// in which the outbound paths from b merge,
	// with no other preds joining them.
	// In these cases, we can reconstruct what the value
	// of any phi in b must be in the successor blocks.

	if len(t.Preds) == 1 && len(t.Succs) == 1 &&
		len(u.Preds) == 1 && len(u.Succs) == 1 &&
		t.Succs[0].b == u.Succs[0].b && len(t.Succs[0].b.Preds) == 2 {
		// p   q
		//  \ /
		//   b
		//  / \
		// t   u
		//  \ /

// phi values by calling b.removePhiArg(v, i).
func (b *Block) removePred(i int) {
	n := len(b.Preds) - 1
	if i != n {
		e := b.Preds[n]
		b.Preds[i] = e
		// Update the other end of the edge we moved.
		e.b.Succs[e.i].i = i
	}
	b.Preds[n] = Edge{}
	b.Preds = b.Preds[:n]
	b.Func.invalidateCFG()
}

// removeSucc removes the ith output edge from b.
// It is the responsibility of the caller to remove

		if b.Func != f {
			f.Fatalf("%s.Func=%s, want %s", b, b.Func.Name, f.Name)
		}

		for i, e := range b.Preds {
			if se := e.b.Succs[e.i]; se.b != b || se.i != i {
				f.Fatalf("block pred/succ not crosslinked correctly %d:%s %d:%s", i, b, se.i, se.b)
			}
		}
		for i, e := range b.Succs {
			if pe := e.b.Preds[e.i]; pe.b != b || pe.i != i {
				f.Fatalf("block succ/pred not crosslinked correctly %d:%s %d:%s", i, b, pe.i, pe.b)
			}

	f.invalidateCFG()
	return true
}

// is this a BlockPlain with one predecessor?
func isLeafPlain(b *Block) bool {
	return b.Kind == BlockPlain && len(b.Preds) == 1
}

func clobberBlock(b *Block) {
	b.Values = nil
	b.Preds = nil
	b.Succs = nil
	b.Aux = nil
	b.ResetControls()
	b.Likely = BranchUnknown
	b.Kind = BlockInvalid
}

		mem1 := sched.NewValue1I(bb.Pos, OpSelectN, types.TypeMem, 0, call)
		sched.AddEdgeTo(h)
		headerMemPhi.AddArg(mem1)

		bb.Succs[p.i] = Edge{test, 0}
		test.Preds = append(test.Preds, Edge{bb, p.i})

		// Must correct all the other phi functions in the header for new incoming edge.
		// Except for mem phis, it will be the same value seen on the original
		// backedge at index i.

		for _, v := range b.Values {
			if v.Op != ssa.OpFwdRef {
				continue
			}
			var_ := v.Aux.(fwdRefAux).N

			// Optimization: look back 1 block for the definition.
			if len(b.Preds) == 1 {
				c := b.Preds[0].Block()
				if w := s.defvars[c.ID][var_]; w != nil {
					v.Op = ssa.OpCopy
					v.Aux = nil
					v.AddArg(w)
					continue
				}
			}

			if _, ok := s.varnum[var_]; ok {

				return false
			}
			for i := range fb.Succs {
				if !checkBlk(fb.Succs[i].b, gb.Succs[i].b) {
					return false
				}
			}
			if len(fb.Preds) != len(gb.Preds) {
				return false
			}
			for i := range fb.Preds {
				if !checkBlk(fb.Preds[i].b, gb.Preds[i].b) {
					return false
				}
			}
			return true

		}
		return blkcor[fb] == gb && blkcor[gb] == fb
	}

				decisionBlock = c
			} else if len(c.Preds) == 1 && len(d.Preds) == 1 && c.Preds[0].Block() == d.Preds[0].Block() {
				decisionBlock = c.Preds[0].Block()
			} else {
				lv.f.Fatalf("can't find write barrier pattern %v", v)
			}
			if len(decisionBlock.Succs) != 2 {
				lv.f.Fatalf("common predecessor block the wrong type %s", decisionBlock.Kind)
			}

			// Flow backwards from the control value to find the

		node := work[len(work)-1]
		work = work[:len(work)-1]

		switch node.op {
		case Work:
			b := node.block

			// First, see if we're dominated by an explicit nil check.
			if len(b.Preds) == 1 {
				p := b.Preds[0].b
				if p.Kind == BlockIf && p.Controls[0].Op == OpIsNonNil && p.Succs[0].b == b {
					if ptr := p.Controls[0].Args[0]; nonNilValues[ptr.ID] == nil {
						nonNilValues[ptr.ID] = ptr

//		do something
//	      nxt = inc + ind
//		goto loop
//
//	 exit_loop:
func findIndVar(f *Func) []indVar {
	var iv []indVar
	sdom := f.Sdom()

	for _, b := range f.Blocks {
		if b.Kind != BlockIf || len(b.Preds) != 2 {
			continue
		}

		var ind *Value   // induction variable
		var init *Value  // starting value
		var limit *Value // ending value

		// Check that the control if it either ind </<= limit or limit </<= ind.