// remove duplicate expressions.
func zcse(f *Func) {
	vals := make(map[vkey]*Value)

	for _, b := range f.Blocks {
		for i := 0; i < len(b.Values); i++ {
			v := b.Values[i]
			if opcodeTable[v.Op].argLen == 0 {
				key := vkey{v.Op, keyFor(v), v.Aux, v.Type}
				if vals[key] == nil {
					vals[key] = v
					if b != f.Entry {
						// Move v to the entry block so it will dominate every block

	// lowering and a subsequent dead code elimination (because lowering
	// rules may leave dead generic ops behind).
	for _, b := range f.Blocks {
		for _, v := range b.Values {
			if !opcodeTable[v.Op].generic {
				continue // lowered
			}
			switch v.Op {

				q = append(q, v)
				if v.Pos.IsStmt() != src.PosNotStmt {
					liveOrderStmts = append(liveOrderStmts, v)
				}
			}
		}
		for _, v := range b.Values {
			if (opcodeTable[v.Op].call || opcodeTable[v.Op].hasSideEffects || opcodeTable[v.Op].nilCheck) && !live[v.ID] {
				live[v.ID] = true
				q = append(q, v)
				if v.Pos.IsStmt() != src.PosNotStmt {
					liveOrderStmts = append(liveOrderStmts, v)
				}
			}

			v := b.Values[j]
			if v.Op == OpInvalid {
				continue
			}
			b.Values[i] = v
			i++
		}
		b.truncateValues(i)
	}
}

func (v *Value) clobbersFlags() bool {
	if opcodeTable[v.Op].clobberFlags {
		return true
	}
	if v.Type.IsTuple() && (v.Type.FieldType(0).IsFlags() || v.Type.FieldType(1).IsFlags()) {
		// This case handles the possibility where a flag value is generated but never used.

// There may be false positives.
func isEmpty(b *Block) bool {
	for _, v := range b.Values {
		if v.Uses > 0 || v.Op.IsCall() || v.Op.HasSideEffects() || v.Type.IsVoid() || opcodeTable[v.Op].nilCheck {
			return false
		}
	}
	return true
}

// fuseBlockPlain handles a run of blocks with length >= 2,
// whose interior has single predecessors and successors,

		// partition can grow in the loop. By not using a range loop here,
		// we process new additions as they arrive, avoiding O(n^2) behavior.
		for i := 0; i < len(partition); i++ {
			e := partition[i]

			if opcodeTable[e[0].Op].commutative {
				// Order the first two args before comparison.
				for _, v := range e {
					if valueEqClass[v.Args[0].ID] > valueEqClass[v.Args[1].ID] {
						v.Args[0], v.Args[1] = v.Args[1], v.Args[0]