src

func.go (24199B)

---

 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package ssa
 
 // This file implements the Function type.
 
 import (
 	"bytes"
 	"fmt"
 	"go/ast"
 	"go/token"
 	"go/types"
 	"io"
 	"iter"
 	"os"
 	"strings"
 
 	"golang.org/x/tools/internal/typeparams"
 )
 
 // Like ObjectOf, but panics instead of returning nil.
 // Only valid during f's create and build phases.
 func (f *Function) objectOf(id *ast.Ident) types.Object {
 	if o := f.info.ObjectOf(id); o != nil {
 		return o
 	}
 	panic(fmt.Sprintf("no types.Object for ast.Ident %s @ %s",
 		id.Name, f.Prog.Fset.Position(id.Pos())))
 }
 
 // Like TypeOf, but panics instead of returning nil.
 // Only valid during f's create and build phases.
 func (f *Function) typeOf(e ast.Expr) types.Type {
 	if T := f.info.TypeOf(e); T != nil {
 		return f.typ(T)
 	}
 	panic(fmt.Sprintf("no type for %T @ %s", e, f.Prog.Fset.Position(e.Pos())))
 }
 
 // typ is the locally instantiated type of T.
 // If f is not an instantiation, then f.typ(T)==T.
 func (f *Function) typ(T types.Type) types.Type {
 	return f.subst.typ(T)
 }
 
 // If id is an Instance, returns info.Instances[id].Type.
 // Otherwise returns f.typeOf(id).
 func (f *Function) instanceType(id *ast.Ident) types.Type {
 	if t, ok := f.info.Instances[id]; ok {
 		return t.Type
 	}
 	return f.typeOf(id)
 }
 
 // selection returns a *selection corresponding to f.info.Selections[selector]
 // with potential updates for type substitution.
 func (f *Function) selection(selector *ast.SelectorExpr) *selection {
 	sel := f.info.Selections[selector]
 	if sel == nil {
 		return nil
 	}
 
 	switch sel.Kind() {
 	case types.MethodExpr, types.MethodVal:
 		if recv := f.typ(sel.Recv()); recv != sel.Recv() {
 			// recv changed during type substitution.
 			pkg := f.declaredPackage().Pkg
 			obj, index, indirect := types.LookupFieldOrMethod(recv, true, pkg, sel.Obj().Name())
 
 			// sig replaces sel.Type(). See (types.Selection).Typ() for details.
 			sig := obj.Type().(*types.Signature)
 			sig = changeRecv(sig, newVar(sig.Recv().Name(), recv))
 			if sel.Kind() == types.MethodExpr {
 				sig = recvAsFirstArg(sig)
 			}
 			return &selection{
 				kind:     sel.Kind(),
 				recv:     recv,
 				typ:      sig,
 				obj:      obj,
 				index:    index,
 				indirect: indirect,
 			}
 		}
 	}
 	return toSelection(sel)
 }
 
 // Destinations associated with unlabelled for/switch/select stmts.
 // We push/pop one of these as we enter/leave each construct and for
 // each BranchStmt we scan for the innermost target of the right type.
 type targets struct {
 	tail         *targets // rest of stack
 	_break       *BasicBlock
 	_continue    *BasicBlock
 	_fallthrough *BasicBlock
 }
 
 // Destinations associated with a labelled block.
 // We populate these as labels are encountered in forward gotos or
 // labelled statements.
 // Forward gotos are resolved once it is known which statement they
 // are associated with inside the Function.
 type lblock struct {
 	label     *types.Label // Label targeted by the blocks.
 	resolved  bool         // _goto block encountered (back jump or resolved fwd jump)
 	_goto     *BasicBlock
 	_break    *BasicBlock
 	_continue *BasicBlock
 }
 
 // label returns the symbol denoted by a label identifier.
 //
 // label should be a non-blank identifier (label.Name != "_").
 func (f *Function) label(label *ast.Ident) *types.Label {
 	return f.objectOf(label).(*types.Label)
 }
 
 // lblockOf returns the branch target associated with the
 // specified label, creating it if needed.
 func (f *Function) lblockOf(label *types.Label) *lblock {
 	lb := f.lblocks[label]
 	if lb == nil {
 		lb = &lblock{
 			label: label,
 			_goto: f.newBasicBlock(label.Name()),
 		}
 		if f.lblocks == nil {
 			f.lblocks = make(map[*types.Label]*lblock)
 		}
 		f.lblocks[label] = lb
 	}
 	return lb
 }
 
 // labelledBlock searches f for the block of the specified label.
 //
 // If f is a yield function, it additionally searches ancestor Functions
 // corresponding to enclosing range-over-func statements within the
 // same source function, so the returned block may belong to a different Function.
 func labelledBlock(f *Function, label *types.Label, tok token.Token) *BasicBlock {
 	if lb := f.lblocks[label]; lb != nil {
 		var block *BasicBlock
 		switch tok {
 		case token.BREAK:
 			block = lb._break
 		case token.CONTINUE:
 			block = lb._continue
 		case token.GOTO:
 			block = lb._goto
 		}
 		if block != nil {
 			return block
 		}
 	}
 	// Search ancestors if this is a yield function.
 	if f.jump != nil {
 		return labelledBlock(f.parent, label, tok)
 	}
 	return nil
 }
 
 // targetedBlock looks for the nearest block in f.targets
 // (and f's ancestors) that matches tok's type, and returns
 // the block and function it was found in.
 func targetedBlock(f *Function, tok token.Token) *BasicBlock {
 	if f == nil {
 		return nil
 	}
 	for t := f.targets; t != nil; t = t.tail {
 		var block *BasicBlock
 		switch tok {
 		case token.BREAK:
 			block = t._break
 		case token.CONTINUE:
 			block = t._continue
 		case token.FALLTHROUGH:
 			block = t._fallthrough
 		}
 		if block != nil {
 			return block
 		}
 	}
 	// Search f's ancestors (in case f is a yield function).
 	return targetedBlock(f.parent, tok)
 }
 
 // instrs returns an iterator that returns each reachable instruction of the SSA function.
 func (f *Function) instrs() iter.Seq[Instruction] {
 	return func(yield func(i Instruction) bool) {
 		for _, block := range f.Blocks {
 			for _, instr := range block.Instrs {
 				if !yield(instr) {
 					return
 				}
 			}
 		}
 	}
 }
 
 // addResultVar adds a result for a variable v to f.results and v to f.returnVars.
 func (f *Function) addResultVar(v *types.Var) {
 	result := emitLocalVar(f, v)
 	f.results = append(f.results, result)
 	f.returnVars = append(f.returnVars, v)
 }
 
 // addParamVar adds a parameter to f.Params.
 func (f *Function) addParamVar(v *types.Var) *Parameter {
 	name := v.Name()
 	if name == "" {
 		name = fmt.Sprintf("arg%d", len(f.Params))
 	}
 	param := &Parameter{
 		name:   name,
 		object: v,
 		typ:    f.typ(v.Type()),
 		parent: f,
 	}
 	f.Params = append(f.Params, param)
 	return param
 }
 
 // addSpilledParam declares a parameter that is pre-spilled to the
 // stack; the function body will load/store the spilled location.
 // Subsequent lifting will eliminate spills where possible.
 func (f *Function) addSpilledParam(obj *types.Var) {
 	param := f.addParamVar(obj)
 	spill := emitLocalVar(f, obj)
 	f.emit(&Store{Addr: spill, Val: param})
 }
 
 // startBody initializes the function prior to generating SSA code for its body.
 // Precondition: f.Type() already set.
 func (f *Function) startBody() {
 	f.currentBlock = f.newBasicBlock("entry")
 	f.vars = make(map[*types.Var]Value) // needed for some synthetics, e.g. init
 }
 
 // createSyntacticParams populates f.Params and generates code (spills
 // and named result locals) for all the parameters declared in the
 // syntax.  In addition it populates the f.objects mapping.
 //
 // Preconditions:
 // f.startBody() was called. f.info != nil.
 // Postcondition:
 // len(f.Params) == len(f.Signature.Params) + (f.Signature.Recv() ? 1 : 0)
 func (f *Function) createSyntacticParams(recv *ast.FieldList, functype *ast.FuncType) {
 	// Receiver (at most one inner iteration).
 	if recv != nil {
 		for _, field := range recv.List {
 			for _, n := range field.Names {
 				f.addSpilledParam(identVar(f, n))
 			}
 			// Anonymous receiver?  No need to spill.
 			if field.Names == nil {
 				f.addParamVar(f.Signature.Recv())
 			}
 		}
 	}
 
 	// Parameters.
 	if functype.Params != nil {
 		n := len(f.Params) // 1 if has recv, 0 otherwise
 		for _, field := range functype.Params.List {
 			for _, n := range field.Names {
 				f.addSpilledParam(identVar(f, n))
 			}
 			// Anonymous parameter?  No need to spill.
 			if field.Names == nil {
 				f.addParamVar(f.Signature.Params().At(len(f.Params) - n))
 			}
 		}
 	}
 
 	// Results.
 	if functype.Results != nil {
 		for _, field := range functype.Results.List {
 			// Implicit "var" decl of locals for named results.
 			for _, n := range field.Names {
 				v := identVar(f, n)
 				f.addResultVar(v)
 			}
 			// Implicit "var" decl of local for an unnamed result.
 			if field.Names == nil {
 				v := f.Signature.Results().At(len(f.results))
 				f.addResultVar(v)
 			}
 		}
 	}
 }
 
 // createDeferStack initializes fn.deferstack to local variable
 // initialized to a ssa:deferstack() call.
 func (fn *Function) createDeferStack() {
 	// Each syntactic function makes a call to ssa:deferstack,
 	// which is spilled to a local. Unused ones are later removed.
 	fn.deferstack = newVar("defer$stack", tDeferStack)
 	call := &Call{Call: CallCommon{Value: vDeferStack}}
 	call.setType(tDeferStack)
 	deferstack := fn.emit(call)
 	spill := emitLocalVar(fn, fn.deferstack)
 	emitStore(fn, spill, deferstack, token.NoPos)
 }
 
 type setNumable interface {
 	setNum(int)
 }
 
 // numberRegisters assigns numbers to all SSA registers
 // (value-defining Instructions) in f, to aid debugging.
 // (Non-Instruction Values are named at construction.)
 func numberRegisters(f *Function) {
 	v := 0
 	for _, b := range f.Blocks {
 		for _, instr := range b.Instrs {
 			switch instr.(type) {
 			case Value:
 				instr.(setNumable).setNum(v)
 				v++
 			}
 		}
 	}
 }
 
 // buildReferrers populates the def/use information in all non-nil
 // Value.Referrers slice.
 // Precondition: all such slices are initially empty.
 func buildReferrers(f *Function) {
 	var rands []*Value
 	for _, b := range f.Blocks {
 		for _, instr := range b.Instrs {
 			rands = instr.Operands(rands[:0]) // recycle storage
 			for _, rand := range rands {
 				if r := *rand; r != nil {
 					if ref := r.Referrers(); ref != nil {
 						*ref = append(*ref, instr)
 					}
 				}
 			}
 		}
 	}
 }
 
 // finishBody() finalizes the contents of the function after SSA code generation of its body.
 //
 // The function is not done being built until done() is called.
 func (f *Function) finishBody() {
 	f.currentBlock = nil
 	f.lblocks = nil
 	f.returnVars = nil
 	f.jump = nil
 	f.source = nil
 	f.exits = nil
 
 	// Remove from f.Locals any Allocs that escape to the heap.
 	j := 0
 	for _, l := range f.Locals {
 		if !l.Heap {
 			f.Locals[j] = l
 			j++
 		}
 	}
 	// Nil out f.Locals[j:] to aid GC.
 	for i := j; i < len(f.Locals); i++ {
 		f.Locals[i] = nil
 	}
 	f.Locals = f.Locals[:j]
 
 	optimizeBlocks(f)
 
 	buildReferrers(f)
 
 	buildDomTree(f)
 
 	if f.Prog.mode&NaiveForm == 0 {
 		// For debugging pre-state of lifting pass:
 		// numberRegisters(f)
 		// f.WriteTo(os.Stderr)
 		lift(f)
 	}
 
 	// clear remaining builder state
 	f.results = nil    // (used by lifting)
 	f.deferstack = nil // (used by lifting)
 	f.vars = nil       // (used by lifting)
 
 	// clear out other function state (keep consistent with buildParamsOnly)
 	f.subst = nil
 
 	numberRegisters(f) // uses f.namedRegisters
 }
 
 // done marks the building of f's SSA body complete,
 // along with any nested functions, and optionally prints them.
 func (f *Function) done() {
 	assert(f.parent == nil, "done called on an anonymous function")
 
 	var visit func(*Function)
 	visit = func(f *Function) {
 		for _, anon := range f.AnonFuncs {
 			visit(anon) // anon is done building before f.
 		}
 
 		f.uniq = 0    // done with uniq
 		f.build = nil // function is built
 
 		if f.Prog.mode&PrintFunctions != 0 {
 			printMu.Lock()
 			f.WriteTo(os.Stdout)
 			printMu.Unlock()
 		}
 
 		if f.Prog.mode&SanityCheckFunctions != 0 {
 			mustSanityCheck(f, nil)
 		}
 	}
 	visit(f)
 }
 
 // removeNilBlocks eliminates nils from f.Blocks and updates each
 // BasicBlock.Index.  Use this after any pass that may delete blocks.
 func (f *Function) removeNilBlocks() {
 	j := 0
 	for _, b := range f.Blocks {
 		if b != nil {
 			b.Index = j
 			f.Blocks[j] = b
 			j++
 		}
 	}
 	// Nil out f.Blocks[j:] to aid GC.
 	for i := j; i < len(f.Blocks); i++ {
 		f.Blocks[i] = nil
 	}
 	f.Blocks = f.Blocks[:j]
 }
 
 // SetDebugMode sets the debug mode for package pkg.  If true, all its
 // functions will include full debug info.  This greatly increases the
 // size of the instruction stream, and causes Functions to depend upon
 // the ASTs, potentially keeping them live in memory for longer.
 func (pkg *Package) SetDebugMode(debug bool) {
 	pkg.debug = debug
 }
 
 // debugInfo reports whether debug info is wanted for this function.
 func (f *Function) debugInfo() bool {
 	// debug info for instantiations follows the debug info of their origin.
 	p := f.declaredPackage()
 	return p != nil && p.debug
 }
 
 // lookup returns the address of the named variable identified by obj
 // that is local to function f or one of its enclosing functions.
 // If escaping, the reference comes from a potentially escaping pointer
 // expression and the referent must be heap-allocated.
 // We assume the referent is a *Alloc or *Phi.
 // (The only Phis at this stage are those created directly by go1.22 "for" loops.)
 func (f *Function) lookup(obj *types.Var, escaping bool) Value {
 	if v, ok := f.vars[obj]; ok {
 		if escaping {
 			switch v := v.(type) {
 			case *Alloc:
 				v.Heap = true
 			case *Phi:
 				for _, edge := range v.Edges {
 					if alloc, ok := edge.(*Alloc); ok {
 						alloc.Heap = true
 					}
 				}
 			}
 		}
 		return v // function-local var (address)
 	}
 
 	// Definition must be in an enclosing function;
 	// plumb it through intervening closures.
 	if f.parent == nil {
 		panic("no ssa.Value for " + obj.String())
 	}
 	outer := f.parent.lookup(obj, true) // escaping
 	v := &FreeVar{
 		name:   obj.Name(),
 		typ:    outer.Type(),
 		pos:    outer.Pos(),
 		outer:  outer,
 		parent: f,
 	}
 	f.vars[obj] = v
 	f.FreeVars = append(f.FreeVars, v)
 	return v
 }
 
 // emit emits the specified instruction to function f.
 func (f *Function) emit(instr Instruction) Value {
 	return f.currentBlock.emit(instr)
 }
 
 // RelString returns the full name of this function, qualified by
 // package name, receiver type, etc.
 //
 // The specific formatting rules are not guaranteed and may change.
 //
 // Examples:
 //
 //	"math.IsNaN"                  // a package-level function
 //	"(*bytes.Buffer).Bytes"       // a declared method or a wrapper
 //	"(*bytes.Buffer).Bytes$thunk" // thunk (func wrapping method; receiver is param 0)
 //	"(*bytes.Buffer).Bytes$bound" // bound (func wrapping method; receiver supplied by closure)
 //	"main.main$1"                 // an anonymous function in main
 //	"main.init#1"                 // a declared init function
 //	"main.init"                   // the synthesized package initializer
 //
 // When these functions are referred to from within the same package
 // (i.e. from == f.Pkg.Object), they are rendered without the package path.
 // For example: "IsNaN", "(*Buffer).Bytes", etc.
 //
 // All non-synthetic functions have distinct package-qualified names.
 // (But two methods may have the same name "(T).f" if one is a synthetic
 // wrapper promoting a non-exported method "f" from another package; in
 // that case, the strings are equal but the identifiers "f" are distinct.)
 func (f *Function) RelString(from *types.Package) string {
 	// Anonymous?
 	if f.parent != nil {
 		// An anonymous function's Name() looks like "parentName$1",
 		// but its String() should include the type/package/etc.
 		parent := f.parent.RelString(from)
 		for i, anon := range f.parent.AnonFuncs {
 			if anon == f {
 				return fmt.Sprintf("%s$%d", parent, 1+i)
 			}
 		}
 
 		return f.name // should never happen
 	}
 
 	// Method (declared or wrapper)?
 	if recv := f.Signature.Recv(); recv != nil {
 		return f.relMethod(from, recv.Type())
 	}
 
 	// Thunk?
 	if f.method != nil {
 		return f.relMethod(from, f.method.recv)
 	}
 
 	// Bound?
 	if len(f.FreeVars) == 1 && strings.HasSuffix(f.name, "$bound") {
 		return f.relMethod(from, f.FreeVars[0].Type())
 	}
 
 	// Package-level function?
 	// Prefix with package name for cross-package references only.
 	if p := f.relPkg(); p != nil && p != from {
 		return fmt.Sprintf("%s.%s", p.Path(), f.name)
 	}
 
 	// Unknown.
 	return f.name
 }
 
 func (f *Function) relMethod(from *types.Package, recv types.Type) string {
 	return fmt.Sprintf("(%s).%s", relType(recv, from), f.name)
 }
 
 // writeSignature writes to buf the signature sig in declaration syntax.
 func writeSignature(buf *bytes.Buffer, from *types.Package, name string, sig *types.Signature) {
 	buf.WriteString("func ")
 	if recv := sig.Recv(); recv != nil {
 		buf.WriteString("(")
 		if name := recv.Name(); name != "" {
 			buf.WriteString(name)
 			buf.WriteString(" ")
 		}
 		types.WriteType(buf, recv.Type(), types.RelativeTo(from))
 		buf.WriteString(") ")
 	}
 	buf.WriteString(name)
 	types.WriteSignature(buf, sig, types.RelativeTo(from))
 }
 
 // declaredPackage returns the package fn is declared in or nil if the
 // function is not declared in a package.
 func (fn *Function) declaredPackage() *Package {
 	switch {
 	case fn.Pkg != nil:
 		return fn.Pkg // non-generic function  (does that follow??)
 	case fn.topLevelOrigin != nil:
 		return fn.topLevelOrigin.Pkg // instance of a named generic function
 	case fn.parent != nil:
 		return fn.parent.declaredPackage() // instance of an anonymous [generic] function
 	default:
 		return nil // function is not declared in a package, e.g. a wrapper.
 	}
 }
 
 // relPkg returns types.Package fn is printed in relationship to.
 func (fn *Function) relPkg() *types.Package {
 	if p := fn.declaredPackage(); p != nil {
 		return p.Pkg
 	}
 	return nil
 }
 
 var _ io.WriterTo = (*Function)(nil) // *Function implements io.Writer
 
 func (f *Function) WriteTo(w io.Writer) (int64, error) {
 	var buf bytes.Buffer
 	WriteFunction(&buf, f)
 	n, err := w.Write(buf.Bytes())
 	return int64(n), err
 }
 
 // WriteFunction writes to buf a human-readable "disassembly" of f.
 func WriteFunction(buf *bytes.Buffer, f *Function) {
 	fmt.Fprintf(buf, "# Name: %s\n", f.String())
 	if f.Pkg != nil {
 		fmt.Fprintf(buf, "# Package: %s\n", f.Pkg.Pkg.Path())
 	}
 	if syn := f.Synthetic; syn != "" {
 		fmt.Fprintln(buf, "# Synthetic:", syn)
 	}
 	if pos := f.Pos(); pos.IsValid() {
 		fmt.Fprintf(buf, "# Location: %s\n", f.Prog.Fset.Position(pos))
 	}
 
 	if f.parent != nil {
 		fmt.Fprintf(buf, "# Parent: %s\n", f.parent.Name())
 	}
 
 	if f.Recover != nil {
 		fmt.Fprintf(buf, "# Recover: %s\n", f.Recover)
 	}
 
 	from := f.relPkg()
 
 	if f.FreeVars != nil {
 		buf.WriteString("# Free variables:\n")
 		for i, fv := range f.FreeVars {
 			fmt.Fprintf(buf, "# % 3d:\t%s %s\n", i, fv.Name(), relType(fv.Type(), from))
 		}
 	}
 
 	if len(f.Locals) > 0 {
 		buf.WriteString("# Locals:\n")
 		for i, l := range f.Locals {
 			fmt.Fprintf(buf, "# % 3d:\t%s %s\n", i, l.Name(), relType(typeparams.MustDeref(l.Type()), from))
 		}
 	}
 	writeSignature(buf, from, f.Name(), f.Signature)
 	buf.WriteString(":\n")
 
 	if f.Blocks == nil {
 		buf.WriteString("\t(external)\n")
 	}
 
 	// NB. column calculations are confused by non-ASCII
 	// characters and assume 8-space tabs.
 	const punchcard = 80 // for old time's sake.
 	const tabwidth = 8
 	for _, b := range f.Blocks {
 		if b == nil {
 			// Corrupt CFG.
 			fmt.Fprintf(buf, ".nil:\n")
 			continue
 		}
 		n, _ := fmt.Fprintf(buf, "%d:", b.Index)
 		// (|predecessors|, |successors|, immediate dominator)
 		bmsg := fmt.Sprintf("%s P:%d S:%d", b.Comment, len(b.Preds), len(b.Succs))
 		if b.Idom() != nil {
 			bmsg = fmt.Sprintf("%s idom:%d", bmsg, b.Idom().Index)
 		}
 		fmt.Fprintf(buf, "%*s%s\n", punchcard-1-n-len(bmsg), "", bmsg)
 
 		if false { // CFG debugging
 			fmt.Fprintf(buf, "\t# CFG: %s --> %s --> %s\n", b.Preds, b, b.Succs)
 		}
 		for _, instr := range b.Instrs {
 			buf.WriteString("\t")
 			switch v := instr.(type) {
 			case Value:
 				l := punchcard - tabwidth
 				// Left-align the instruction.
 				if name := v.Name(); name != "" {
 					n, _ := fmt.Fprintf(buf, "%s = ", name)
 					l -= n
 				}
 				n, _ := buf.WriteString(instr.String())
 				l -= n
 				// Right-align the type if there's space.
 				if t := v.Type(); t != nil {
 					buf.WriteByte(' ')
 					ts := relType(t, from)
 					l -= len(ts) + len("  ") // (spaces before and after type)
 					if l > 0 {
 						fmt.Fprintf(buf, "%*s", l, "")
 					}
 					buf.WriteString(ts)
 				}
 			case nil:
 				// Be robust against bad transforms.
 				buf.WriteString("<deleted>")
 			default:
 				buf.WriteString(instr.String())
 			}
 			// -mode=S: show line numbers
 			if f.Prog.mode&LogSource != 0 {
 				if pos := instr.Pos(); pos.IsValid() {
 					fmt.Fprintf(buf, " L%d", f.Prog.Fset.Position(pos).Line)
 				}
 			}
 			buf.WriteString("\n")
 		}
 	}
 	fmt.Fprintf(buf, "\n")
 }
 
 // newBasicBlock adds to f a new basic block and returns it.  It does
 // not automatically become the current block for subsequent calls to emit.
 // comment is an optional string for more readable debugging output.
 func (f *Function) newBasicBlock(comment string) *BasicBlock {
 	b := &BasicBlock{
 		Index:   len(f.Blocks),
 		Comment: comment,
 		parent:  f,
 	}
 	b.Succs = b.succs2[:0]
 	f.Blocks = append(f.Blocks, b)
 	return b
 }
 
 // NewFunction returns a new synthetic Function instance belonging to
 // prog, with its name and signature fields set as specified.
 //
 // The caller is responsible for initializing the remaining fields of
 // the function object, e.g. Pkg, Params, Blocks.
 //
 // It is practically impossible for clients to construct well-formed
 // SSA functions/packages/programs directly, so we assume this is the
 // job of the Builder alone.  NewFunction exists to provide clients a
 // little flexibility.  For example, analysis tools may wish to
 // construct fake Functions for the root of the callgraph, a fake
 // "reflect" package, etc.
 //
 // TODO(adonovan): think harder about the API here.
 func (prog *Program) NewFunction(name string, sig *types.Signature, provenance string) *Function {
 	return &Function{Prog: prog, name: name, Signature: sig, Synthetic: provenance}
 }
 
 // Syntax returns the function's syntax (*ast.Func{Decl,Lit})
 // if it was produced from syntax or an *ast.RangeStmt if
 // it is a range-over-func yield function.
 func (f *Function) Syntax() ast.Node { return f.syntax }
 
 // identVar returns the variable defined by id.
 func identVar(fn *Function, id *ast.Ident) *types.Var {
 	return fn.info.Defs[id].(*types.Var)
 }
 
 // unique returns a unique positive int within the source tree of f.
 // The source tree of f includes all of f's ancestors by parent and all
 // of the AnonFuncs contained within these.
 func unique(f *Function) int64 {
 	f.uniq++
 	return f.uniq
 }
 
 // exit is a change of control flow going from a range-over-func
 // yield function to an ancestor function caused by a break, continue,
 // goto, or return statement.
 //
 // There are 3 types of exits:
 // * return from the source function (from ReturnStmt),
 // * jump to a block (from break and continue statements [labelled/unlabelled]),
 // * go to a label (from goto statements).
 //
 // As the builder does one pass over the ast, it is unclear whether
 // a forward goto statement will leave a range-over-func body.
 // The function being exited to is unresolved until the end
 // of building the range-over-func body.
 type exit struct {
 	id   int64     // unique value for exit within from and to
 	from *Function // the function the exit starts from
 	to   *Function // the function being exited to (nil if unresolved)
 	pos  token.Pos
 
 	block *BasicBlock  // basic block within to being jumped to.
 	label *types.Label // forward label being jumped to via goto.
 	// block == nil && label == nil => return
 }
 
 // storeVar emits to function f code to store a value v to a *types.Var x.
 func storeVar(f *Function, x *types.Var, v Value, pos token.Pos) {
 	emitStore(f, f.lookup(x, true), v, pos)
 }
 
 // labelExit creates a new exit to a yield fn to exit the function using a label.
 func labelExit(fn *Function, label *types.Label, pos token.Pos) *exit {
 	e := &exit{
 		id:    unique(fn),
 		from:  fn,
 		to:    nil,
 		pos:   pos,
 		label: label,
 	}
 	fn.exits = append(fn.exits, e)
 	return e
 }
 
 // blockExit creates a new exit to a yield fn that jumps to a basic block.
 func blockExit(fn *Function, block *BasicBlock, pos token.Pos) *exit {
 	e := &exit{
 		id:    unique(fn),
 		from:  fn,
 		to:    block.parent,
 		pos:   pos,
 		block: block,
 	}
 	fn.exits = append(fn.exits, e)
 	return e
 }
 
 // returnExit creates a new exit to a yield fn that returns the source function.
 func returnExit(fn *Function, pos token.Pos) *exit {
 	e := &exit{
 		id:   unique(fn),
 		from: fn,
 		to:   fn.source,
 		pos:  pos,
 	}
 	fn.exits = append(fn.exits, e)
 	return e
 }