go/ssa/sanity.go - tools - Git at Google

 // 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

 // An optional pass for sanity-checking invariants of the SSA representation.
 // Currently it checks CFG invariants but little at the instruction level.

 import (
 	"fmt"
 	"go/types"
 	"io"
 	"os"
 	"strings"
 )

 type sanity struct {
 	reporter io.Writer
 	fn       *Function
 	block    *BasicBlock
 	instrs   map[Instruction]struct{}
 	insane   bool
 }

 // sanityCheck performs integrity checking of the SSA representation
 // of the function fn and returns true if it was valid.  Diagnostics
 // are written to reporter if non-nil, os.Stderr otherwise.  Some
 // diagnostics are only warnings and do not imply a negative result.
 //
 // Sanity-checking is intended to facilitate the debugging of code
 // transformation passes.
 //
 func sanityCheck(fn *Function, reporter io.Writer) bool {
 	if reporter == nil {
 		reporter = os.Stderr
 	}
 	return (&sanity{reporter: reporter}).checkFunction(fn)
 }

 // mustSanityCheck is like sanityCheck but panics instead of returning
 // a negative result.
 //
 func mustSanityCheck(fn *Function, reporter io.Writer) {
 	if !sanityCheck(fn, reporter) {
 		fn.WriteTo(os.Stderr)
 		panic("SanityCheck failed")
 	}
 }

 func (s *sanity) diagnostic(prefix, format string, args ...interface{}) {
 	fmt.Fprintf(s.reporter, "%s: function %s", prefix, s.fn)
 	if s.block != nil {
 		fmt.Fprintf(s.reporter, ", block %s", s.block)
 	}
 	io.WriteString(s.reporter, ": ")
 	fmt.Fprintf(s.reporter, format, args...)
 	io.WriteString(s.reporter, "\n")
 }

 func (s *sanity) errorf(format string, args ...interface{}) {
 	s.insane = true
 	s.diagnostic("Error", format, args...)
 }

 func (s *sanity) warnf(format string, args ...interface{}) {
 	s.diagnostic("Warning", format, args...)
 }

 // findDuplicate returns an arbitrary basic block that appeared more
 // than once in blocks, or nil if all were unique.
 func findDuplicate(blocks []*BasicBlock) *BasicBlock {
 	if len(blocks) < 2 {
 		return nil
 	}
 	if blocks[0] == blocks[1] {
 		return blocks[0]
 	}
 	// Slow path:
 	m := make(map[*BasicBlock]bool)
 	for _, b := range blocks {
 		if m[b] {
 			return b
 		}
 		m[b] = true
 	}
 	return nil
 }

 func (s *sanity) checkInstr(idx int, instr Instruction) {
 	switch instr := instr.(type) {
 	case *If, *Jump, *Return, *Panic:
 		s.errorf("control flow instruction not at end of block")
 	case *Phi:
 		if idx == 0 {
 			// It suffices to apply this check to just the first phi node.
 			if dup := findDuplicate(s.block.Preds); dup != nil {
 				s.errorf("phi node in block with duplicate predecessor %s", dup)
 			}
 		} else {
 			prev := s.block.Instrs[idx-1]
 			if _, ok := prev.(*Phi); !ok {
 				s.errorf("Phi instruction follows a non-Phi: %T", prev)
 			}
 		}
 		if ne, np := len(instr.Edges), len(s.block.Preds); ne != np {
 			s.errorf("phi node has %d edges but %d predecessors", ne, np)

 		} else {
 			for i, e := range instr.Edges {
 				if e == nil {
 					s.errorf("phi node '%s' has no value for edge #%d from %s", instr.Comment, i, s.block.Preds[i])
 				}
 			}
 		}

 	case *Alloc:
 		if !instr.Heap {
 			found := false
 			for _, l := range s.fn.Locals {
 				if l == instr {
 					found = true
 					break
 				}
 			}
 			if !found {
 				s.errorf("local alloc %s = %s does not appear in Function.Locals", instr.Name(), instr)
 			}
 		}

 	case *BinOp:
 	case *Call:
 	case *ChangeInterface:
 	case *ChangeType:
 	case *Convert:
 		if _, ok := instr.X.Type().Underlying().(*types.Basic); !ok {
 			if _, ok := instr.Type().Underlying().(*types.Basic); !ok {
 				s.errorf("convert %s -> %s: at least one type must be basic", instr.X.Type(), instr.Type())
 			}
 		}

 	case *Defer:
 	case *Extract:
 	case *Field:
 	case *FieldAddr:
 	case *Go:
 	case *Index:
 	case *IndexAddr:
 	case *Lookup:
 	case *MakeChan:
 	case *MakeClosure:
 		numFree := len(instr.Fn.(*Function).FreeVars)
 		numBind := len(instr.Bindings)
 		if numFree != numBind {
 			s.errorf("MakeClosure has %d Bindings for function %s with %d free vars",
 				numBind, instr.Fn, numFree)

 		}
 		if recv := instr.Type().(*types.Signature).Recv(); recv != nil {
 			s.errorf("MakeClosure's type includes receiver %s", recv.Type())
 		}

 	case *MakeInterface:
 	case *MakeMap:
 	case *MakeSlice:
 	case *MapUpdate:
 	case *Next:
 	case *Range:
 	case *RunDefers:
 	case *Select:
 	case *Send:
 	case *Slice:
 	case *Store:
 	case *TypeAssert:
 	case *UnOp:
 	case *DebugRef:
 		// TODO(adonovan): implement checks.
 	default:
 		panic(fmt.Sprintf("Unknown instruction type: %T", instr))
 	}

 	if call, ok := instr.(CallInstruction); ok {
 		if call.Common().Signature() == nil {
 			s.errorf("nil signature: %s", call)
 		}
 	}

 	// Check that value-defining instructions have valid types
 	// and a valid referrer list.
 	if v, ok := instr.(Value); ok {
 		t := v.Type()
 		if t == nil {
 			s.errorf("no type: %s = %s", v.Name(), v)
 		} else if t == tRangeIter {
 			// not a proper type; ignore.
 		} else if b, ok := t.Underlying().(*types.Basic); ok && b.Info()&types.IsUntyped != 0 {
 			s.errorf("instruction has 'untyped' result: %s = %s : %s", v.Name(), v, t)
 		}
 		s.checkReferrerList(v)
 	}

 	// Untyped constants are legal as instruction Operands(),
 	// for example:
 	//   _ = "foo"[0]
 	// or:
 	//   if wordsize==64 {...}

 	// All other non-Instruction Values can be found via their
 	// enclosing Function or Package.
 }

 func (s *sanity) checkFinalInstr(instr Instruction) {
 	switch instr := instr.(type) {
 	case *If:
 		if nsuccs := len(s.block.Succs); nsuccs != 2 {
 			s.errorf("If-terminated block has %d successors; expected 2", nsuccs)
 			return
 		}
 		if s.block.Succs[0] == s.block.Succs[1] {
 			s.errorf("If-instruction has same True, False target blocks: %s", s.block.Succs[0])
 			return
 		}

 	case *Jump:
 		if nsuccs := len(s.block.Succs); nsuccs != 1 {
 			s.errorf("Jump-terminated block has %d successors; expected 1", nsuccs)
 			return
 		}

 	case *Return:
 		if nsuccs := len(s.block.Succs); nsuccs != 0 {
 			s.errorf("Return-terminated block has %d successors; expected none", nsuccs)
 			return
 		}
 		if na, nf := len(instr.Results), s.fn.Signature.Results().Len(); nf != na {
 			s.errorf("%d-ary return in %d-ary function", na, nf)
 		}

 	case *Panic:
 		if nsuccs := len(s.block.Succs); nsuccs != 0 {
 			s.errorf("Panic-terminated block has %d successors; expected none", nsuccs)
 			return
 		}

 	default:
 		s.errorf("non-control flow instruction at end of block")
 	}
 }

 func (s *sanity) checkBlock(b *BasicBlock, index int) {
 	s.block = b

 	if b.Index != index {
 		s.errorf("block has incorrect Index %d", b.Index)
 	}
 	if b.parent != s.fn {
 		s.errorf("block has incorrect parent %s", b.parent)
 	}

 	// Check all blocks are reachable.
 	// (The entry block is always implicitly reachable,
 	// as is the Recover block, if any.)
 	if (index > 0 && b != b.parent.Recover) && len(b.Preds) == 0 {
 		s.warnf("unreachable block")
 		if b.Instrs == nil {
 			// Since this block is about to be pruned,
 			// tolerating transient problems in it
 			// simplifies other optimizations.
 			return
 		}
 	}

 	// Check predecessor and successor relations are dual,
 	// and that all blocks in CFG belong to same function.
 	for _, a := range b.Preds {
 		found := false
 		for _, bb := range a.Succs {
 			if bb == b {
 				found = true
 				break
 			}
 		}
 		if !found {
 			s.errorf("expected successor edge in predecessor %s; found only: %s", a, a.Succs)
 		}
 		if a.parent != s.fn {
 			s.errorf("predecessor %s belongs to different function %s", a, a.parent)
 		}
 	}
 	for _, c := range b.Succs {
 		found := false
 		for _, bb := range c.Preds {
 			if bb == b {
 				found = true
 				break
 			}
 		}
 		if !found {
 			s.errorf("expected predecessor edge in successor %s; found only: %s", c, c.Preds)
 		}
 		if c.parent != s.fn {
 			s.errorf("successor %s belongs to different function %s", c, c.parent)
 		}
 	}

 	// Check each instruction is sane.
 	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)
 			continue
 		}
 		if b2 := instr.Block(); b2 == nil {
 			s.errorf("nil Block() for instruction at index %d", j)
 			continue
 		} else if b2 != b {
 			s.errorf("wrong Block() (%s) for instruction at index %d ", b2, j)
 			continue
 		}
 		if j < n-1 {
 			s.checkInstr(j, instr)
 		} else {
 			s.checkFinalInstr(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 "untyped" types only appear on constant operands.
 			if _, ok := (*op).(*Const); !ok {
 				if basic, ok := (*op).Type().(*types.Basic); ok {
 					if basic.Info()&types.IsUntyped != 0 {
 						s.errorf("operand #%d of %s is untyped: %s", i, instr, basic)
 					}
 				}
 			}

 			// 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.Block() == nil {
 					s.errorf("operand %d of %s is an instruction (%s) that belongs to no block", i, instr, val)
 				} else 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.parent == nil {
 					continue // only anon functions are local
 				}
 			}

 			// TODO(adonovan): check val.Parent() != nil <=> val.Referrers() is defined.

 			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)
 			}
 		}
 	}
 }

 func (s *sanity) checkReferrerList(v Value) {
 	refs := v.Referrers()
 	if refs == nil {
 		s.errorf("%s has missing referrer list", v.Name())
 		return
 	}
 	for i, ref := range *refs {
 		if _, ok := s.instrs[ref]; !ok {
 			s.errorf("%s.Referrers()[%d] = %s is not an instruction belonging to this function", v.Name(), i, ref)
 		}
 	}
 }

 func (s *sanity) checkFunction(fn *Function) bool {
 	// TODO(adonovan): check Function invariants:
 	// - check params match signature
 	// - check transient fields are nil
 	// - warn if any fn.Locals do not appear among block instructions.
 	s.fn = fn
 	if fn.Prog == nil {
 		s.errorf("nil Prog")
 	}

 	fn.String()            // must not crash
 	fn.RelString(fn.pkg()) // must not crash

 	// All functions have a package, except delegates (which are
 	// shared across packages, or duplicated as weak symbols in a
 	// separate-compilation model), and error.Error.
 	if fn.Pkg == nil {
 		if strings.HasPrefix(fn.Synthetic, "wrapper ") ||
 			strings.HasPrefix(fn.Synthetic, "bound ") ||
 			strings.HasPrefix(fn.Synthetic, "thunk ") ||
 			strings.HasSuffix(fn.name, "Error") {
 			// ok
 		} else {
 			s.errorf("nil Pkg")
 		}
 	}
 	if src, syn := fn.Synthetic == "", fn.Syntax() != nil; src != syn {
 		s.errorf("got fromSource=%t, hasSyntax=%t; want same values", src, syn)
 	}
 	for i, l := range fn.Locals {
 		if l.Parent() != fn {
 			s.errorf("Local %s at index %d has wrong parent", l.Name(), i)
 		}
 		if l.Heap {
 			s.errorf("Local %s at index %d has Heap flag set", l.Name(), i)
 		}
 	}
 	// Build the set of valid referrers.
 	s.instrs = make(map[Instruction]struct{})
 	for _, b := range fn.Blocks {
 		for _, instr := range b.Instrs {
 			s.instrs[instr] = struct{}{}
 		}
 	}
 	for i, p := range fn.Params {
 		if p.Parent() != fn {
 			s.errorf("Param %s at index %d has wrong parent", p.Name(), i)
 		}
 		s.checkReferrerList(p)
 	}
 	for i, fv := range fn.FreeVars {
 		if fv.Parent() != fn {
 			s.errorf("FreeVar %s at index %d has wrong parent", fv.Name(), i)
 		}
 		s.checkReferrerList(fv)
 	}

 	if fn.Blocks != nil && len(fn.Blocks) == 0 {
 		// Function _had_ blocks (so it's not external) but
 		// they were "optimized" away, even the entry block.
 		s.errorf("Blocks slice is non-nil but empty")
 	}
 	for i, b := range fn.Blocks {
 		if b == nil {
 			s.warnf("nil *BasicBlock at f.Blocks[%d]", i)
 			continue
 		}
 		s.checkBlock(b, i)
 	}
 	if fn.Recover != nil && fn.Blocks[fn.Recover.Index] != fn.Recover {
 		s.errorf("Recover block is not in Blocks slice")
 	}

 	s.block = nil
 	for i, anon := range fn.AnonFuncs {
 		if anon.Parent() != fn {
 			s.errorf("AnonFuncs[%d]=%s but %s.Parent()=%s", i, anon, anon, anon.Parent())
 		}
 	}
 	s.fn = nil
 	return !s.insane
 }

 // sanityCheckPackage checks invariants of packages upon creation.
 // It does not require that the package is built.
 // Unlike sanityCheck (for functions), it just panics at the first error.
 func sanityCheckPackage(pkg *Package) {
 	if pkg.Pkg == nil {
 		panic(fmt.Sprintf("Package %s has no Object", pkg))
 	}
 	pkg.String() // must not crash

 	for name, mem := range pkg.Members {
 		if name != mem.Name() {
 			panic(fmt.Sprintf("%s: %T.Name() = %s, want %s",
 				pkg.Pkg.Path(), mem, mem.Name(), name))
 		}
 		obj := mem.Object()
 		if obj == nil {
 			// This check is sound because fields
 			// {Global,Function}.object have type
 			// types.Object.  (If they were declared as
 			// *types.{Var,Func}, we'd have a non-empty
 			// interface containing a nil pointer.)

 			continue // not all members have typechecker objects
 		}
 		if obj.Name() != name {
 			if obj.Name() == "init" && strings.HasPrefix(mem.Name(), "init#") {
 				// Ok.  The name of a declared init function varies between
 				// its types.Func ("init") and its ssa.Function ("init#%d").
 			} else {
 				panic(fmt.Sprintf("%s: %T.Object().Name() = %s, want %s",
 					pkg.Pkg.Path(), mem, obj.Name(), name))
 			}
 		}
 		if obj.Pos() != mem.Pos() {
 			panic(fmt.Sprintf("%s Pos=%d obj.Pos=%d", mem, mem.Pos(), obj.Pos()))
 		}
 	}
 }
	// 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

	// An optional pass for sanity-checking invariants of the SSA representation.
	// Currently it checks CFG invariants but little at the instruction level.

	import (
	"fmt"
	"go/types"
	"io"
	"os"
	"strings"
	)

	type sanity struct {
	reporter io.Writer
	fn *Function
	block *BasicBlock
	instrs map[Instruction]struct{}
	insane bool
	}

	// sanityCheck performs integrity checking of the SSA representation
	// of the function fn and returns true if it was valid. Diagnostics
	// are written to reporter if non-nil, os.Stderr otherwise. Some
	// diagnostics are only warnings and do not imply a negative result.
	//
	// Sanity-checking is intended to facilitate the debugging of code
	// transformation passes.
	//
	func sanityCheck(fn *Function, reporter io.Writer) bool {
	if reporter == nil {
	reporter = os.Stderr
	}
	return (&sanity{reporter: reporter}).checkFunction(fn)
	}

	// mustSanityCheck is like sanityCheck but panics instead of returning
	// a negative result.
	//
	func mustSanityCheck(fn *Function, reporter io.Writer) {
	if !sanityCheck(fn, reporter) {
	fn.WriteTo(os.Stderr)
	panic("SanityCheck failed")
	}
	}

	func (s *sanity) diagnostic(prefix, format string, args ...interface{}) {
	fmt.Fprintf(s.reporter, "%s: function %s", prefix, s.fn)
	if s.block != nil {
	fmt.Fprintf(s.reporter, ", block %s", s.block)
	}
	io.WriteString(s.reporter, ": ")
	fmt.Fprintf(s.reporter, format, args...)
	io.WriteString(s.reporter, "\n")
	}

	func (s *sanity) errorf(format string, args ...interface{}) {
	s.insane = true
	s.diagnostic("Error", format, args...)
	}

	func (s *sanity) warnf(format string, args ...interface{}) {
	s.diagnostic("Warning", format, args...)
	}

	// findDuplicate returns an arbitrary basic block that appeared more
	// than once in blocks, or nil if all were unique.
	func findDuplicate(blocks []BasicBlock) BasicBlock {
	if len(blocks) < 2 {
	return nil
	}
	if blocks[0] == blocks[1] {
	return blocks[0]
	}
	// Slow path:
	m := make(map[*BasicBlock]bool)
	for _, b := range blocks {
	if m[b] {
	return b
	}
	m[b] = true
	}
	return nil
	}

	func (s *sanity) checkInstr(idx int, instr Instruction) {
	switch instr := instr.(type) {
	case If, Jump, Return, Panic:
	s.errorf("control flow instruction not at end of block")
	case *Phi:
	if idx == 0 {
	// It suffices to apply this check to just the first phi node.
	if dup := findDuplicate(s.block.Preds); dup != nil {
	s.errorf("phi node in block with duplicate predecessor %s", dup)
	}
	} else {
	prev := s.block.Instrs[idx-1]
	if _, ok := prev.(*Phi); !ok {
	s.errorf("Phi instruction follows a non-Phi: %T", prev)
	}
	}
	if ne, np := len(instr.Edges), len(s.block.Preds); ne != np {
	s.errorf("phi node has %d edges but %d predecessors", ne, np)

	} else {
	for i, e := range instr.Edges {
	if e == nil {
	s.errorf("phi node '%s' has no value for edge #%d from %s", instr.Comment, i, s.block.Preds[i])
	}
	}
	}

	case *Alloc:
	if !instr.Heap {
	found := false
	for _, l := range s.fn.Locals {
	if l == instr {
	found = true
	break
	}
	}
	if !found {
	s.errorf("local alloc %s = %s does not appear in Function.Locals", instr.Name(), instr)
	}
	}

	case *BinOp:
	case *Call:
	case *ChangeInterface:
	case *ChangeType:
	case *Convert:
	if _, ok := instr.X.Type().Underlying().(*types.Basic); !ok {
	if _, ok := instr.Type().Underlying().(*types.Basic); !ok {
	s.errorf("convert %s -> %s: at least one type must be basic", instr.X.Type(), instr.Type())
	}
	}

	case *Defer:
	case *Extract:
	case *Field:
	case *FieldAddr:
	case *Go:
	case *Index:
	case *IndexAddr:
	case *Lookup:
	case *MakeChan:
	case *MakeClosure:
	numFree := len(instr.Fn.(*Function).FreeVars)
	numBind := len(instr.Bindings)
	if numFree != numBind {
	s.errorf("MakeClosure has %d Bindings for function %s with %d free vars",
	numBind, instr.Fn, numFree)

	}
	if recv := instr.Type().(*types.Signature).Recv(); recv != nil {
	s.errorf("MakeClosure's type includes receiver %s", recv.Type())
	}

	case *MakeInterface:
	case *MakeMap:
	case *MakeSlice:
	case *MapUpdate:
	case *Next:
	case *Range:
	case *RunDefers:
	case *Select:
	case *Send:
	case *Slice:
	case *Store:
	case *TypeAssert:
	case *UnOp:
	case *DebugRef:
	// TODO(adonovan): implement checks.
	default:
	panic(fmt.Sprintf("Unknown instruction type: %T", instr))
	}

	if call, ok := instr.(CallInstruction); ok {
	if call.Common().Signature() == nil {
	s.errorf("nil signature: %s", call)
	}
	}

	// Check that value-defining instructions have valid types
	// and a valid referrer list.
	if v, ok := instr.(Value); ok {
	t := v.Type()
	if t == nil {
	s.errorf("no type: %s = %s", v.Name(), v)
	} else if t == tRangeIter {
	// not a proper type; ignore.
	} else if b, ok := t.Underlying().(*types.Basic); ok && b.Info()&types.IsUntyped != 0 {
	s.errorf("instruction has 'untyped' result: %s = %s : %s", v.Name(), v, t)
	}
	s.checkReferrerList(v)
	}

	// Untyped constants are legal as instruction Operands(),
	// for example:
	// _ = "foo"[0]
	// or:
	// if wordsize==64 {...}

	// All other non-Instruction Values can be found via their
	// enclosing Function or Package.
	}

	func (s *sanity) checkFinalInstr(instr Instruction) {
	switch instr := instr.(type) {
	case *If:
	if nsuccs := len(s.block.Succs); nsuccs != 2 {
	s.errorf("If-terminated block has %d successors; expected 2", nsuccs)
	return
	}
	if s.block.Succs[0] == s.block.Succs[1] {
	s.errorf("If-instruction has same True, False target blocks: %s", s.block.Succs[0])
	return
	}

	case *Jump:
	if nsuccs := len(s.block.Succs); nsuccs != 1 {
	s.errorf("Jump-terminated block has %d successors; expected 1", nsuccs)
	return
	}

	case *Return:
	if nsuccs := len(s.block.Succs); nsuccs != 0 {
	s.errorf("Return-terminated block has %d successors; expected none", nsuccs)
	return
	}
	if na, nf := len(instr.Results), s.fn.Signature.Results().Len(); nf != na {
	s.errorf("%d-ary return in %d-ary function", na, nf)
	}

	case *Panic:
	if nsuccs := len(s.block.Succs); nsuccs != 0 {
	s.errorf("Panic-terminated block has %d successors; expected none", nsuccs)
	return
	}

	default:
	s.errorf("non-control flow instruction at end of block")
	}
	}

	func (s sanity) checkBlock(b BasicBlock, index int) {
	s.block = b

	if b.Index != index {
	s.errorf("block has incorrect Index %d", b.Index)
	}
	if b.parent != s.fn {
	s.errorf("block has incorrect parent %s", b.parent)
	}

	// Check all blocks are reachable.
	// (The entry block is always implicitly reachable,
	// as is the Recover block, if any.)
	if (index > 0 && b != b.parent.Recover) && len(b.Preds) == 0 {
	s.warnf("unreachable block")
	if b.Instrs == nil {
	// Since this block is about to be pruned,
	// tolerating transient problems in it
	// simplifies other optimizations.
	return
	}
	}

	// Check predecessor and successor relations are dual,
	// and that all blocks in CFG belong to same function.
	for _, a := range b.Preds {
	found := false
	for _, bb := range a.Succs {
	if bb == b {
	found = true
	break
	}
	}
	if !found {
	s.errorf("expected successor edge in predecessor %s; found only: %s", a, a.Succs)
	}
	if a.parent != s.fn {
	s.errorf("predecessor %s belongs to different function %s", a, a.parent)
	}
	}
	for _, c := range b.Succs {
	found := false
	for _, bb := range c.Preds {
	if bb == b {
	found = true
	break
	}
	}
	if !found {
	s.errorf("expected predecessor edge in successor %s; found only: %s", c, c.Preds)
	}
	if c.parent != s.fn {
	s.errorf("successor %s belongs to different function %s", c, c.parent)
	}
	}

	// Check each instruction is sane.
	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)
	continue
	}
	if b2 := instr.Block(); b2 == nil {
	s.errorf("nil Block() for instruction at index %d", j)
	continue
	} else if b2 != b {
	s.errorf("wrong Block() (%s) for instruction at index %d ", b2, j)
	continue
	}
	if j < n-1 {
	s.checkInstr(j, instr)
	} else {
	s.checkFinalInstr(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 "untyped" types only appear on constant operands.
	if _, ok := (op).(Const); !ok {
	if basic, ok := (op).Type().(types.Basic); ok {
	if basic.Info()&types.IsUntyped != 0 {
	s.errorf("operand #%d of %s is untyped: %s", i, instr, basic)
	}
	}
	}

	// 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.Block() == nil {
	s.errorf("operand %d of %s is an instruction (%s) that belongs to no block", i, instr, val)
	} else 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.parent == nil {
	continue // only anon functions are local
	}
	}

	// TODO(adonovan): check val.Parent() != nil <=> val.Referrers() is defined.

	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)
	}
	}
	}
	}

	func (s *sanity) checkReferrerList(v Value) {
	refs := v.Referrers()
	if refs == nil {
	s.errorf("%s has missing referrer list", v.Name())
	return
	}
	for i, ref := range *refs {
	if _, ok := s.instrs[ref]; !ok {
	s.errorf("%s.Referrers()[%d] = %s is not an instruction belonging to this function", v.Name(), i, ref)
	}
	}
	}

	func (s sanity) checkFunction(fn Function) bool {
	// TODO(adonovan): check Function invariants:
	// - check params match signature
	// - check transient fields are nil
	// - warn if any fn.Locals do not appear among block instructions.
	s.fn = fn
	if fn.Prog == nil {
	s.errorf("nil Prog")
	}

	fn.String() // must not crash
	fn.RelString(fn.pkg()) // must not crash

	// All functions have a package, except delegates (which are
	// shared across packages, or duplicated as weak symbols in a
	// separate-compilation model), and error.Error.
	if fn.Pkg == nil {
	if strings.HasPrefix(fn.Synthetic, "wrapper ") \|\|
	strings.HasPrefix(fn.Synthetic, "bound ") \|\|
	strings.HasPrefix(fn.Synthetic, "thunk ") \|\|
	strings.HasSuffix(fn.name, "Error") {
	// ok
	} else {
	s.errorf("nil Pkg")
	}
	}
	if src, syn := fn.Synthetic == "", fn.Syntax() != nil; src != syn {
	s.errorf("got fromSource=%t, hasSyntax=%t; want same values", src, syn)
	}
	for i, l := range fn.Locals {
	if l.Parent() != fn {
	s.errorf("Local %s at index %d has wrong parent", l.Name(), i)
	}
	if l.Heap {
	s.errorf("Local %s at index %d has Heap flag set", l.Name(), i)
	}
	}
	// Build the set of valid referrers.
	s.instrs = make(map[Instruction]struct{})
	for _, b := range fn.Blocks {
	for _, instr := range b.Instrs {
	s.instrs[instr] = struct{}{}
	}
	}
	for i, p := range fn.Params {
	if p.Parent() != fn {
	s.errorf("Param %s at index %d has wrong parent", p.Name(), i)
	}
	s.checkReferrerList(p)
	}
	for i, fv := range fn.FreeVars {
	if fv.Parent() != fn {
	s.errorf("FreeVar %s at index %d has wrong parent", fv.Name(), i)
	}
	s.checkReferrerList(fv)
	}

	if fn.Blocks != nil && len(fn.Blocks) == 0 {
	// Function _had_ blocks (so it's not external) but
	// they were "optimized" away, even the entry block.
	s.errorf("Blocks slice is non-nil but empty")
	}
	for i, b := range fn.Blocks {
	if b == nil {
	s.warnf("nil *BasicBlock at f.Blocks[%d]", i)
	continue
	}
	s.checkBlock(b, i)
	}
	if fn.Recover != nil && fn.Blocks[fn.Recover.Index] != fn.Recover {
	s.errorf("Recover block is not in Blocks slice")
	}

	s.block = nil
	for i, anon := range fn.AnonFuncs {
	if anon.Parent() != fn {
	s.errorf("AnonFuncs[%d]=%s but %s.Parent()=%s", i, anon, anon, anon.Parent())
	}
	}
	s.fn = nil
	return !s.insane
	}

	// sanityCheckPackage checks invariants of packages upon creation.
	// It does not require that the package is built.
	// Unlike sanityCheck (for functions), it just panics at the first error.
	func sanityCheckPackage(pkg *Package) {
	if pkg.Pkg == nil {
	panic(fmt.Sprintf("Package %s has no Object", pkg))
	}
	pkg.String() // must not crash

	for name, mem := range pkg.Members {
	if name != mem.Name() {
	panic(fmt.Sprintf("%s: %T.Name() = %s, want %s",
	pkg.Pkg.Path(), mem, mem.Name(), name))
	}
	obj := mem.Object()
	if obj == nil {
	// This check is sound because fields
	// {Global,Function}.object have type
	// types.Object. (If they were declared as
	// *types.{Var,Func}, we'd have a non-empty
	// interface containing a nil pointer.)

	continue // not all members have typechecker objects
	}
	if obj.Name() != name {
	if obj.Name() == "init" && strings.HasPrefix(mem.Name(), "init#") {
	// Ok. The name of a declared init function varies between
	// its types.Func ("init") and its ssa.Function ("init#%d").
	} else {
	panic(fmt.Sprintf("%s: %T.Object().Name() = %s, want %s",
	pkg.Pkg.Path(), mem, obj.Name(), name))
	}
	}
	if obj.Pos() != mem.Pos() {
	panic(fmt.Sprintf("%s Pos=%d obj.Pos=%d", mem, mem.Pos(), obj.Pos()))
	}
	}
	}