func (f *Function) removeNilBlocks() {
	j := 0
	for _, b := range f.Blocks {
		if b != nil {
			b.Index = j
			f.Blocks[j] = b
			j++
		}
	}
	for i := j; i < len(f.Blocks); i++ {
		f.Blocks[i] = nil
	}
	f.Blocks = f.Blocks[:j]
}
removeNilBlocks eliminates nils from f.Blocks and updates each
BasicBlock.Index.  Use this after any pass that may delete blocks.
package ssa
import (
	"bytes"
	"fmt"
	"go/ast"
	"go/token"
	"go/types"
	"io"
	"os"
	"strings"
)
func addEdge(from, to *BasicBlock) {
	from.Succs = append(from.Succs, to)
	to.Preds = append(to.Preds, from)
}
func (b *BasicBlock) Parent() *Function { return b.parent }
func (b *BasicBlock) String() string {
	return fmt.Sprintf("%d", b.Index)
}
func (b *BasicBlock) emit(i Instruction) Value {
	i.setBlock(b)
	b.Instrs = append(b.Instrs, i)
	v, _ := i.(Value)
	return v
}
func (b *BasicBlock) predIndex(c *BasicBlock) int {
	for i, pred := range b.Preds {
		if pred == c {
			return i
		}
	}
	panic(fmt.Sprintf("no edge %s -> %s", c, b))
}
func (b *BasicBlock) hasPhi() bool {
	_, ok := b.Instrs[0].(*Phi)
	return ok
}
func (b *BasicBlock) phis() []Instruction {
	for i, instr := range b.Instrs {
		if _, ok := instr.(*Phi); !ok {
			return b.Instrs[:i]
		}
	}
	return nil  
}
func (b *BasicBlock) replacePred(p, q *BasicBlock) {
	for i, pred := range b.Preds {
		if pred == p {
			b.Preds[i] = q
		}
	}
}
func (b *BasicBlock) replaceSucc(p, q *BasicBlock) {
	for i, succ := range b.Succs {
		if succ == p {
			b.Succs[i] = q
		}
	}
}
func (b *BasicBlock) removePred(p *BasicBlock) {
	phis := b.phis()
	j := 0
	for i, pred := range b.Preds {
		if pred != p {
			b.Preds[j] = b.Preds[i]
			for _, instr := range phis {
				phi := instr.(*Phi)
				phi.Edges[j] = phi.Edges[i]
			}
			j++
		}
	}
	for i := j; i < len(b.Preds); i++ {
		b.Preds[i] = nil
		for _, instr := range phis {
			instr.(*Phi).Edges[i] = nil
		}
	}
	b.Preds = b.Preds[:j]
	for _, instr := range phis {
		phi := instr.(*Phi)
		phi.Edges = phi.Edges[:j]
	}
}
type targets struct {
	tail         *targets  
	_break       *BasicBlock
	_continue    *BasicBlock
	_fallthrough *BasicBlock
}
type lblock struct {
	_goto     *BasicBlock
	_break    *BasicBlock
	_continue *BasicBlock
}
func (f *Function) labelledBlock(label *ast.Ident) *lblock {
	lb := f.lblocks[label.Obj]
	if lb == nil {
		lb = &lblock{_goto: f.newBasicBlock(label.Name)}
		if f.lblocks == nil {
			f.lblocks = make(map[*ast.Object]*lblock)
		}
		f.lblocks[label.Obj] = lb
	}
	return lb
}
func (f *Function) addParam(name string, typ types.Type, pos token.Pos) *Parameter {
	v := &Parameter{
		name:   name,
		typ:    typ,
		pos:    pos,
		parent: f,
	}
	f.Params = append(f.Params, v)
	return v
}
func (f *Function) addParamObj(obj types.Object) *Parameter {
	name := obj.Name()
	if name == "" {
		name = fmt.Sprintf("arg%d", len(f.Params))
	}
	param := f.addParam(name, obj.Type(), obj.Pos())
	param.object = obj
	return param
}
func (f *Function) addSpilledParam(obj types.Object) {
	param := f.addParamObj(obj)
	spill := &Alloc{Comment: obj.Name()}
	spill.setType(types.NewPointer(obj.Type()))
	spill.setPos(obj.Pos())
	f.objects[obj] = spill
	f.Locals = append(f.Locals, spill)
	f.emit(spill)
	f.emit(&Store{Addr: spill, Val: param})
}
func (f *Function) startBody() {
	f.currentBlock = f.newBasicBlock("entry")
	f.objects = make(map[types.Object]Value)  
}
func (f *Function) createSyntacticParams(recv *ast.FieldList, functype *ast.FuncType) {
	if recv != nil {
		for _, field := range recv.List {
			for _, n := range field.Names {
				f.addSpilledParam(f.Pkg.info.Defs[n])
			}
			if field.Names == nil {
				f.addParamObj(f.Signature.Recv())
			}
		}
	}
	if functype.Params != nil {
		n := len(f.Params)  
		for _, field := range functype.Params.List {
			for _, n := range field.Names {
				f.addSpilledParam(f.Pkg.info.Defs[n])
			}
			if field.Names == nil {
				f.addParamObj(f.Signature.Params().At(len(f.Params) - n))
			}
		}
	}
	if functype.Results != nil {
		for _, field := range functype.Results.List {
			for _, n := range field.Names {
				f.namedResults = append(f.namedResults, f.addLocalForIdent(n))
			}
		}
	}
}
func numberRegisters(f *Function) {
	v := 0
	for _, b := range f.Blocks {
		for _, instr := range b.Instrs {
			switch instr.(type) {
			case Value:
				instr.(interface {
					setNum(int)
				}).setNum(v)
				v++
			}
		}
	}
}
func buildReferrers(f *Function) {
	var rands []*Value
	for _, b := range f.Blocks {
		for _, instr := range b.Instrs {
			rands = instr.Operands(rands[:0])  
			for _, rand := range rands {
				if r := *rand; r != nil {
					if ref := r.Referrers(); ref != nil {
						*ref = append(*ref, instr)
					}
				}
			}
		}
	}
}
func (f *Function) finishBody() {
	f.objects = nil
	f.currentBlock = nil
	f.lblocks = nil
	if n := f.syntax; n != nil && !f.debugInfo() {
		f.syntax = extentNode{n.Pos(), n.End()}
	}
	j := 0
	for _, l := range f.Locals {
		if !l.Heap {
			f.Locals[j] = l
			j++
		}
	}
	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 {
		lift(f)
	}
	f.namedResults = nil  
	numberRegisters(f)
	if f.Prog.mode&PrintFunctions != 0 {
		printMu.Lock()
		f.WriteTo(os.Stdout)
		printMu.Unlock()
	}
	if f.Prog.mode&SanityCheckFunctions != 0 {
		mustSanityCheck(f, nil)
	}
}