v.Args[0], v.Args[1] = v.Args[1], v.Args[0]
		}
		v.AddArg(dom.Controls[0])
	}

	// Put all of the instructions into 'dom'
	// and update the CFG appropriately.
	dom.Kind = post.Kind
	dom.CopyControls(post)
	dom.Aux = post.Aux
	dom.Succs = append(dom.Succs[:0], post.Succs...)
	for i := range dom.Succs {
		e := dom.Succs[i]
		e.b.Preds[e.i].b = dom
	}

func (b *Block) ReplaceControl(i int, v *Value) {
	b.Controls[i].Uses--
	b.Controls[i] = v
	v.Uses++
}

// CopyControls replaces the controls for this block with those from the
// provided block. The provided block is not modified.
func (b *Block) CopyControls(from *Block) {
	if b == from {
		return
	}
	b.ResetControls()
	for _, c := range from.ControlValues() {
		b.AddControl(c)
	}
}

		}

		// Build branch point.
		bThen := f.NewBlock(BlockPlain)
		bEnd := f.NewBlock(b.Kind)
		bThen.Pos = pos
		bEnd.Pos = b.Pos
		b.Pos = pos

		// Set up control flow for end block.
		bEnd.CopyControls(b)
		bEnd.Likely = b.Likely
		for _, e := range b.Succs {
			bEnd.Succs = append(bEnd.Succs, e)
			e.b.Preds[e.i].b = bEnd
		}

		// set up control flow for write barrier test