for n := 0; n < 2; n++ {
		for _, b := range po {
			// Walk values backwards to figure out what flag
			// value we want in the flag register at the start
			// of the block.
			var flag *Value
			for _, c := range b.ControlValues() {
				if c.Type.IsFlags() {
					if flag != nil {
						panic("cannot have multiple controls using flags")
					}
					flag = c
				}
			}
			if flag == nil {
				flag = end[b.ID]
			}

		return 0
	}
	if b.Controls[1] == nil {
		return 1
	}
	return 2
}

// ControlValues returns a slice containing the non-nil control
// values of the block. The index of each control value will be
// the same as it is in the Controls property and can be used
// in ReplaceControl calls.
func (b *Block) ControlValues() []*Value {
	if b.Controls[0] == nil {
		return b.Controls[:0]
	}
	if b.Controls[1] == nil {

func copyelim(f *Func) {
	phielim(f)

	// loop of copyelimValue(v) process has been done in phielim() pass.
	// Update block control values.
	for _, b := range f.Blocks {
		for i, v := range b.ControlValues() {
			if v.Op == OpCopy {
				b.ReplaceControl(i, v.Args[0])
			}
		}
	}

	// Update named values.
	for _, name := range f.Names {
		values := f.NamedValues[*name]
		for i, v := range values {

		for _, v := range b.Values {
			for i, a := range v.Args {
				if !valueMark[a.ID] {
					f.Fatalf("%v, arg %d of %s, is missing", a, i, v.LongString())
				}
			}
		}
		for _, c := range b.ControlValues() {
			if !valueMark[c.ID] {
				f.Fatalf("control value for %s is missing: %v", b, c)
			}
		}
	}
	for b := f.freeBlocks; b != nil; b = b.succstorage[0].b {
		if blockMark[b.ID] {

					if _, exist := t.defUse[arg]; !exist {
						t.defUse[arg] = make([]*Value, 0, arg.Uses)
					}
					t.defUse[arg] = append(t.defUse[arg], val)
				}
			}
		}
		for _, ctl := range block.ControlValues() {
			// for control values that can become constants, find their use blocks
			if possibleConst(ctl) {
				t.defBlock[ctl] = append(t.defBlock[ctl], block)
			}
		}
	}
}

				for _, a := range v.Args {
					if a.isFlagOp() {
						score[v.ID] = ScoreReadFlags
					}
				}
			}
		}
		for _, c := range b.ControlValues() {
			// Force the control values to be scheduled at the end,
			// unless they have other special priority.
			if c.Block != b || score[c.ID] < ScoreReadTuple {
				continue
			}

						use = b.Preds[i].b
					}
					if target[a.ID] == nil {
						target[a.ID] = use
					} else {
						target[a.ID] = lca.find(target[a.ID], use)
					}
				}
			}
			for _, c := range b.ControlValues() {
				if !canMove[c.ID] {
					continue
				}
				if target[c.ID] == nil {
					target[c.ID] = b
				} else {
					target[c.ID] = lca.find(target[c.ID], b)
				}
			}
		}

							v.Pos = v.Pos.WithIsStmt()
							w.Pos = w.Pos.WithNotStmt()
						} // TODO and if this fails?
					}
					v.SetArg(i, x)
					rewrites++
				}
			}
		}
		for i, v := range b.ControlValues() {
			if x := rewrite[v.ID]; x != nil {
				if v.Op == OpNilCheck {
					// nilcheck pass will remove the nil checks and log
					// them appropriately, so don't mess with them here.
					continue
				}

	// Calls are live (because callee can observe the memory state).
	for _, b := range f.Blocks {
		if !reachable[b.ID] {
			continue
		}
		for _, v := range b.ControlValues() {
			if !live[v.ID] {
				live[v.ID] = true
				q = append(q, v)
				if v.Pos.IsStmt() != src.PosNotStmt {
					liveOrderStmts = append(liveOrderStmts, v)
				}
			}
		}

		changed := false
		for _, b := range f.Blocks {
			for _, v := range b.Values {
				changed = visit(v) || changed
			}
			// keep the auto if its address reaches a control value
			for _, c := range b.ControlValues() {
				if n, ok := addr[c]; ok && !used.Has(n) {
					used.Add(n)
					changed = true
				}
			}
		}
		if !changed {
			break
		}
	}

	// Eliminate stores to unread autos.