diff --git a/ssa/blockopt.go b/ssa/blockopt.go
index bea292016f2..0253b350b63 100644
--- a/ssa/blockopt.go
+++ b/ssa/blockopt.go
@@ -85,7 +85,7 @@ func jumpThreading(f *Function, b *BasicBlock) bool {
 		// If a now has two edges to c, replace its degenerate If by Jump.
 		if len(a.Succs) == 2 && a.Succs[0] == c && a.Succs[1] == c {
 			jump := new(Jump)
-			jump.SetBlock(a)
+			jump.setBlock(a)
 			a.Instrs[len(a.Instrs)-1] = jump
 			a.Succs = a.Succs[:1]
 			c.removePred(b)
@@ -120,7 +120,7 @@ func fuseBlocks(f *Function, a *BasicBlock) bool {
 	// Eliminate jump at end of A, then copy all of B across.
 	a.Instrs = append(a.Instrs[:len(a.Instrs)-1], b.Instrs...)
 	for _, instr := range b.Instrs {
-		instr.SetBlock(a)
+		instr.setBlock(a)
 	}
 
 	// A inherits B's successors
diff --git a/ssa/builder.go b/ssa/builder.go
index 491f65ac5cf..3d1c6325103 100644
--- a/ssa/builder.go
+++ b/ssa/builder.go
@@ -2040,8 +2040,6 @@ start:
 
 		fn.currentBlock = done
 
-		// TODO statement debug info.
-
 	case *ast.SwitchStmt:
 		b.switchStmt(fn, s, label)
 
diff --git a/ssa/func.go b/ssa/func.go
index cd94e8d97d9..cf1aa6561d5 100644
--- a/ssa/func.go
+++ b/ssa/func.go
@@ -37,7 +37,7 @@ func (b *BasicBlock) String() string {
 // If the instruction defines a Value, it is returned.
 //
 func (b *BasicBlock) emit(i Instruction) Value {
-	i.SetBlock(b)
+	i.setBlock(b)
 	b.Instrs = append(b.Instrs, i)
 	v, _ := i.(Value)
 	return v
diff --git a/ssa/sanity.go b/ssa/sanity.go
index 81f33516d7d..f6afb062649 100644
--- a/ssa/sanity.go
+++ b/ssa/sanity.go
@@ -291,14 +291,11 @@ func (s *sanity) checkBlock(b *BasicBlock, index int) {
 	}
 
 	// Check each instruction is sane.
-	// TODO(adonovan): check Instruction invariants:
-	// - check Operands is dual to Value.Referrers.
-	// - check all Operands that are also Instructions belong to s.fn too
-	//   (and for bonus marks, that their block dominates block b).
 	n := len(b.Instrs)
 	if n == 0 {
 		s.errorf("basic block contains no instructions")
 	}
+	var rands [10]*Value // reuse storage
 	for j, instr := range b.Instrs {
 		if instr == nil {
 			s.errorf("nil instruction at index %d", j)
@@ -316,6 +313,48 @@ func (s *sanity) checkBlock(b *BasicBlock, index int) {
 		} else {
 			s.checkFinalInstr(j, instr)
 		}
+
+		// Check Instruction.Operands.
+	operands:
+		for i, op := range instr.Operands(rands[:0]) {
+			if op == nil {
+				s.errorf("nil operand pointer %d of %s", i, instr)
+				continue
+			}
+			val := *op
+			if val == nil {
+				continue // a nil operand is ok
+			}
+			// Check that Operands that are also Instructions belong to same function.
+			// TODO(adonovan): also check their block dominates block b.
+			if val, ok := val.(Instruction); ok {
+				if val.Parent() != s.fn {
+					s.errorf("operand %d of %s is an instruction (%s) from function %s", i, instr, val, val.Parent())
+				}
+			}
+
+			// Check that each function-local operand of
+			// instr refers back to instr.  (NB: quadratic)
+			switch val := val.(type) {
+			case *Const, *Global, *Builtin:
+				continue // not local
+			case *Function:
+				if val.Enclosing == nil {
+					continue // only anon functions are local
+				}
+			}
+
+			if refs := val.Referrers(); refs != nil {
+				for _, ref := range *refs {
+					if ref == instr {
+						continue operands
+					}
+				}
+				s.errorf("operand %d of %s (%s) does not refer to us", i, instr, val)
+			} else {
+				s.errorf("operand %d of %s (%s) has no referrers", i, instr, val)
+			}
+		}
 	}
 }
 
diff --git a/ssa/ssa.go b/ssa/ssa.go
index c463664ee6d..a4f968eaa1a 100644
--- a/ssa/ssa.go
+++ b/ssa/ssa.go
@@ -127,8 +127,9 @@ type Value interface {
 	// caller may perform mutations to the object's state.
 	//
 	// Referrers is currently only defined for the function-local
-	// values Capture, Parameter and all value-defining instructions.
-	// It returns nil for Function, Builtin, Const and Global.
+	// values Capture, Parameter, Functions (iff anonymous) and
+	// all value-defining instructions.
+	// It returns nil for named Functions, Builtin, Const and Global.
 	//
 	// Instruction.Operands contains the inverse of this relation.
 	Referrers() *[]Instruction
@@ -184,10 +185,8 @@ type Instruction interface {
 	// belongs.
 	Block() *BasicBlock
 
-	// SetBlock sets the basic block to which this instruction
-	// belongs.
-	// TODO(adonovan): unexport this.
-	SetBlock(*BasicBlock)
+	// setBlock sets the basic block to which this instruction belongs.
+	setBlock(*BasicBlock)
 
 	// Operands returns the operands of this instruction: the
 	// set of Values it references.
@@ -279,6 +278,7 @@ type Function struct {
 	Blocks    []*BasicBlock // basic blocks of the function; nil => external
 	Recover   *BasicBlock   // optional; control transfers here after recovered panic
 	AnonFuncs []*Function   // anonymous functions directly beneath this one
+	referrers []Instruction // referring instructions (iff Enclosing != nil)
 
 	// The following fields are set transiently during building,
 	// then cleared.
@@ -1212,7 +1212,7 @@ type register struct {
 }
 
 // anInstruction is a mix-in embedded by all Instructions.
-// It provides the implementations of the Block and SetBlock methods.
+// It provides the implementations of the Block and setBlock methods.
 type anInstruction struct {
 	block *BasicBlock // the basic block of this instruction
 }
@@ -1368,12 +1368,17 @@ func (*Global) Referrers() *[]Instruction { return nil }
 func (v *Global) Token() token.Token      { return token.VAR }
 func (v *Global) Object() types.Object    { return v.object }
 
-func (v *Function) Name() string            { return v.name }
-func (v *Function) Type() types.Type        { return v.Signature }
-func (v *Function) Pos() token.Pos          { return v.pos }
-func (*Function) Referrers() *[]Instruction { return nil }
-func (v *Function) Token() token.Token      { return token.FUNC }
-func (v *Function) Object() types.Object    { return v.object }
+func (v *Function) Name() string         { return v.name }
+func (v *Function) Type() types.Type     { return v.Signature }
+func (v *Function) Pos() token.Pos       { return v.pos }
+func (v *Function) Token() token.Token   { return token.FUNC }
+func (v *Function) Object() types.Object { return v.object }
+func (v *Function) Referrers() *[]Instruction {
+	if v.Enclosing != nil {
+		return &v.referrers
+	}
+	return nil
+}
 
 func (v *Parameter) Type() types.Type          { return v.typ }
 func (v *Parameter) Name() string              { return v.name }
@@ -1396,7 +1401,7 @@ func (v *register) setPos(pos token.Pos)      { v.pos = pos }
 
 func (v *anInstruction) Parent() *Function          { return v.block.parent }
 func (v *anInstruction) Block() *BasicBlock         { return v.block }
-func (v *anInstruction) SetBlock(block *BasicBlock) { v.block = block }
+func (v *anInstruction) setBlock(block *BasicBlock) { v.block = block }
 
 func (t *Type) Name() string         { return t.object.Name() }
 func (t *Type) Pos() token.Pos       { return t.object.Pos() }