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.