func (b *Block) AddControl(v *Value) {
	i := b.NumControls()
	b.Controls[i] = v // panics if array is full
	v.Uses++
}

// ReplaceControl exchanges the existing control value at the index provided
// for the new value. The index must refer to a valid control value.
func (b *Block) ReplaceControl(i int, v *Value) {
	b.Controls[i].Uses--
	b.Controls[i] = v
	v.Uses++
}

	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 {
			if v.Op == OpCopy {
				values[i] = v.Args[0]

				if a.Op == OpSPanchored || opcodeTable[a.Op].nilCheck {
					v.SetArg(i, a.Args[0])
				}
			}
		}
		for i, c := range b.ControlValues() {
			if c.Op == OpSPanchored || opcodeTable[c.Op].nilCheck {
				b.ReplaceControl(i, c.Args[0])
			}
		}
	}
	for _, b := range f.Blocks {
		i := 0
		for _, v := range b.Values {
			if v.Op == OpSPanchored {
				// Free this value
				if v.Uses != 0 {

				b0 = new(Block)
				*b0 = *b
				b0.Succs = append([]Edge{}, b.Succs...) // make a new copy, not aliasing
			}
			for i, c := range b.ControlValues() {
				for c.Op == OpCopy {
					c = c.Args[0]
					b.ReplaceControl(i, c)
				}
			}
			if rb(b) {
				change = true
				if debug > 1 {
					fmt.Printf("rewriting %s  ->  %s\n", b0.LongString(), b.LongString())
				}
			}
			for j, v := range b.Values {

			}
			// We assume that a control input can be passed in any
			// type-compatible register. If this turns out not to be true,
			// we'll need to introduce a regspec for a block's control value.
			b.ReplaceControl(i, s.allocValToReg(v, s.compatRegs(v.Type), false, b.Pos))
		}

		// Reduce the uses of the control values once registers have been loaded.
		// This loop is equivalent to the advanceUses method.