src/cmd/compile/internal/ssa/stackalloc.go - go - Git at Google

 // Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // TODO: live at start of block instead?

 package ssa

 import "fmt"

 type stackAllocState struct {
 	f *Func

 	// live is the output of stackalloc.
 	// live[b.id] = live values at the end of block b.
 	live [][]ID

 	// The following slices are reused across multiple users
 	// of stackAllocState.
 	values    []stackValState
 	interfere [][]ID // interfere[v.id] = values that interfere with v.
 	names     []LocalSlot
 	slots     []int
 	used      []bool

 	nArgSlot, // Number of Values sourced to arg slot
 	nNotNeed, // Number of Values not needing a stack slot
 	nNamedSlot, // Number of Values using a named stack slot
 	nReuse, // Number of values reusing a stack slot
 	nAuto, // Number of autos allocated for stack slots.
 	nSelfInterfere int32 // Number of self-interferences
 }

 func newStackAllocState(f *Func) *stackAllocState {
 	s := f.Config.stackAllocState
 	if s == nil {
 		return new(stackAllocState)
 	}
 	if s.f != nil {
 		f.Config.Fatalf(0, "newStackAllocState called without previous free")
 	}
 	return s
 }

 func putStackAllocState(s *stackAllocState) {
 	for i := range s.values {
 		s.values[i] = stackValState{}
 	}
 	for i := range s.interfere {
 		s.interfere[i] = nil
 	}
 	for i := range s.names {
 		s.names[i] = LocalSlot{}
 	}
 	for i := range s.slots {
 		s.slots[i] = 0
 	}
 	for i := range s.used {
 		s.used[i] = false
 	}
 	s.f.Config.stackAllocState = s
 	s.f = nil
 	s.live = nil
 	s.nArgSlot, s.nNotNeed, s.nNamedSlot, s.nReuse, s.nAuto, s.nSelfInterfere = 0, 0, 0, 0, 0, 0
 }

 type stackValState struct {
 	typ      Type
 	spill    *Value
 	needSlot bool
 }

 // stackalloc allocates storage in the stack frame for
 // all Values that did not get a register.
 // Returns a map from block ID to the stack values live at the end of that block.
 func stackalloc(f *Func, spillLive [][]ID) [][]ID {
 	if f.pass.debug > stackDebug {
 		fmt.Println("before stackalloc")
 		fmt.Println(f.String())
 	}
 	s := newStackAllocState(f)
 	s.init(f, spillLive)
 	defer putStackAllocState(s)

 	s.stackalloc()
 	if f.pass.stats > 0 {
 		f.LogStat("stack_alloc_stats",
 			s.nArgSlot, "arg_slots", s.nNotNeed, "slot_not_needed",
 			s.nNamedSlot, "named_slots", s.nAuto, "auto_slots",
 			s.nReuse, "reused_slots", s.nSelfInterfere, "self_interfering")
 	}

 	return s.live
 }

 func (s *stackAllocState) init(f *Func, spillLive [][]ID) {
 	s.f = f

 	// Initialize value information.
 	if n := f.NumValues(); cap(s.values) >= n {
 		s.values = s.values[:n]
 	} else {
 		s.values = make([]stackValState, n)
 	}
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			s.values[v.ID].typ = v.Type
 			s.values[v.ID].needSlot = !v.Type.IsMemory() && !v.Type.IsVoid() && !v.Type.IsFlags() && f.getHome(v.ID) == nil && !v.rematerializeable()
 			if f.pass.debug > stackDebug && s.values[v.ID].needSlot {
 				fmt.Printf("%s needs a stack slot\n", v)
 			}
 			if v.Op == OpStoreReg {
 				s.values[v.Args[0].ID].spill = v
 			}
 		}
 	}

 	// Compute liveness info for values needing a slot.
 	s.computeLive(spillLive)

 	// Build interference graph among values needing a slot.
 	s.buildInterferenceGraph()
 }

 func (s *stackAllocState) stackalloc() {
 	f := s.f

 	// Build map from values to their names, if any.
 	// A value may be associated with more than one name (e.g. after
 	// the assignment i=j). This step picks one name per value arbitrarily.
 	if n := f.NumValues(); cap(s.names) >= n {
 		s.names = s.names[:n]
 	} else {
 		s.names = make([]LocalSlot, n)
 	}
 	names := s.names
 	for _, name := range f.Names {
 		// Note: not "range f.NamedValues" above, because
 		// that would be nondeterministic.
 		for _, v := range f.NamedValues[name] {
 			names[v.ID] = name
 		}
 	}

 	// Allocate args to their assigned locations.
 	for _, v := range f.Entry.Values {
 		if v.Op != OpArg {
 			continue
 		}
 		loc := LocalSlot{v.Aux.(GCNode), v.Type, v.AuxInt}
 		if f.pass.debug > stackDebug {
 			fmt.Printf("stackalloc %s to %s\n", v, loc.Name())
 		}
 		f.setHome(v, loc)
 	}

 	// For each type, we keep track of all the stack slots we
 	// have allocated for that type.
 	// TODO: share slots among equivalent types. We would need to
 	// only share among types with the same GC signature. See the
 	// type.Equal calls below for where this matters.
 	locations := map[Type][]LocalSlot{}

 	// Each time we assign a stack slot to a value v, we remember
 	// the slot we used via an index into locations[v.Type].
 	slots := s.slots
 	if n := f.NumValues(); cap(slots) >= n {
 		slots = slots[:n]
 	} else {
 		slots = make([]int, n)
 		s.slots = slots
 	}
 	for i := range slots {
 		slots[i] = -1
 	}

 	// Pick a stack slot for each value needing one.
 	var used []bool
 	if n := f.NumValues(); cap(s.used) >= n {
 		used = s.used[:n]
 	} else {
 		used = make([]bool, n)
 		s.used = used
 	}
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if !s.values[v.ID].needSlot {
 				s.nNotNeed++
 				continue
 			}
 			if v.Op == OpArg {
 				s.nArgSlot++
 				continue // already picked
 			}

 			// If this is a named value, try to use the name as
 			// the spill location.
 			var name LocalSlot
 			if v.Op == OpStoreReg {
 				name = names[v.Args[0].ID]
 			} else {
 				name = names[v.ID]
 			}
 			if name.N != nil && v.Type.Compare(name.Type) == CMPeq {
 				for _, id := range s.interfere[v.ID] {
 					h := f.getHome(id)
 					if h != nil && h.(LocalSlot).N == name.N && h.(LocalSlot).Off == name.Off {
 						// A variable can interfere with itself.
 						// It is rare, but but it can happen.
 						s.nSelfInterfere++
 						goto noname
 					}
 				}
 				if f.pass.debug > stackDebug {
 					fmt.Printf("stackalloc %s to %s\n", v, name.Name())
 				}
 				s.nNamedSlot++
 				f.setHome(v, name)
 				continue
 			}

 		noname:
 			// Set of stack slots we could reuse.
 			locs := locations[v.Type]
 			// Mark all positions in locs used by interfering values.
 			for i := 0; i < len(locs); i++ {
 				used[i] = false
 			}
 			for _, xid := range s.interfere[v.ID] {
 				slot := slots[xid]
 				if slot >= 0 {
 					used[slot] = true
 				}
 			}
 			// Find an unused stack slot.
 			var i int
 			for i = 0; i < len(locs); i++ {
 				if !used[i] {
 					s.nReuse++
 					break
 				}
 			}
 			// If there is no unused stack slot, allocate a new one.
 			if i == len(locs) {
 				s.nAuto++
 				locs = append(locs, LocalSlot{N: f.Config.fe.Auto(v.Type), Type: v.Type, Off: 0})
 				locations[v.Type] = locs
 			}
 			// Use the stack variable at that index for v.
 			loc := locs[i]
 			if f.pass.debug > stackDebug {
 				fmt.Printf("stackalloc %s to %s\n", v, loc.Name())
 			}
 			f.setHome(v, loc)
 			slots[v.ID] = i
 		}
 	}
 }

 // computeLive computes a map from block ID to a list of
 // stack-slot-needing value IDs live at the end of that block.
 // TODO: this could be quadratic if lots of variables are live across lots of
 // basic blocks. Figure out a way to make this function (or, more precisely, the user
 // of this function) require only linear size & time.
 func (s *stackAllocState) computeLive(spillLive [][]ID) {
 	s.live = make([][]ID, s.f.NumBlocks())
 	var phis []*Value
 	live := s.f.newSparseSet(s.f.NumValues())
 	defer s.f.retSparseSet(live)
 	t := s.f.newSparseSet(s.f.NumValues())
 	defer s.f.retSparseSet(t)

 	// Instead of iterating over f.Blocks, iterate over their postordering.
 	// Liveness information flows backward, so starting at the end
 	// increases the probability that we will stabilize quickly.
 	po := s.f.postorder()
 	for {
 		changed := false
 		for _, b := range po {
 			// Start with known live values at the end of the block
 			live.clear()
 			live.addAll(s.live[b.ID])

 			// Propagate backwards to the start of the block
 			phis = phis[:0]
 			for i := len(b.Values) - 1; i >= 0; i-- {
 				v := b.Values[i]
 				live.remove(v.ID)
 				if v.Op == OpPhi {
 					// Save phi for later.
 					// Note: its args might need a stack slot even though
 					// the phi itself doesn't. So don't use needSlot.
 					if !v.Type.IsMemory() && !v.Type.IsVoid() {
 						phis = append(phis, v)
 					}
 					continue
 				}
 				for _, a := range v.Args {
 					if s.values[a.ID].needSlot {
 						live.add(a.ID)
 					}
 				}
 			}

 			// for each predecessor of b, expand its list of live-at-end values
 			// invariant: s contains the values live at the start of b (excluding phi inputs)
 			for i, e := range b.Preds {
 				p := e.b
 				t.clear()
 				t.addAll(s.live[p.ID])
 				t.addAll(live.contents())
 				t.addAll(spillLive[p.ID])
 				for _, v := range phis {
 					a := v.Args[i]
 					if s.values[a.ID].needSlot {
 						t.add(a.ID)
 					}
 					if spill := s.values[a.ID].spill; spill != nil {
 						//TODO: remove?  Subsumed by SpillUse?
 						t.add(spill.ID)
 					}
 				}
 				if t.size() == len(s.live[p.ID]) {
 					continue
 				}
 				// grow p's live set
 				s.live[p.ID] = append(s.live[p.ID][:0], t.contents()...)
 				changed = true
 			}
 		}

 		if !changed {
 			break
 		}
 	}
 	if s.f.pass.debug > stackDebug {
 		for _, b := range s.f.Blocks {
 			fmt.Printf("stacklive %s %v\n", b, s.live[b.ID])
 		}
 	}
 }

 func (f *Func) getHome(vid ID) Location {
 	if int(vid) >= len(f.RegAlloc) {
 		return nil
 	}
 	return f.RegAlloc[vid]
 }

 func (f *Func) setHome(v *Value, loc Location) {
 	for v.ID >= ID(len(f.RegAlloc)) {
 		f.RegAlloc = append(f.RegAlloc, nil)
 	}
 	f.RegAlloc[v.ID] = loc
 }

 func (s *stackAllocState) buildInterferenceGraph() {
 	f := s.f
 	if n := f.NumValues(); cap(s.interfere) >= n {
 		s.interfere = s.interfere[:n]
 	} else {
 		s.interfere = make([][]ID, n)
 	}
 	live := f.newSparseSet(f.NumValues())
 	defer f.retSparseSet(live)
 	for _, b := range f.Blocks {
 		// Propagate liveness backwards to the start of the block.
 		// Two values interfere if one is defined while the other is live.
 		live.clear()
 		live.addAll(s.live[b.ID])
 		for i := len(b.Values) - 1; i >= 0; i-- {
 			v := b.Values[i]
 			if s.values[v.ID].needSlot {
 				live.remove(v.ID)
 				for _, id := range live.contents() {
 					if s.values[v.ID].typ.Compare(s.values[id].typ) == CMPeq {
 						s.interfere[v.ID] = append(s.interfere[v.ID], id)
 						s.interfere[id] = append(s.interfere[id], v.ID)
 					}
 				}
 			}
 			for _, a := range v.Args {
 				if s.values[a.ID].needSlot {
 					live.add(a.ID)
 				}
 			}
 			if v.Op == OpArg && s.values[v.ID].needSlot {
 				// OpArg is an input argument which is pre-spilled.
 				// We add back v.ID here because we want this value
 				// to appear live even before this point. Being live
 				// all the way to the start of the entry block prevents other
 				// values from being allocated to the same slot and clobbering
 				// the input value before we have a chance to load it.
 				live.add(v.ID)
 			}
 		}
 	}
 	if f.pass.debug > stackDebug {
 		for vid, i := range s.interfere {
 			if len(i) > 0 {
 				fmt.Printf("v%d interferes with", vid)
 				for _, x := range i {
 					fmt.Printf(" v%d", x)
 				}
 				fmt.Println()
 			}
 		}
 	}
 }
	// Copyright 2015 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// TODO: live at start of block instead?

	package ssa

	import "fmt"

	type stackAllocState struct {
	f *Func

	// live is the output of stackalloc.
	// live[b.id] = live values at the end of block b.
	live [][]ID

	// The following slices are reused across multiple users
	// of stackAllocState.
	values []stackValState
	interfere [][]ID // interfere[v.id] = values that interfere with v.
	names []LocalSlot
	slots []int
	used []bool

	nArgSlot, // Number of Values sourced to arg slot
	nNotNeed, // Number of Values not needing a stack slot
	nNamedSlot, // Number of Values using a named stack slot
	nReuse, // Number of values reusing a stack slot
	nAuto, // Number of autos allocated for stack slots.
	nSelfInterfere int32 // Number of self-interferences
	}

	func newStackAllocState(f Func) stackAllocState {
	s := f.Config.stackAllocState
	if s == nil {
	return new(stackAllocState)
	}
	if s.f != nil {
	f.Config.Fatalf(0, "newStackAllocState called without previous free")
	}
	return s
	}

	func putStackAllocState(s *stackAllocState) {
	for i := range s.values {
	s.values[i] = stackValState{}
	}
	for i := range s.interfere {
	s.interfere[i] = nil
	}
	for i := range s.names {
	s.names[i] = LocalSlot{}
	}
	for i := range s.slots {
	s.slots[i] = 0
	}
	for i := range s.used {
	s.used[i] = false
	}
	s.f.Config.stackAllocState = s
	s.f = nil
	s.live = nil
	s.nArgSlot, s.nNotNeed, s.nNamedSlot, s.nReuse, s.nAuto, s.nSelfInterfere = 0, 0, 0, 0, 0, 0
	}

	type stackValState struct {
	typ Type
	spill *Value
	needSlot bool
	}

	// stackalloc allocates storage in the stack frame for
	// all Values that did not get a register.
	// Returns a map from block ID to the stack values live at the end of that block.
	func stackalloc(f *Func, spillLive [][]ID) [][]ID {
	if f.pass.debug > stackDebug {
	fmt.Println("before stackalloc")
	fmt.Println(f.String())
	}
	s := newStackAllocState(f)
	s.init(f, spillLive)
	defer putStackAllocState(s)

	s.stackalloc()
	if f.pass.stats > 0 {
	f.LogStat("stack_alloc_stats",
	s.nArgSlot, "arg_slots", s.nNotNeed, "slot_not_needed",
	s.nNamedSlot, "named_slots", s.nAuto, "auto_slots",
	s.nReuse, "reused_slots", s.nSelfInterfere, "self_interfering")
	}

	return s.live
	}

	func (s stackAllocState) init(f Func, spillLive [][]ID) {
	s.f = f

	// Initialize value information.
	if n := f.NumValues(); cap(s.values) >= n {
	s.values = s.values[:n]
	} else {
	s.values = make([]stackValState, n)
	}
	for _, b := range f.Blocks {
	for _, v := range b.Values {
	s.values[v.ID].typ = v.Type
	s.values[v.ID].needSlot = !v.Type.IsMemory() && !v.Type.IsVoid() && !v.Type.IsFlags() && f.getHome(v.ID) == nil && !v.rematerializeable()
	if f.pass.debug > stackDebug && s.values[v.ID].needSlot {
	fmt.Printf("%s needs a stack slot\n", v)
	}
	if v.Op == OpStoreReg {
	s.values[v.Args[0].ID].spill = v
	}
	}
	}

	// Compute liveness info for values needing a slot.
	s.computeLive(spillLive)

	// Build interference graph among values needing a slot.
	s.buildInterferenceGraph()
	}

	func (s *stackAllocState) stackalloc() {
	f := s.f

	// Build map from values to their names, if any.
	// A value may be associated with more than one name (e.g. after
	// the assignment i=j). This step picks one name per value arbitrarily.
	if n := f.NumValues(); cap(s.names) >= n {
	s.names = s.names[:n]
	} else {
	s.names = make([]LocalSlot, n)
	}
	names := s.names
	for _, name := range f.Names {
	// Note: not "range f.NamedValues" above, because
	// that would be nondeterministic.
	for _, v := range f.NamedValues[name] {
	names[v.ID] = name
	}
	}

	// Allocate args to their assigned locations.
	for _, v := range f.Entry.Values {
	if v.Op != OpArg {
	continue
	}
	loc := LocalSlot{v.Aux.(GCNode), v.Type, v.AuxInt}
	if f.pass.debug > stackDebug {
	fmt.Printf("stackalloc %s to %s\n", v, loc.Name())
	}
	f.setHome(v, loc)
	}

	// For each type, we keep track of all the stack slots we
	// have allocated for that type.
	// TODO: share slots among equivalent types. We would need to
	// only share among types with the same GC signature. See the
	// type.Equal calls below for where this matters.
	locations := map[Type][]LocalSlot{}

	// Each time we assign a stack slot to a value v, we remember
	// the slot we used via an index into locations[v.Type].
	slots := s.slots
	if n := f.NumValues(); cap(slots) >= n {
	slots = slots[:n]
	} else {
	slots = make([]int, n)
	s.slots = slots
	}
	for i := range slots {
	slots[i] = -1
	}

	// Pick a stack slot for each value needing one.
	var used []bool
	if n := f.NumValues(); cap(s.used) >= n {
	used = s.used[:n]
	} else {
	used = make([]bool, n)
	s.used = used
	}
	for _, b := range f.Blocks {
	for _, v := range b.Values {
	if !s.values[v.ID].needSlot {
	s.nNotNeed++
	continue
	}
	if v.Op == OpArg {
	s.nArgSlot++
	continue // already picked
	}

	// If this is a named value, try to use the name as
	// the spill location.
	var name LocalSlot
	if v.Op == OpStoreReg {
	name = names[v.Args[0].ID]
	} else {
	name = names[v.ID]
	}
	if name.N != nil && v.Type.Compare(name.Type) == CMPeq {
	for _, id := range s.interfere[v.ID] {
	h := f.getHome(id)
	if h != nil && h.(LocalSlot).N == name.N && h.(LocalSlot).Off == name.Off {
	// A variable can interfere with itself.
	// It is rare, but but it can happen.
	s.nSelfInterfere++
	goto noname
	}
	}
	if f.pass.debug > stackDebug {
	fmt.Printf("stackalloc %s to %s\n", v, name.Name())
	}
	s.nNamedSlot++
	f.setHome(v, name)
	continue
	}

	noname:
	// Set of stack slots we could reuse.
	locs := locations[v.Type]
	// Mark all positions in locs used by interfering values.
	for i := 0; i < len(locs); i++ {
	used[i] = false
	}
	for _, xid := range s.interfere[v.ID] {
	slot := slots[xid]
	if slot >= 0 {
	used[slot] = true
	}
	}
	// Find an unused stack slot.
	var i int
	for i = 0; i < len(locs); i++ {
	if !used[i] {
	s.nReuse++
	break
	}
	}
	// If there is no unused stack slot, allocate a new one.
	if i == len(locs) {
	s.nAuto++
	locs = append(locs, LocalSlot{N: f.Config.fe.Auto(v.Type), Type: v.Type, Off: 0})
	locations[v.Type] = locs
	}
	// Use the stack variable at that index for v.
	loc := locs[i]
	if f.pass.debug > stackDebug {
	fmt.Printf("stackalloc %s to %s\n", v, loc.Name())
	}
	f.setHome(v, loc)
	slots[v.ID] = i
	}
	}
	}

	// computeLive computes a map from block ID to a list of
	// stack-slot-needing value IDs live at the end of that block.
	// TODO: this could be quadratic if lots of variables are live across lots of
	// basic blocks. Figure out a way to make this function (or, more precisely, the user
	// of this function) require only linear size & time.
	func (s *stackAllocState) computeLive(spillLive [][]ID) {
	s.live = make([][]ID, s.f.NumBlocks())
	var phis []*Value
	live := s.f.newSparseSet(s.f.NumValues())
	defer s.f.retSparseSet(live)
	t := s.f.newSparseSet(s.f.NumValues())
	defer s.f.retSparseSet(t)

	// Instead of iterating over f.Blocks, iterate over their postordering.
	// Liveness information flows backward, so starting at the end
	// increases the probability that we will stabilize quickly.
	po := s.f.postorder()
	for {
	changed := false
	for _, b := range po {
	// Start with known live values at the end of the block
	live.clear()
	live.addAll(s.live[b.ID])

	// Propagate backwards to the start of the block
	phis = phis[:0]
	for i := len(b.Values) - 1; i >= 0; i-- {
	v := b.Values[i]
	live.remove(v.ID)
	if v.Op == OpPhi {
	// Save phi for later.
	// Note: its args might need a stack slot even though
	// the phi itself doesn't. So don't use needSlot.
	if !v.Type.IsMemory() && !v.Type.IsVoid() {
	phis = append(phis, v)
	}
	continue
	}
	for _, a := range v.Args {
	if s.values[a.ID].needSlot {
	live.add(a.ID)
	}
	}
	}

	// for each predecessor of b, expand its list of live-at-end values
	// invariant: s contains the values live at the start of b (excluding phi inputs)
	for i, e := range b.Preds {
	p := e.b
	t.clear()
	t.addAll(s.live[p.ID])
	t.addAll(live.contents())
	t.addAll(spillLive[p.ID])
	for _, v := range phis {
	a := v.Args[i]
	if s.values[a.ID].needSlot {
	t.add(a.ID)
	}
	if spill := s.values[a.ID].spill; spill != nil {
	//TODO: remove? Subsumed by SpillUse?
	t.add(spill.ID)
	}
	}
	if t.size() == len(s.live[p.ID]) {
	continue
	}
	// grow p's live set
	s.live[p.ID] = append(s.live[p.ID][:0], t.contents()...)
	changed = true
	}
	}

	if !changed {
	break
	}
	}
	if s.f.pass.debug > stackDebug {
	for _, b := range s.f.Blocks {
	fmt.Printf("stacklive %s %v\n", b, s.live[b.ID])
	}
	}
	}

	func (f *Func) getHome(vid ID) Location {
	if int(vid) >= len(f.RegAlloc) {
	return nil
	}
	return f.RegAlloc[vid]
	}

	func (f Func) setHome(v Value, loc Location) {
	for v.ID >= ID(len(f.RegAlloc)) {
	f.RegAlloc = append(f.RegAlloc, nil)
	}
	f.RegAlloc[v.ID] = loc
	}

	func (s *stackAllocState) buildInterferenceGraph() {
	f := s.f
	if n := f.NumValues(); cap(s.interfere) >= n {
	s.interfere = s.interfere[:n]
	} else {
	s.interfere = make([][]ID, n)
	}
	live := f.newSparseSet(f.NumValues())
	defer f.retSparseSet(live)
	for _, b := range f.Blocks {
	// Propagate liveness backwards to the start of the block.
	// Two values interfere if one is defined while the other is live.
	live.clear()
	live.addAll(s.live[b.ID])
	for i := len(b.Values) - 1; i >= 0; i-- {
	v := b.Values[i]
	if s.values[v.ID].needSlot {
	live.remove(v.ID)
	for _, id := range live.contents() {
	if s.values[v.ID].typ.Compare(s.values[id].typ) == CMPeq {
	s.interfere[v.ID] = append(s.interfere[v.ID], id)
	s.interfere[id] = append(s.interfere[id], v.ID)
	}
	}
	}
	for _, a := range v.Args {
	if s.values[a.ID].needSlot {
	live.add(a.ID)
	}
	}
	if v.Op == OpArg && s.values[v.ID].needSlot {
	// OpArg is an input argument which is pre-spilled.
	// We add back v.ID here because we want this value
	// to appear live even before this point. Being live
	// all the way to the start of the entry block prevents other
	// values from being allocated to the same slot and clobbering
	// the input value before we have a chance to load it.
	live.add(v.ID)
	}
	}
	}
	if f.pass.debug > stackDebug {
	for vid, i := range s.interfere {
	if len(i) > 0 {
	fmt.Printf("v%d interferes with", vid)
	for _, x := range i {
	fmt.Printf(" v%d", x)
	}
	fmt.Println()
	}
	}
	}
	}