go/ssa/promote.go

package ssa

// This file defines utilities for method-set computation, synthesis
// of wrapper methods, and desugaring of implicit field selections.
//
// Wrappers include:
// - promotion wrappers for methods of embedded fields.
// - interface method wrappers for closures of I.f.
// - bound method wrappers, for uncalled obj.Method closures.
// - indirection wrappers, for calls to T-methods on a *T receiver.

// TODO(adonovan): rename to wrappers.go when promotion logic has evaporated.

import (
	"code.google.com/p/go.tools/go/types"
	"fmt"
	"go/token"
)

// anonFieldPath is a linked list of anonymous fields that
// breadth-first traversal has entered, rightmost (outermost) first.
// e.g. "e.f" denoting "e.A.B.C.f" would have a path [C, B, A].
// Common tails may be shared.
//
// It is used by various "promotion"-related algorithms.
//
type anonFieldPath struct {
	tail  *anonFieldPath
	index int // index of field within enclosing types.Struct.Fields
	field *types.Var
}

func (p *anonFieldPath) contains(f *types.Var) bool {
	for ; p != nil; p = p.tail {
		if p.field == f {
			return true
		}
	}
	return false
}

// reverse returns the linked list reversed, as a slice.
func (p *anonFieldPath) reverse() []*anonFieldPath {
	n := 0
	for q := p; q != nil; q = q.tail {
		n++
	}
	s := make([]*anonFieldPath, n)
	n = 0
	for ; p != nil; p = p.tail {
		s[len(s)-1-n] = p
		n++
	}
	return s
}

// isIndirect returns true if the path indirects a pointer.
func (p *anonFieldPath) isIndirect() bool {
	for ; p != nil; p = p.tail {
		if isPointer(p.field.Type()) {
			return true
		}
	}
	return false
}

// Method Set construction ----------------------------------------

// A candidate is a method eligible for promotion: a method of an
// abstract (interface) or concrete (anonymous struct or named) type,
// along with the anonymous field path via which it is implicitly
// reached.  If there is exactly one candidate for a given id, it will
// be promoted to membership of the original type's method-set.
//
// Candidates with path=nil are trivially members of the original
// type's method-set.
//
type candidate struct {
	method *types.Func    // method object of abstract or concrete type
	path   *anonFieldPath // desugared selector path
}

func (c candidate) String() string {
	s := ""
	// Inefficient!
	for p := c.path; p != nil; p = p.tail {
		s = "." + p.field.Name() + s
	}
	return s + "." + c.method.Name()
}

func (c candidate) isConcrete() bool {
	return c.method.Type().(*types.Signature).Recv() != nil
}

// ptrRecv returns true if this candidate is a concrete method with a
// pointer receiver.
//
func (c candidate) ptrRecv() bool {
	recv := c.method.Type().(*types.Signature).Recv()
	return recv != nil && isPointer(recv.Type())
}

// MethodSet returns the method set for type typ, building wrapper
// methods as needed for embedded field promotion, and indirection for
// *T receiver types, etc.
// A nil result indicates an empty set.
//
// Thread-safe.
//
func (p *Program) MethodSet(typ types.Type) MethodSet {
	if !canHaveConcreteMethods(typ, true) {
		return nil
	}

	p.methodsMu.Lock()
	defer p.methodsMu.Unlock()

	mset := p.methodSets.At(typ)
	if mset == nil {
		mset = buildMethodSet(p, typ)
		p.methodSets.Set(typ, mset)
	}
	return mset.(MethodSet)
}

// buildMethodSet computes the concrete method set for type typ.
// It is the implementation of Program.MethodSet.
//
// TODO(adonovan): use go/types.MethodSet(typ) when it's ready.
//
// EXCLUSIVE_LOCKS_REQUIRED(meth.Prog.methodsMu)
//
func buildMethodSet(prog *Program, typ types.Type) MethodSet {
	if prog.mode&LogSource != 0 {
		defer logStack("buildMethodSet %s", typ)()
	}

	// cands maps ids (field and method names) encountered at any
	// level of of the breadth-first traversal to a unique
	// promotion candidate.  A nil value indicates a "blocked" id
	// (i.e. a field or ambiguous method).
	//
	// nextcands is the same but carries just the level in progress.
	cands, nextcands := make(map[Id]*candidate), make(map[Id]*candidate)

	var next, list []*anonFieldPath
	list = append(list, nil) // hack: nil means "use typ"

	// For each level of the type graph...
	for len(list) > 0 {
		// Invariant: next=[], nextcands={}.

		// Collect selectors from one level into 'nextcands'.
		// Record the next levels into 'next'.
		for _, node := range list {
			t := typ // first time only
			if node != nil {
				t = node.field.Type()
			}
			t = deref(t)

			if nt, ok := t.(*types.Named); ok {
				for i, n := 0, nt.NumMethods(); i < n; i++ {
					addCandidate(nextcands, nt.Method(i), node)
				}
				t = nt.Underlying()
			}

			switch t := t.(type) {
			case *types.Interface:
				for i, n := 0, t.NumMethods(); i < n; i++ {
					addCandidate(nextcands, t.Method(i), node)
				}

			case *types.Struct:
				for i, n := 0, t.NumFields(); i < n; i++ {
					f := t.Field(i)
					nextcands[makeId(f.Name(), f.Pkg())] = nil // a field: block id
					// Queue up anonymous fields for next iteration.
					// Break cycles to ensure termination.
					if f.Anonymous() && !node.contains(f) {
						next = append(next, &anonFieldPath{node, i, f})
					}
				}
			}
		}

		// Examine collected selectors.
		// Promote unique, non-blocked ones to cands.
		for id, cand := range nextcands {
			delete(nextcands, id)
			if cand == nil {
				// Update cands so we ignore it at all deeper levels.
				// Don't clobber existing (shallower) binding!
				if _, ok := cands[id]; !ok {
					cands[id] = nil // block id
				}
				continue
			}
			if _, ok := cands[id]; ok {
				// Ignore candidate: a shallower binding exists.
			} else {
				cands[id] = cand
			}
		}
		list, next = next, list[:0] // reuse array
	}

	// Build method sets and wrapper methods.
	mset := make(MethodSet)
	for id, cand := range cands {
		if cand == nil {
			continue // blocked; ignore
		}
		if cand.ptrRecv() && !isPointer(typ) && !cand.path.isIndirect() {
			// A candidate concrete method f with receiver
			// *C is promoted into the method set of
			// (non-pointer) E iff the implicit path selection
			// is indirect, e.g. e.A->B.C.f
			continue
		}
		var method *Function
		if cand.path == nil {
			// Trivial member of method-set; no promotion needed.
			method = prog.concreteMethods[cand.method]

			if !cand.ptrRecv() && isPointer(typ) {
				// Call to method on T from receiver of type *T.
				method = indirectionWrapper(method)
			}
		} else {
			method = promotionWrapper(prog, typ, cand)
		}
		if method == nil {
			panic("unexpected nil method in method set")
		}
		mset[id] = method
	}
	return mset
}

// addCandidate adds the promotion candidate (method, node) to m[(name, package)].
// If a map entry already exists (whether nil or not), its value is set to nil.
//
func addCandidate(m map[Id]*candidate, method *types.Func, node *anonFieldPath) {
	id := makeId(method.Name(), method.Pkg())
	prev, found := m[id]
	switch {
	case prev != nil:
		// Two candidates for same selector: ambiguous; block it.
		m[id] = nil
	case found:
		// Already blocked.
	default:
		// A viable candidate.
		m[id] = &candidate{method, node}
	}
}

// promotionWrapper returns a synthetic Function that delegates to a
// "promoted" method.  For example, given these decls:
//
//    type A struct {B}
//    type B struct {*C}
//    type C ...
//    func (*C) f()
//
// then promotionWrapper(typ=A, cand={method:(*C).f, path:[B,*C]}) will
// synthesize this wrapper method:
//
//    func (a A) f() { return a.B.C->f() }
//
// prog is the program to which the synthesized method will belong.
// typ is the receiver type of the wrapper method.  cand is the
// candidate method to be promoted; it may be concrete or an interface
// method.
//
// EXCLUSIVE_LOCKS_REQUIRED(meth.Prog.methodsMu)
//
func promotionWrapper(prog *Program, typ types.Type, cand *candidate) *Function {
	old := cand.method.Type().(*types.Signature)
	sig := types.NewSignature(types.NewVar(token.NoPos, nil, "recv", typ), old.Params(), old.Results(), old.IsVariadic())

	// TODO(adonovan): consult memoization cache keyed by (typ, cand).
	// Needs typemap.  Also needs hash/eq functions for 'candidate'.
	if prog.mode&LogSource != 0 {
		defer logStack("promotionWrapper (%s)%s, type %s", typ, cand, sig)()
	}
	// TODO(adonovan): is there a *types.Func for this function?
	fn := &Function{
		name:      cand.method.Name(),
		Signature: sig,
		Synthetic: fmt.Sprintf("promotion wrapper for (%s)%s", typ, cand),
		Prog:      prog,
		pos:       cand.method.Pos(),
	}
	fn.startBody()
	fn.addSpilledParam(sig.Recv())
	createParams(fn)

	// Each promotion wrapper performs a sequence of selections,
	// then tailcalls the promoted method.
	// We use pointer arithmetic (FieldAddr possibly followed by
	// Load) in preference to value extraction (Field possibly
	// preceded by Load).
	var v Value = fn.Locals[0] // spilled receiver
	if isPointer(typ) {
		v = emitLoad(fn, v)
	}
	// Iterate over selections e.A.B.C.f in the natural order [A,B,C].
	for _, p := range cand.path.reverse() {
		// Loop invariant: v holds a pointer to a struct.
		if _, ok := deref(v.Type()).Underlying().(*types.Struct); !ok {
			panic(fmt.Sprint("not a *struct: ", v.Type(), p.field.Type))
		}
		sel := &FieldAddr{
			X:     v,
			Field: p.index,
		}
		sel.setType(types.NewPointer(p.field.Type()))
		v = fn.emit(sel)
		if isPointer(p.field.Type()) {
			v = emitLoad(fn, v)
		}
	}
	if !cand.ptrRecv() {
		v = emitLoad(fn, v)
	}

	var c Call
	if cand.isConcrete() {
		c.Call.Func = prog.concreteMethods[cand.method]
		c.Call.Args = append(c.Call.Args, v)
	} else {
		iface := v.Type().Underlying().(*types.Interface)
		id := makeId(cand.method.Name(), cand.method.Pkg())
		c.Call.Method, _ = interfaceMethodIndex(iface, id)
		c.Call.Recv = v
	}
	for _, arg := range fn.Params[1:] {
		c.Call.Args = append(c.Call.Args, arg)
	}
	emitTailCall(fn, &c)
	fn.finishBody()
	return fn
}

// createParams creates parameters for wrapper method fn based on its
// Signature.Params, which do not include the receiver.
//
func createParams(fn *Function) {
	var last *Parameter
	tparams := fn.Signature.Params()
	for i, n := 0, tparams.Len(); i < n; i++ {
		last = fn.addParamObj(tparams.At(i))
	}
	if fn.Signature.IsVariadic() {
		last.typ = types.NewSlice(last.typ)
	}
}

// Wrappers for standalone interface methods ----------------------------------

// interfaceMethodWrapper returns a synthetic wrapper function permitting a
// method id of interface typ to be called like a standalone function,
// e.g.:
//
//   type I interface { f(x int) R }
//   m := I.f  // wrapper
//   var i I
//   m(i, 0)
//
// The wrapper is defined as if by:
//
//   func I.f(i I, x int, ...) R {
//     return i.f(x, ...)
//   }
//
// TODO(adonovan): opt: currently the stub is created even when used
// in call position: I.f(i, 0).  Clearly this is suboptimal.
//
// EXCLUSIVE_LOCKS_ACQUIRED(meth.Prog.methodsMu)
//
func interfaceMethodWrapper(prog *Program, typ types.Type, id Id) *Function {
	index, meth := interfaceMethodIndex(typ.Underlying().(*types.Interface), id)
	prog.methodsMu.Lock()
	defer prog.methodsMu.Unlock()
	// If one interface embeds another they'll share the same
	// wrappers for common methods.  This is safe, but it might
	// confuse some tools because of the implicit interface
	// conversion applied to the first argument.  If this becomes
	// a problem, we should include 'typ' in the memoization key.
	fn, ok := prog.ifaceMethodWrappers[meth]
	if !ok {
		if prog.mode&LogSource != 0 {
			defer logStack("interfaceMethodWrapper %s.%s", typ, id)()
		}
		fn = &Function{
			name:      meth.Name(),
			object:    meth,
			Signature: meth.Type().(*types.Signature),
			Synthetic: fmt.Sprintf("interface method wrapper for %s.%s", typ, id),
			pos:       meth.Pos(),
			Prog:      prog,
		}
		fn.startBody()
		fn.addParam("recv", typ, token.NoPos)
		createParams(fn)
		var c Call
		c.Call.Method = index
		c.Call.Recv = fn.Params[0]
		for _, arg := range fn.Params[1:] {
			c.Call.Args = append(c.Call.Args, arg)
		}
		emitTailCall(fn, &c)
		fn.finishBody()

		prog.ifaceMethodWrappers[meth] = fn
	}
	return fn
}

// Wrappers for bound methods -------------------------------------------------

// boundMethodWrapper returns a synthetic wrapper function that
// delegates to a concrete method.  The wrapper has one free variable,
// the method's receiver.  Use MakeClosure with such a wrapper to
// construct a bound-method closure.
// e.g.:
//
//   type T int
//   func (t T) meth()
//   var t T
//   f := t.meth
//   f() // calls t.meth()
//
// f is a closure of a synthetic wrapper defined as if by:
//
//   f := func() { return t.meth() }
//
// EXCLUSIVE_LOCKS_ACQUIRED(meth.Prog.methodsMu)
//
func boundMethodWrapper(meth *Function) *Function {
	prog := meth.Prog
	prog.methodsMu.Lock()
	defer prog.methodsMu.Unlock()
	fn, ok := prog.boundMethodWrappers[meth]
	if !ok {
		if prog.mode&LogSource != 0 {
			defer logStack("boundMethodWrapper %s", meth)()
		}
		s := meth.Signature
		fn = &Function{
			name:      "bound$" + meth.String(),
			Signature: types.NewSignature(nil, s.Params(), s.Results(), s.IsVariadic()), // drop recv
			Synthetic: "bound method wrapper for " + meth.String(),
			Prog:      prog,
			pos:       meth.Pos(),
		}

		cap := &Capture{name: "recv", typ: s.Recv().Type(), parent: fn}
		fn.FreeVars = []*Capture{cap}
		fn.startBody()
		createParams(fn)
		var c Call
		c.Call.Func = meth
		c.Call.Args = []Value{cap}
		for _, arg := range fn.Params {
			c.Call.Args = append(c.Call.Args, arg)
		}
		emitTailCall(fn, &c)
		fn.finishBody()

		prog.boundMethodWrappers[meth] = fn
	}
	return fn
}

// Receiver indirection wrapper ------------------------------------

// indirectionWrapper returns a synthetic method with *T receiver
// that delegates to meth, which has a T receiver.
//
//      func (recv *T) f(...) ... {
//              return (*recv).f(...)
//      }
//
// EXCLUSIVE_LOCKS_REQUIRED(meth.Prog.methodsMu)
//
func indirectionWrapper(meth *Function) *Function {
	prog := meth.Prog
	fn, ok := prog.indirectionWrappers[meth]
	if !ok {
		if prog.mode&LogSource != 0 {
			defer logStack("makeIndirectionWrapper %s", meth)()
		}

		s := meth.Signature
		recv := types.NewVar(token.NoPos, meth.Pkg.Object, "recv",
			types.NewPointer(s.Recv().Type()))
		// TODO(adonovan): is there a *types.Func for this method?
		fn = &Function{
			name:      meth.Name(),
			Signature: types.NewSignature(recv, s.Params(), s.Results(), s.IsVariadic()),
			Prog:      prog,
			Synthetic: "receiver indirection wrapper for " + meth.String(),
			pos:       meth.Pos(),
		}

		fn.startBody()
		fn.addParamObj(recv)
		createParams(fn)
		// TODO(adonovan): consider emitting a nil-pointer check here
		// with a nice error message, like gc does.
		var c Call
		c.Call.Func = meth
		c.Call.Args = append(c.Call.Args, emitLoad(fn, fn.Params[0]))
		for _, arg := range fn.Params[1:] {
			c.Call.Args = append(c.Call.Args, arg)
		}
		emitTailCall(fn, &c)
		fn.finishBody()

		prog.indirectionWrappers[meth] = fn
	}
	return fn
}