:   fn.targets,
  1462  				_break: done,
  1463  			}
  1464  			b.stmtList(fn, clause.Body)
  1465  			fn.targets = fn.targets.tail
  1466  			emitJump(fn, done)
  1467  			fn.currentBlock = done
  1468  			return
  1469  		}
  1470  	}
  1471  
  1472  	// First evaluate all channels in all cases, and find
  1473  	// the directions of each state.
  1474  	var states []*SelectState
  1475  	blocking := true
  1476  	debugInfo := fn.debugInfo()
  1477  	for _, clause := range s.Body.List {
  1478  		var st *SelectState
  1479  		switch comm := clause.(*ast.CommClause).Comm.(type) {
  1480  		case nil: // default case
  1481  			blocking = false
  1482  			continue
  1483  
  1484  		case *ast.SendStmt: // ch<- i
  1485  			ch := b.expr(fn, comm.Chan)
  1486  			st = &SelectState{
  1487  				Dir:  types.SendOnly,
  1488  				Chan: ch,
  1489  				Send: emitConv(fn, b.expr(fn, comm.Value),
  1490  					ch.Type().Underlying().(*types.Chan).Elem()),
  1491  				Pos: comm.Arrow,
  1492  			}
  1493  			if debugInfo {
  1494  				st.DebugNode = comm
  1495  			}
  1496  
  1497  		case *ast.AssignStmt: // x := <-ch
  1498  			recv := unparen(comm.Rhs[0]).(*ast.UnaryExpr)
  1499  			st = &SelectState{
  1500  				Dir:  types.RecvOnly,
  1501  				Chan: b.expr(fn, recv.X),
  1502  				Pos:  recv.OpPos,
  1503  			}
  1504  			if debugInfo {
  1505  				st.DebugNode = recv
  1506  			}
  1507  
  1508  		case *ast.ExprStmt: // <-ch
  1509  			recv := unparen(comm.X).(*ast.UnaryExpr)
  1510  			st = &SelectState{
  1511  				Dir:  types.RecvOnly,
  1512  				Chan: b.expr(fn, recv.X),
  1513  				Pos:  recv.OpPos,
  1514  			}
  1515  			if debugInfo {
  1516  				st.DebugNode = recv
  1517  			}
  1518  		}
  1519  		states = append(states, st)
  1520  	}
  1521  
  1522  	// We dispatch on the (fair) result of Select using a
  1523  	// sequential if-else chain, in effect:
  1524  	//
  1525  	// idx, recvOk, r0...r_n-1 := select(...)
  1526  	// if idx == 0 {  // receive on channel 0  (first receive => r0)
  1527  	//     x, ok := r0, recvOk
  1528  	//     ...state0...
  1529  	// } else if v == 1 {   // send on channel 1
  1530  	//     ...state1...
  1531  	// } else {
  1532  	//     ...default...
  1533  	// }
  1534  	sel := &Select{
  1535  		States:   states,
  1536  		Blocking: blocking,
  1537  	}
  1538  	sel.setPos(s.Select)
  1539  	var vars []*types.Var
  1540  	vars = append(vars, varIndex, varOk)
  1541  	for _, st := range states {
  1542  		if st.Dir == types.RecvOnly {
  1543  			tElem := st.Chan.Type().Underlying().(*types.Chan).Elem()
  1544  			vars = append(vars, anonVar(tElem))
  1545  		}
  1546  	}
  1547  	sel.setType(types.NewTuple(vars...))
  1548  
  1549  	fn.emit(sel)
  1550  	idx := emitExtract(fn, sel, 0)
  1551  
  1552  	done := fn.newBasicBlock("select.done")
  1553  	if label != nil {
  1554  		label._break = done
  1555  	}
  1556  
  1557  	var defaultBody *[]ast.Stmt
  1558  	state := 0
  1559  	r := 2 // index in 'sel' tuple of value; increments if st.Dir==RECV
  1560  	for _, cc := range s.Body.List {
  1561  		clause := cc.(*ast.CommClause)
  1562  		if clause.Comm == nil {
  1563  			defaultBody = &clause.Body
  1564  			continue
  1565  		}
  1566  		body := fn.newBasicBlock("select.body")
  1567  		next := fn.newBasicBlock("select.next")
  1568  		emitIf(fn, emitCompare(fn, token.EQL, idx, intConst(int64(state)), token.NoPos), body, next)
  1569  		fn.currentBlock = body
  1570  		fn.targets = &targets{
  1571  			tail:   fn.targets,
  1572  			_break: done,
  1573  		}
  1574  		switch comm := clause.Comm.(type) {
  1575  		case *ast.ExprStmt: // <-ch
  1576  			if debugInfo {
  1577  				v := emitExtract(fn, sel, r)
  1578  				emitDebugRef(fn, states[state].DebugNode.(ast.Expr), v, false)
  1579  			}
  1580  			r++
  1581  
  1582  		case *ast.AssignStmt: // x := <-states[state].Chan
  1583  			if comm.Tok == token.DEFINE {
  1584  				fn.addLocalForIdent(comm.Lhs[0].(*ast.Ident))
  1585  			}
  1586  			x := b.addr(fn, comm.Lhs[0], false) // non-escaping
  1587  			v := emitExtract(fn, sel, r)
  1588  			if debugInfo {
  1589  				emitDebugRef(fn, states[state].DebugNode.(ast.Expr), v, false)
  1590  			}
  1591  			x.store(fn, v)
  1592  
  1593  			if len(comm.Lhs) == 2 { // x, ok := ...
  1594  				if comm.Tok == token.DEFINE {
  1595  					fn.addLocalForIdent(comm.Lhs[1].(*ast.Ident))
  1596  				}
  1597  				ok := b.addr(fn, comm.Lhs[1], false) // non-escaping
  1598  				ok.store(fn, emitExtract(fn, sel, 1))
  1599  			}
  1600  			r++
  1601  		}
  1602  		b.stmtList(fn, clause.Body)
  1603  		fn.targets = fn.targets.tail
  1604  		emitJump(fn, done)
  1605  		fn.currentBlock = next
  1606  		state++
  1607  	}
  1608  	if defaultBody != nil {
  1609  		fn.targets = &targets{
  1610  			tail:   fn.targets,
  1611  			_break: done,
  1612  		}
  1613  		b.stmtList(fn, *defaultBody)
  1614  		fn.targets = fn.targets.tail
  1615  	} else {
  1616  		// A blocking select must match some case.
  1617  		// (This should really be a runtime.errorString, not a string.)
  1618  		fn.emit(&Panic{
  1619  			X: emitConv(fn, stringConst("blocking select matched no case"), tEface),
  1620  		})
  1621  		fn.currentBlock = fn.newBasicBlock("unreachable")
  1622  	}
  1623  	emitJump(fn, done)
  1624  	fn.currentBlock = done
  1625  }
  1626  
  1627  // forStmt emits to fn code for the for statement s, optionally
  1628  // labelled by label.
  1629  //
  1630  func (b *builder) forStmt(fn *Function, s *ast.ForStmt, label *lblock) {
  1631  	//	...init...
  1632  	//      jump loop
  1633  	// loop:
  1634  	//      if cond goto body else done
  1635  	// body:
  1636  	//      ...body...
  1637  	//      jump post
  1638  	// post:				 (target of continue)
  1639  	//      ...post...
  1640  	//      jump loop
  1641  	// done:                                 (target of break)
  1642  	if s.Init != nil {
  1643  		b.stmt(fn, s.Init)
  1644  	}
  1645  	body := fn.newBasicBlock("for.body")
  1646  	done := fn.newBasicBlock("for.done") // target of 'break'
  1647  	loop := body                         // target of back-edge
  1648  	if s.Cond != nil {
  1649  		loop = fn.newBasicBlock("for.loop")
  1650  	}
  1651  	cont := loop // target of 'continue'
  1652  	if s.Post != nil {
  1653  		cont = fn.newBasicBlock("for.post")
  1654  	}
  1655  	if label != nil {
  1656  		label._break = done
  1657  		label._continue = cont
  1658  	}
  1659  	emitJump(fn, loop)
  1660  	fn.currentBlock = loop
  1661  	if loop != body {
  1662  		b.cond(fn, s.Cond, body, done)
  1663  		fn.currentBlock = body
  1664  	}
  1665  	fn.targets = &targets{
  1666  		tail:      fn.targets,
  1667  		_break:    done,
  1668  		_continue: cont,
  1669  	}
  1670  	b.stmt(fn, s.Body)
  1671  	fn.targets = fn.targets.tail
  1672  	emitJump(fn, cont)
  1673  
  1674  	if s.Post != nil {
  1675  		fn.currentBlock = cont
  1676  		b.stmt(fn, s.Post)
  1677  		emitJump(fn, loop) // back-edge
  1678  	}
  1679  	fn.currentBlock = done
  1680  }
  1681  
  1682  // rangeIndexed emits to fn the header for an integer-indexed loop
  1683  // over array, *array or slice value x.
  1684  // The v result is defined only if tv is non-nil.
  1685  // forPos is the position of the "for" token.
  1686  //
  1687  func (b *builder) rangeIndexed(fn *Function, x Value, tv types.Type, pos token.Pos) (k, v Value, loop, done *BasicBlock) {
  1688  	//
  1689  	//      length = len(x)
  1690  	//      index = -1
  1691  	// loop:                                   (target of continue)
  1692  	//      index++
  1693  	// 	if index < length goto body else done
  1694  	// body:
  1695  	//      k = index
  1696  	//      v = x[index]
  1697  	//      ...body...
  1698  	// 	jump loop
  1699  	// done:                                   (target of break)
  1700  
  1701  	// Determine number of iterations.
  1702  	var length Value
  1703  	if arr, ok := deref(x.Type()).Underlying().(*types.Array); ok {
  1704  		// For array or *array, the number of iterations is
  1705  		// known statically thanks to the type.  We avoid a
  1706  		// data dependence upon x, permitting later dead-code
  1707  		// elimination if x is pure, static unrolling, etc.
  1708  		// Ranging over a nil *array may have >0 iterations.
  1709  		// We still generate code for x, in case it has effects.
  1710  		length = intConst(arr.Len())
  1711  	} else {
  1712  		// length = len(x).
  1713  		var c Call
  1714  		c.Call.Value = makeLen(x.Type())
  1715  		c.Call.Args = []Value{x}
  1716  		c.setType(tInt)
  1717  		length = fn.emit(&c)
  1718  	}
  1719  
  1720  	index := fn.addLocal(tInt, token.NoPos)
  1721  	emitStore(fn, index, intConst(-1), pos)
  1722  
  1723  	loop = fn.newBasicBlock("rangeindex.loop")
  1724  	emitJump(fn, loop)
  1725  	fn.currentBlock = loop
  1726  
  1727  	incr := &BinOp{
  1728  		Op: token.ADD,
  1729  		X:  emitLoad(fn, index),
  1730  		Y:  vOne,
  1731  	}
  1732  	incr.setType(tInt)
  1733  	emitStore(fn, index, fn.emit(incr), pos)
  1734  
  1735  	body := fn.newBasicBlock("rangeindex.body")
  1736  	done = fn.newBasicBlock("rangeindex.done")
  1737  	emitIf(fn, emitCompare(fn, token.LSS, incr, length, token.NoPos), body, done)
  1738  	fn.currentBlock = body
  1739  
  1740  	k = emitLoad(fn, index)
  1741  	if tv != nil {
  1742  		switch t := x.Type().Underlying().(type) {
  1743  		case *types.Array:
  1744  			instr := &Index{
  1745  				X:     x,
  1746  				Index: k,
  1747  			}
  1748  			instr.setType(t.Elem())
  1749  			instr.setPos(x.Pos())
  1750  			v = fn.emit(instr)
  1751  
  1752  		case *types.Pointer: // *array
  1753  			instr := &IndexAddr{
  1754  				X:     x,
  1755  				Index: k,
  1756  			}
  1757  			instr.setType(types.NewPointer(t.Elem().Underlying().(*types.Array).Elem()))
  1758  			instr.setPos(x.Pos())
  1759  			v = emitLoad(fn, fn.emit(instr))
  1760  
  1761  		case *types.Slice:
  1762  			instr := &IndexAddr{
  1763  				X:     x,
  1764  				Index: k,
  1765  			}
  1766  			instr.setType(types.NewPointer(t.Elem()))
  1767  			instr.setPos(x.Pos())
  1768  			v = emitLoad(fn, fn.emit(instr))
  1769  
  1770  		default:
  1771  			panic("rangeIndexed x:" + t.String())
  1772  		}
  1773  	}
  1774  	return
  1775  }
  1776  
  1777  // rangeIter emits to fn the header for a loop using
  1778  // Range/Next/Extract to iterate over map or string value x.
  1779  // tk and tv are the types of the key/value results k and v, or nil
  1780  // if the respective component is not wanted.
  1781  //
  1782  func (b *builder) rangeIter(fn *Function, x Value, tk, tv types.Type, pos token.Pos) (k, v Value, loop, done *BasicBlock) {
  1783  	//
  1784  	//	it = range x
  1785  	// loop:                                   (target of continue)
  1786  	//	okv = next it                      (ok, key, value)
  1787  	//  	ok = extract okv #0
  1788  	// 	if ok goto body else done
  1789  	// body:
  1790  	// 	k = extract okv #1
  1791  	// 	v = extract okv #2
  1792  	//      ...body...
  1793  	// 	jump loop
  1794  	// done:                                   (target of break)
  1795  	//
  1796  
  1797  	if tk == nil {
  1798  		tk = tInvalid
  1799  	}
  1800  	if tv == nil {
  1801  		tv = tInvalid
  1802  	}
  1803  
  1804  	rng := &Range{X: x}
  1805  	rng.setPos(pos)
  1806  	rng.setType(tRangeIter)
  1807  	it := fn.emit(rng)
  1808  
  1809  	loop = fn.newBasicBlock("rangeiter.loop")
  1810  	emitJump(fn, loop)
  1811  	fn.currentBlock = loop
  1812  
  1813  	_, isString := x.Type().Underlying().(*types.Basic)
  1814  
  1815  	okv := &Next{
  1816  		Iter:     it,
  1817  		IsString: isString,
  1818  	}
  1819  	okv.setType(types.NewTuple(
  1820  		varOk,
  1821  		newVar("k", tk),
  1822  		newVar("v", tv),
  1823  	))
  1824  	fn.emit(okv)
  1825  
  1826  	body := fn.newBasicBlock("rangeiter.body")
  1827  	done = fn.newBasicBlock("rangeiter.done")
  1828  	emitIf(fn, emitExtract(fn, okv, 0), body, done)
  1829  	fn.currentBlock = body
  1830  
  1831  	if tk != tInvalid {
  1832  		k = emitExtract(fn, okv, 1)
  1833  	}
  1834  	if tv != tInvalid {
  1835  		v = emitExtract(fn, okv, 2)
  1836  	}
  1837  	return
  1838  }
  1839  
  1840  // rangeChan emits to fn the header for a loop that receives from
  1841  // channel x until it fails.
  1842  // tk is the channel's element type, or nil if the k result is
  1843  // not wanted
  1844  // pos is the position of the '=' or ':=' token.
  1845  //
  1846  func (b *builder) rangeChan(fn *Function, x Value, tk types.Type, pos token.Pos) (k Value, loop, done *BasicBlock) {
  1847  	//
  1848  	// loop:                                   (target of continue)
  1849  	//      ko = <-x                           (key, ok)
  1850  	//      ok = extract ko #1
  1851  	//      if ok goto body else done
  1852  	// body:
  1853  	//      k = extract ko #0
  1854  	//      ...
  1855  	//      goto loop
  1856  	// done:                                   (target of break)
  1857  
  1858  	loop = fn.newBasicBlock("rangechan.loop")
  1859  	emitJump(fn, loop)
  1860  	fn.currentBlock = loop
  1861  	recv := &UnOp{
  1862  		Op:      token.ARROW,
  1863  		X:       x,
  1864  		CommaOk: true,
  1865  	}
  1866  	recv.setPos(pos)
  1867  	recv.setType(types.NewTuple(
  1868  		newVar("k", x.Type().Underlying().(*types.Chan).Elem()),
  1869  		varOk,
  1870  	))
  1871  	ko := fn.emit(recv)
  1872  	body := fn.newBasicBlock("rangechan.body")
  1873  	done = fn.newBasicBlock("rangechan.done")
  1874  	emitIf(fn, emitExtract(fn, ko, 1), body, done)
  1875  	fn.currentBlock = body
  1876  	if tk != nil {
  1877  		k = emitExtract(fn, ko, 0)
  1878  	}
  1879  	return
  1880  }
  1881  
  1882  // rangeStmt emits to fn code for the range statement s, optionally
  1883  // labelled by label.
  1884  //
  1885  func (b *builder) rangeStmt(fn *Function, s *ast.RangeStmt, label *lblock) {
  1886  	var tk, tv types.Type
  1887  	if s.Key != nil && !isBlankIdent(s.Key) {
  1888  		tk = fn.Pkg.typeOf(s.Key)
  1889  	}
  1890  	if s.Value != nil && !isBlankIdent(s.Value) {
  1891  		tv = fn.Pkg.typeOf(s.Value)
  1892  	}
  1893  
  1894  	// If iteration variables are defined (:=), this
  1895  	// occurs once outside the loop.
  1896  	//
  1897  	// Unlike a short variable declaration, a RangeStmt
  1898  	// using := never redeclares an existing variable; it
  1899  	// always creates a new one.
  1900  	if s.Tok == token.DEFINE {
  1901  		if tk != nil {
  1902  			fn.addLocalForIdent(s.Key.(*ast.Ident))
  1903  		}
  1904  		if tv != nil {
  1905  			fn.addLocalForIdent(s.Value.(*ast.Ident))
  1906  		}
  1907  	}
  1908  
  1909  	x := b.expr(fn, s.X)
  1910  
  1911  	var k, v Value
  1912  	var loop, done *BasicBlock
  1913  	switch rt := x.Type().Underlying().(type) {
  1914  	case *types.Slice, *types.Array, *types.Pointer: // *array
  1915  		k, v, loop, done = b.rangeIndexed(fn, x, tv, s.For)
  1916  
  1917  	case *types.Chan:
  1918  		k, loop, done = b.rangeChan(fn, x, tk, s.For)
  1919  
  1920  	case *types.Map, *types.Basic: // string
  1921  		k, v, loop, done = b.rangeIter(fn, x, tk, tv, s.For)
  1922  
  1923  	default:
  1924  		panic("Cannot range over: " + rt.String())
  1925  	}
  1926  
  1927  	// Evaluate both LHS expressions before we update either.
  1928  	var kl, vl lvalue
  1929  	if tk != nil {
  1930  		kl = b.addr(fn, s.Key, false) // non-escaping
  1931  	}
  1932  	if tv != nil {
  1933  		vl = b.addr(fn, s.Value, false) // non-escaping
  1934  	}
  1935  	if tk != nil {
  1936  		kl.store(fn, k)
  1937  	}
  1938  	if tv != nil {
  1939  		vl.store(fn, v)
  1940  	}
  1941  
  1942  	if label != nil {
  1943  		label._break = done
  1944  		label._continue = loop
  1945  	}
  1946  
  1947  	fn.targets = &targets{
  1948  		tail:      fn.targets,
  1949  		_break:    done,
  1950  		_continue: loop,
  1951  	}
  1952  	b.stmt(fn, s.Body)
  1953  	fn.targets = fn.targets.tail
  1954  	emitJump(fn, loop) // back-edge
  1955  	fn.currentBlock = done
  1956  }
  1957  
  1958  // stmt lowers statement s to SSA form, emitting code to fn.
  1959  func (b *builder) stmt(fn *Function, _s ast.Stmt) {
  1960  	// The label of the current statement.  If non-nil, its _goto
  1961  	// target is always set; its _break and _continue are set only
  1962  	// within the body of switch/typeswitch/select/for/range.
  1963  	// It is effectively an additional default-nil parameter of stmt().
  1964  	var label *lblock
  1965  start:
  1966  	switch s := _s.(type) {
  1967  	case *ast.EmptyStmt:
  1968  		// ignore.  (Usually removed by gofmt.)
  1969  
  1970  	case *ast.DeclStmt: // Con, Var or Typ
  1971  		d := s.Decl.(*ast.GenDecl)
  1972  		if d.Tok == token.VAR {
  1973  			for _, spec := range d.Specs {
  1974  				if vs, ok := spec.(*ast.ValueSpec); ok {
  1975  					b.localValueSpec(fn, vs)
  1976  				}
  1977  			}
  1978  		}
  1979  
  1980  	case *ast.LabeledStmt:
  1981  		label = fn.labelledBlock(s.Label)
  1982  		emitJump(fn, label._goto)
  1983  		fn.currentBlock = label._goto
  1984  		_s = s.Stmt
  1985  		goto start // effectively: tailcall stmt(fn, s.Stmt, label)
  1986  
  1987  	case *ast.ExprStmt:
  1988  		b.expr(fn, s.X)
  1989  
  1990  	case *ast.SendStmt:
  1991  		fn.emit(&Send{
  1992  			Chan: b.expr(fn, s.Chan),
  1993  			X: emitConv(fn, b.expr(fn, s.Value),
  1994  				fn.Pkg.typeOf(s.Chan).Underlying().(*types.Chan).Elem()),
  1995  			pos: s.Arrow,
  1996  		})
  1997  
  1998  	case *ast.IncDecStmt:
  1999  		op := token.ADD
  2000  		if s.Tok == token.DEC {
  2001  			op = token.SUB
  2002  		}
  2003  		loc := b.addr(fn, s.X, false)
  2004  		b.assignOp(fn, loc, NewConst(constant.MakeInt64(1), loc.typ()), op, s.Pos())
  2005  
  2006  	case *ast.AssignStmt:
  2007  		switch s.Tok {
  2008  		case token.ASSIGN, token.DEFINE:
  2009  			b.assignStmt(fn, s.Lhs, s.Rhs, s.Tok == token.DEFINE)
  2010  
  2011  		default: // +=, etc.
  2012  			op := s.Tok + token.ADD - token.ADD_ASSIGN
  2013  			b.assignOp(fn, b.addr(fn, s.Lhs[0], false), b.expr(fn, s.Rhs[0]), op, s.Pos())
  2014  		}
  2015  
  2016  	case *ast.GoStmt:
  2017  		// The "intrinsics" new/make/len/cap are forbidden here.
  2018  		// panic is treated like an ordinary function call.
  2019  		v := Go{pos: s.Go}
  2020  		b.setCall(fn, s.Call, &v.Call)
  2021  		fn.emit(&v)
  2022  
  2023  	case *ast.DeferStmt:
  2024  		// The "intrinsics" new/make/len/cap are forbidden here.
  2025  		// panic is treated like an ordinary function call.
  2026  		v := Defer{pos: s.Defer}
  2027  		b.setCall(fn, s.Call, &v.Call)
  2028  		fn.emit(&v)
  2029  
  2030  		// A deferred call can cause recovery from panic,
  2031  		// and control resumes at the Recover block.
  2032  		createRecoverBlock(fn)
  2033  
  2034  	case *ast.ReturnStmt:
  2035  		var results []Value
  2036  		if len(s.Results) == 1 && fn.Signature.Results().Len() > 1 {
  2037  			// Return of one expression in a multi-valued function.
  2038  			tuple := b.exprN(fn, s.Results[0])
  2039  			ttuple := tuple.Type().(*types.Tuple)
  2040  			for i, n := 0, ttuple.Len(); i < n; i++ {
  2041  				results = append(results,
  2042  					emitConv(fn, emitExtract(fn, tuple, i),
  2043  						fn.Signature.Results().At(i).Type()))
  2044  			}
  2045  		} else {
  2046  			// 1:1 return, or no-arg return in non-void function.
  2047  			for i, r := range s.Results {
  2048  				v := emitConv(fn, b.expr(fn, r), fn.Signature.Results().At(i).Type())
  2049  				results = append(results, v)
  2050  			}
  2051  		}
  2052  		if fn.namedResults != nil {
  2053  			// Function has named result parameters (NRPs).
  2054  			// Perform parallel assignment of return operands to NRPs.
  2055  			for i, r := range results {
  2056  				emitStore(fn, fn.namedResults[i], r, s.Return)
  2057  			}
  2058  		}
  2059  		// Run function calls deferred in this
  2060  		// function when explicitly returning from it.
  2061  		fn.emit(new(RunDefers))
  2062  		if fn.namedResults != nil {
  2063  			// Reload NRPs to form the result tuple.
  2064  			results = results[:0]
  2065  			for _, r := range fn.namedResults {
  2066  				results = append(results, emitLoad(fn, r))
  2067  			}
  2068  		}
  2069  		fn.emit(&Return{Results: results, pos: s.Return})
  2070  		fn.currentBlock = fn.newBasicBlock("unreachable")
  2071  
  2072  	case *ast.BranchStmt:
  2073  		var block *BasicBlock
  2074  		switch s.Tok {
  2075  		case token.BREAK:
  2076  			if s.Label != nil {
  2077  				block = fn.labelledBlock(s.Label)._break
  2078  			} else {
  2079  				for t := fn.targets; t != nil && block == nil; t = t.tail {
  2080  					block = t._break
  2081  				}
  2082  			}
  2083  
  2084  		case token.CONTINUE:
  2085  			if s.Label != nil {
  2086  				block = fn.labelledBlock(s.Label)._continue
  2087  			} else {
  2088  				for t := fn.targets; t != nil && block == nil; t = t.tail {
  2089  					block = t._continue
  2090  				}
  2091  			}
  2092  
  2093  		case token.FALLTHROUGH:
  2094  			for t := fn.targets; t != nil && block == nil; t = t.tail {
  2095  				block = t._fallthrough
  2096  			}
  2097  
  2098  		case token.GOTO:
  2099  			block = fn.labelledBlock(s.Label)._goto
  2100  		}
  2101  		emitJump(fn, block)
  2102  		fn.currentBlock = fn.newBasicBlock("unreachable")
  2103  
  2104  	case *ast.BlockStmt:
  2105  		b.stmtList(fn, s.List)
  2106  
  2107  	case *ast.IfStmt:
  2108  		if s.Init != nil {
  2109  			b.stmt(fn, s.Init)
  2110  		}
  2111  		then := fn.newBasicBlock("if.then")
  2112  		done := fn.newBasicBlock("if.done")
  2113  		els := done
  2114  		if s.Else != nil {
  2115  			els = fn.newBasicBlock("if.else")
  2116  		}
  2117  		b.cond(fn, s.Cond, then, els)
  2118  		fn.currentBlock = then
  2119  		b.stmt(fn, s.Body)
  2120  		emitJump(fn, done)
  2121  
  2122  		if s.Else != nil {
  2123  			fn.currentBlock = els
  2124  			b.stmt(fn, s.Else)
  2125  			emitJump(fn, done)
  2126  		}
  2127  
  2128  		fn.currentBlock = done
  2129  
  2130  	case *ast.SwitchStmt:
  2131  		b.switchStmt(fn, s, label)
  2132  
  2133  	case *ast.TypeSwitchStmt:
  2134  		b.typeSwitchStmt(fn, s, label)
  2135  
  2136  	case *ast.SelectStmt:
  2137  		b.selectStmt(fn, s, label)
  2138  
  2139  	case *ast.ForStmt:
  2140  		b.forStmt(fn, s, label)
  2141  
  2142  	case *ast.RangeStmt:
  2143  		b.rangeStmt(fn, s, label)
  2144  
  2145  	default:
  2146  		panic(fmt.Sprintf("unexpected statement kind: %T", s))
  2147  	}
  2148  }
  2149  
  2150  // buildFunction builds SSA code for the body of function fn.  Idempotent.
  2151  func (b *builder) buildFunction(fn *Function) {
  2152  	if fn.Blocks != nil {
  2153  		return // building already started
  2154  	}
  2155  
  2156  	var recvField *ast.FieldList
  2157  	var body *ast.BlockStmt
  2158  	var functype *ast.FuncType
  2159  	switch n := fn.syntax.(type) {
  2160  	case nil:
  2161  		return // not a Go source function.  (Synthetic, or from object file.)
  2162  	case *ast.FuncDecl:
  2163  		functype = n.Type
  2164  		recvField = n.Recv
  2165  		body = n.Body
  2166  	case *ast.FuncLit:
  2167  		functype = n.Type
  2168  		body = n.Body
  2169  	default:
  2170  		panic(n)
  2171  	}
  2172  
  2173  	if body == nil {
  2174  		// External function.
  2175  		if fn.Params == nil {
  2176  			// This condition ensures we add a non-empty
  2177  			// params list once only, but we may attempt
  2178  			// the degenerate empty case repeatedly.
  2179  			// TODO(adonovan): opt: don't do that.
  2180  
  2181  			// We set Function.Params even though there is no body
  2182  			// code to reference them.  This simplifies clients.
  2183  			if recv := fn.Signature.Recv(); recv != nil {
  2184  				fn.addParamObj(recv)
  2185  			}
  2186  			params := fn.Signature.Params()
  2187  			for i, n := 0, params.Len(); i < n; i++ {
  2188  				fn.addParamObj(params.At(i))
  2189  			}
  2190  		}
  2191  		return
  2192  	}
  2193  	if fn.Prog.mode&LogSource != 0 {
  2194  		defer logStack("build function %s @ %s", fn, fn.Prog.Fset.Position(fn.pos))()
  2195  	}
  2196  	fn.startBody()
  2197  	fn.createSyntacticParams(recvField, functype)
  2198  	b.stmt(fn, body)
  2199  	if cb := fn.currentBlock; cb != nil && (cb == fn.Blocks[0] || cb == fn.Recover || cb.Preds != nil) {
  2200  		// Control fell off the end of the function's body block.
  2201  		//
  2202  		// Block optimizations eliminate the current block, if
  2203  		// unreachable.  It is a builder invariant that
  2204  		// if this no-arg return is ill-typed for
  2205  		// fn.Signature.Results, this block must be
  2206  		// unreachable.  The sanity checker checks this.
  2207  		fn.emit(new(RunDefers))
  2208  		fn.emit(new(Return))
  2209  	}
  2210  	fn.finishBody()
  2211  }
  2212  
  2213  // buildFuncDecl builds SSA code for the function or method declared
  2214  // by decl in package pkg.
  2215  //
  2216  func (b *builder) buildFuncDecl(pkg *Package, decl *ast.FuncDecl) {
  2217  	id := decl.Name
  2218  	if isBlankIdent(id) {
  2219  		return // discard
  2220  	}
  2221  	fn := pkg.values[pkg.info.Defs[id]].(*Function)
  2222  	if decl.Recv == nil && id.Name == "init" {
  2223  		var v Call
  2224  		v.Call.Value = fn
  2225  		v.setType(types.NewTuple())
  2226  		pkg.init.emit(&v)
  2227  	}
  2228  	b.buildFunction(fn)
  2229  }
  2230  
  2231  // Build calls Package.Build for each package in prog.
  2232  // Building occurs in parallel unless the BuildSerially mode flag was set.
  2233  //
  2234  // Build is intended for whole-program analysis; a typical compiler
  2235  // need only build a single package.
  2236  //
  2237  // Build is idempotent and thread-safe.
  2238  //
  2239  func (prog *Program) Build() {
  2240  	var wg sync.WaitGroup
  2241  	for _, p := range prog.packages {
  2242  		if prog.mode&BuildSerially != 0 {
  2243  			p.Build()
  2244  		} else {
  2245  			wg.Add(1)
  2246  			go func(p *Package) {
  2247  				p.Build()
  2248  				wg.Done()
  2249  			}(p)
  2250  		}
  2251  	}
  2252  	wg.Wait()
  2253  }
  2254  
  2255  // Build builds SSA code for all functions and vars in package p.
  2256  //
  2257  // Precondition: CreatePackage must have been called for all of p's
  2258  // direct imports (and hence its direct imports must have been
  2259  // error-free).
  2260  //
  2261  // Build is idempotent and thread-safe.
  2262  //
  2263  func (p *Package) Build() { p.buildOnce.Do(p.build) }
  2264  
  2265  func (p *Package) build() {
  2266  	if p.info == nil {
  2267  		return // synthetic package, e.g. "testmain"
  2268  	}
  2269  
  2270  	// Ensure we have runtime type info for all exported members.
  2271  	// TODO(adonovan): ideally belongs in memberFromObject, but
  2272  	// that would require package creation in topological order.
  2273  	for name, mem := range p.Members {
  2274  		if ast.IsExported(name) {
  2275  			p.Prog.needMethodsOf(mem.Type())
  2276  		}
  2277  	}
  2278  	if p.Prog.mode&LogSource != 0 {
  2279  		defer logStack("build %s", p)()
  2280  	}
  2281  	init := p.init
  2282  	init.startBody()
  2283  
  2284  	var done *BasicBlock
  2285  
  2286  	if p.Prog.mode&BareInits == 0 {
  2287  		// Make init() skip if package is already initialized.
  2288  		initguard := p.Var("init$guard")
  2289  		doinit := init.newBasicBlock("init.start")
  2290  		done = init.newBasicBlock("init.done")
  2291  		emitIf(init, emitLoad(init, initguard), done, doinit)
  2292  		init.currentBlock = doinit
  2293  		emitStore(init, initguard, vTrue, token.NoPos)
  2294  
  2295  		// Call the init() function of each package we import.
  2296  		for _, pkg := range p.Pkg.Imports() {
  2297  			prereq := p.Prog.packages[pkg]
  2298  			if prereq == nil {
  2299  				panic(fmt.Sprintf("Package(%q).Build(): unsatisfied import: Program.CreatePackage(%q) was not called", p.Pkg.Path(), pkg.Path()))
  2300  			}
  2301  			var v Call
  2302  			v.Call.Value = prereq.init
  2303  			v.Call.pos = init.pos
  2304  			v.setType(types.NewTuple())
  2305  			init.emit(&v)
  2306  		}
  2307  	}
  2308  
  2309  	var b builder
  2310  
  2311  	// Initialize package-level vars in correct order.
  2312  	for _, varinit := range p.info.InitOrder {
  2313  		if init.Prog.mode&LogSource != 0 {
  2314  			fmt.Fprintf(os.Stderr, "build global initializer %v @ %s\n",
  2315  				varinit.Lhs, p.Prog.Fset.Position(varinit.Rhs.Pos()))
  2316  		}
  2317  		if len(varinit.Lhs) == 1 {
  2318  			// 1:1 initialization: var x, y = a(), b()
  2319  			var lval lvalue
  2320  			if v := varinit.Lhs[0]; v.Name() != "_" {
  2321  				lval = &address{addr: p.values[v].(*Global), pos: v.Pos()}
  2322  			} else {
  2323  				lval = blank{}
  2324  			}
  2325  			b.assign(init, lval, varinit.Rhs, true, nil)
  2326  		} else {
  2327  			// n:1 initialization: var x, y :=  f()
  2328  			tuple := b.exprN(init, varinit.Rhs)
  2329  			for i, v := range varinit.Lhs {
  2330  				if v.Name() == "_" {
  2331  					continue
  2332  				}
  2333  				emitStore(init, p.values[v].(*Global), emitExtract(init, tuple, i), v.Pos())
  2334  			}
  2335  		}
  2336  	}
  2337  
  2338  	// Build all package-level functions, init functions
  2339  	// and methods, including unreachable/blank ones.
  2340  	// We build them in source order, but it's not significant.
  2341  	for _, file := range p.files {
  2342  		for _, decl := range file.Decls {
  2343  			if decl, ok := decl.(*ast.FuncDecl); ok {
  2344  				b.buildFuncDecl(p, decl)
  2345  			}
  2346  		}
  2347  	}
  2348  
  2349  	// Finish up init().
  2350  	if p.Prog.mode&BareInits == 0 {
  2351  		emitJump(init, done)
  2352  		init.currentBlock = done
  2353  	}
  2354  	init.emit(new(Return))
  2355  	init.finishBody()
  2356  
  2357  	p.info = nil // We no longer need ASTs or go/types deductions.
  2358  
  2359  	if p.Prog.mode&SanityCheckFunctions != 0 {
  2360  		sanityCheckPackage(p)
  2361  	}
  2362  }
  2363  
  2364  // Like ObjectOf, but panics instead of returning nil.
  2365  // Only valid during p's create and build phases.
  2366  func (p *Package) objectOf(id *ast.Ident) types.Object {
  2367  	if o := p.info.ObjectOf(id); o != nil {
  2368  		return o
  2369  	}
  2370  	panic(fmt.Sprintf("no types.Object for ast.Ident %s @ %s",
  2371  		id.Name, p.Prog.Fset.Position(id.Pos())))
  2372  }
  2373  
  2374  // Like TypeOf, but panics instead of returning nil.
  2375  // Only valid during p's create and build phases.
  2376  func (p *Package) typeOf(e ast.Expr) types.Type {
  2377  	if T := p.info.TypeOf(e); T != nil {
  2378  		return T
  2379  	}
  2380  	panic(fmt.Sprintf("no type for %T @ %s",
  2381  		e, p.Prog.Fset.Position(e.Pos())))
  2382  }