src/cmd/compile/internal/ssa/schedule.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.

 package ssa

 import (
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/types"
 	"container/heap"
 	"sort"
 )

 const (
 	ScorePhi       = iota // towards top of block
 	ScoreArg              // must occur at the top of the entry block
 	ScoreInitMem          // after the args - used as mark by debug info generation
 	ScoreReadTuple        // must occur immediately after tuple-generating insn (or call)
 	ScoreNilCheck
 	ScoreMemory
 	ScoreReadFlags
 	ScoreDefault
 	ScoreFlags
 	ScoreControl // towards bottom of block
 )

 type ValHeap struct {
 	a           []*Value
 	score       []int8
 	inBlockUses []bool
 }

 func (h ValHeap) Len() int      { return len(h.a) }
 func (h ValHeap) Swap(i, j int) { a := h.a; a[i], a[j] = a[j], a[i] }

 func (h *ValHeap) Push(x interface{}) {
 	// Push and Pop use pointer receivers because they modify the slice's length,
 	// not just its contents.
 	v := x.(*Value)
 	h.a = append(h.a, v)
 }
 func (h *ValHeap) Pop() interface{} {
 	old := h.a
 	n := len(old)
 	x := old[n-1]
 	h.a = old[0 : n-1]
 	return x
 }
 func (h ValHeap) Less(i, j int) bool {
 	x := h.a[i]
 	y := h.a[j]
 	sx := h.score[x.ID]
 	sy := h.score[y.ID]
 	if c := sx - sy; c != 0 {
 		return c < 0 // lower scores come earlier.
 	}
 	// Note: only scores are required for correct scheduling.
 	// Everything else is just heuristics.

 	ix := h.inBlockUses[x.ID]
 	iy := h.inBlockUses[y.ID]
 	if ix != iy {
 		return ix // values with in-block uses come earlier
 	}

 	if x.Pos != y.Pos { // Favor in-order line stepping
 		return x.Pos.Before(y.Pos)
 	}
 	if x.Op != OpPhi {
 		if c := len(x.Args) - len(y.Args); c != 0 {
 			return c > 0 // smaller args come later
 		}
 	}
 	if c := x.Uses - y.Uses; c != 0 {
 		return c > 0 // smaller uses come later
 	}
 	// These comparisons are fairly arbitrary.
 	// The goal here is stability in the face
 	// of unrelated changes elsewhere in the compiler.
 	if c := x.AuxInt - y.AuxInt; c != 0 {
 		return c < 0
 	}
 	if cmp := x.Type.Compare(y.Type); cmp != types.CMPeq {
 		return cmp == types.CMPlt
 	}
 	return x.ID < y.ID
 }

 func (op Op) isLoweredGetClosurePtr() bool {
 	switch op {
 	case OpAMD64LoweredGetClosurePtr, OpPPC64LoweredGetClosurePtr, OpARMLoweredGetClosurePtr, OpARM64LoweredGetClosurePtr,
 		Op386LoweredGetClosurePtr, OpMIPS64LoweredGetClosurePtr, OpLOONG64LoweredGetClosurePtr, OpS390XLoweredGetClosurePtr, OpMIPSLoweredGetClosurePtr,
 		OpRISCV64LoweredGetClosurePtr, OpWasmLoweredGetClosurePtr:
 		return true
 	}
 	return false
 }

 // Schedule the Values in each Block. After this phase returns, the
 // order of b.Values matters and is the order in which those values
 // will appear in the assembly output. For now it generates a
 // reasonable valid schedule using a priority queue. TODO(khr):
 // schedule smarter.
 func schedule(f *Func) {
 	// reusable priority queue
 	priq := new(ValHeap)

 	// "priority" for a value
 	score := f.Cache.allocInt8Slice(f.NumValues())
 	defer f.Cache.freeInt8Slice(score)

 	// maps mem values to the next live memory value
 	nextMem := f.Cache.allocValueSlice(f.NumValues())
 	defer f.Cache.freeValueSlice(nextMem)

 	// inBlockUses records whether a value is used in the block
 	// in which it lives. (block control values don't count as uses.)
 	inBlockUses := f.Cache.allocBoolSlice(f.NumValues())
 	defer f.Cache.freeBoolSlice(inBlockUses)
 	if f.Config.optimize {
 		for _, b := range f.Blocks {
 			for _, v := range b.Values {
 				for _, a := range v.Args {
 					if a.Block == b {
 						inBlockUses[a.ID] = true
 					}
 				}
 			}
 		}
 	}
 	priq.inBlockUses = inBlockUses

 	for _, b := range f.Blocks {
 		// Compute score. Larger numbers are scheduled closer to the end of the block.
 		for _, v := range b.Values {
 			switch {
 			case v.Op.isLoweredGetClosurePtr():
 				// We also score GetLoweredClosurePtr as early as possible to ensure that the
 				// context register is not stomped. GetLoweredClosurePtr should only appear
 				// in the entry block where there are no phi functions, so there is no
 				// conflict or ambiguity here.
 				if b != f.Entry {
 					f.Fatalf("LoweredGetClosurePtr appeared outside of entry block, b=%s", b.String())
 				}
 				score[v.ID] = ScorePhi
 			case opcodeTable[v.Op].nilCheck:
 				// Nil checks must come before loads from the same address.
 				score[v.ID] = ScoreNilCheck
 			case v.Op == OpPhi:
 				// We want all the phis first.
 				score[v.ID] = ScorePhi
 			case v.Op == OpArgIntReg || v.Op == OpArgFloatReg:
 				// In-register args must be scheduled as early as possible to ensure that they
 				// are not stomped (similar to the closure pointer above).
 				// In particular, they need to come before regular OpArg operations because
 				// of how regalloc places spill code (see regalloc.go:placeSpills:mustBeFirst).
 				if b != f.Entry {
 					f.Fatalf("%s appeared outside of entry block, b=%s", v.Op, b.String())
 				}
 				score[v.ID] = ScorePhi
 			case v.Op == OpArg || v.Op == OpSP || v.Op == OpSB:
 				// We want all the args as early as possible, for better debugging.
 				score[v.ID] = ScoreArg
 			case v.Op == OpInitMem:
 				// Early, but after args. See debug.go:buildLocationLists
 				score[v.ID] = ScoreInitMem
 			case v.Type.IsMemory():
 				// Schedule stores as early as possible. This tends to
 				// reduce register pressure.
 				score[v.ID] = ScoreMemory
 			case v.Op == OpSelect0 || v.Op == OpSelect1 || v.Op == OpSelectN:
 				// Tuple selectors need to appear immediately after the instruction
 				// that generates the tuple.
 				score[v.ID] = ScoreReadTuple
 			case v.hasFlagInput():
 				// Schedule flag-reading ops earlier, to minimize the lifetime
 				// of flag values.
 				score[v.ID] = ScoreReadFlags
 			case v.isFlagOp():
 				// Schedule flag register generation as late as possible.
 				// This makes sure that we only have one live flags
 				// value at a time.
 				// Note that this case is after the case above, so values
 				// which both read and generate flags are given ScoreReadFlags.
 				score[v.ID] = ScoreFlags
 			default:
 				score[v.ID] = ScoreDefault
 				// If we're reading flags, schedule earlier to keep flag lifetime short.
 				for _, a := range v.Args {
 					if a.isFlagOp() {
 						score[v.ID] = ScoreReadFlags
 					}
 				}
 			}
 		}
 		for _, c := range b.ControlValues() {
 			// Force the control values to be scheduled at the end,
 			// unless they have other special priority.
 			if c.Block != b || score[c.ID] < ScoreReadTuple {
 				continue
 			}
 			if score[c.ID] == ScoreReadTuple {
 				score[c.Args[0].ID] = ScoreControl
 				continue
 			}
 			score[c.ID] = ScoreControl
 		}
 	}
 	priq.score = score

 	// An edge represents a scheduling constraint that x must appear before y in the schedule.
 	type edge struct {
 		x, y *Value
 	}
 	edges := make([]edge, 0, 64)

 	// inEdges is the number of scheduling edges incoming from values that haven't been scheduled yet.
 	// i.e. inEdges[y.ID] = |e in edges where e.y == y and e.x is not in the schedule yet|.
 	inEdges := f.Cache.allocInt32Slice(f.NumValues())
 	defer f.Cache.freeInt32Slice(inEdges)

 	for _, b := range f.Blocks {
 		edges = edges[:0]
 		// Standard edges: from the argument of a value to that value.
 		for _, v := range b.Values {
 			if v.Op == OpPhi {
 				// If a value is used by a phi, it does not induce
 				// a scheduling edge because that use is from the
 				// previous iteration.
 				continue
 			}
 			for _, a := range v.Args {
 				if a.Block == b {
 					edges = append(edges, edge{a, v})
 				}
 			}
 		}

 		// Find store chain for block.
 		// Store chains for different blocks overwrite each other, so
 		// the calculated store chain is good only for this block.
 		for _, v := range b.Values {
 			if v.Op != OpPhi && v.Op != OpInitMem && v.Type.IsMemory() {
 				nextMem[v.MemoryArg().ID] = v
 			}
 		}

 		// Add edges to enforce that any load must come before the following store.
 		for _, v := range b.Values {
 			if v.Op == OpPhi || v.Type.IsMemory() {
 				continue
 			}
 			w := v.MemoryArg()
 			if w == nil {
 				continue
 			}
 			if s := nextMem[w.ID]; s != nil && s.Block == b {
 				edges = append(edges, edge{v, s})
 			}
 		}

 		// Sort all the edges by source Value ID.
 		sort.Slice(edges, func(i, j int) bool {
 			return edges[i].x.ID < edges[j].x.ID
 		})
 		// Compute inEdges for values in this block.
 		for _, e := range edges {
 			inEdges[e.y.ID]++
 		}

 		// Initialize priority queue with schedulable values.
 		priq.a = priq.a[:0]
 		for _, v := range b.Values {
 			if inEdges[v.ID] == 0 {
 				heap.Push(priq, v)
 			}
 		}

 		// Produce the schedule. Pick the highest priority scheduleable value,
 		// add it to the schedule, add any of its uses that are now scheduleable
 		// to the queue, and repeat.
 		nv := len(b.Values)
 		b.Values = b.Values[:0]
 		for priq.Len() > 0 {
 			// Schedule the next schedulable value in priority order.
 			v := heap.Pop(priq).(*Value)
 			b.Values = append(b.Values, v)

 			// Find all the scheduling edges out from this value.
 			i := sort.Search(len(edges), func(i int) bool {
 				return edges[i].x.ID >= v.ID
 			})
 			j := sort.Search(len(edges), func(i int) bool {
 				return edges[i].x.ID > v.ID
 			})
 			// Decrement inEdges for each target of edges from v.
 			for _, e := range edges[i:j] {
 				inEdges[e.y.ID]--
 				if inEdges[e.y.ID] == 0 {
 					heap.Push(priq, e.y)
 				}
 			}
 		}
 		if len(b.Values) != nv {
 			f.Fatalf("schedule does not include all values in block %s", b)
 		}
 	}

 	// Remove SPanchored now that we've scheduled.
 	// Also unlink nil checks now that ordering is assured
 	// between the nil check and the uses of the nil-checked pointer.
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			for i, a := range v.Args {
 				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 {
 					base.Fatalf("SPAnchored still has %d uses", v.Uses)
 				}
 				v.resetArgs()
 				f.freeValue(v)
 			} else {
 				if opcodeTable[v.Op].nilCheck {
 					if v.Uses != 0 {
 						base.Fatalf("nilcheck still has %d uses", v.Uses)
 					}
 					// We can't delete the nil check, but we mark
 					// it as having void type so regalloc won't
 					// try to allocate a register for it.
 					v.Type = types.TypeVoid
 				}
 				b.Values[i] = v
 				i++
 			}
 		}
 		b.truncateValues(i)
 	}

 	f.scheduled = true
 }

 // storeOrder orders values with respect to stores. That is,
 // if v transitively depends on store s, v is ordered after s,
 // otherwise v is ordered before s.
 // Specifically, values are ordered like
 //
 //	store1
 //	NilCheck that depends on store1
 //	other values that depends on store1
 //	store2
 //	NilCheck that depends on store2
 //	other values that depends on store2
 //	...
 //
 // The order of non-store and non-NilCheck values are undefined
 // (not necessarily dependency order). This should be cheaper
 // than a full scheduling as done above.
 // Note that simple dependency order won't work: there is no
 // dependency between NilChecks and values like IsNonNil.
 // Auxiliary data structures are passed in as arguments, so
 // that they can be allocated in the caller and be reused.
 // This function takes care of reset them.
 func storeOrder(values []*Value, sset *sparseSet, storeNumber []int32) []*Value {
 	if len(values) == 0 {
 		return values
 	}

 	f := values[0].Block.Func

 	// find all stores

 	// Members of values that are store values.
 	// A constant bound allows this to be stack-allocated. 64 is
 	// enough to cover almost every storeOrder call.
 	stores := make([]*Value, 0, 64)
 	hasNilCheck := false
 	sset.clear() // sset is the set of stores that are used in other values
 	for _, v := range values {
 		if v.Type.IsMemory() {
 			stores = append(stores, v)
 			if v.Op == OpInitMem || v.Op == OpPhi {
 				continue
 			}
 			sset.add(v.MemoryArg().ID) // record that v's memory arg is used
 		}
 		if v.Op == OpNilCheck {
 			hasNilCheck = true
 		}
 	}
 	if len(stores) == 0 || !hasNilCheck && f.pass.name == "nilcheckelim" {
 		// there is no store, the order does not matter
 		return values
 	}

 	// find last store, which is the one that is not used by other stores
 	var last *Value
 	for _, v := range stores {
 		if !sset.contains(v.ID) {
 			if last != nil {
 				f.Fatalf("two stores live simultaneously: %v and %v", v, last)
 			}
 			last = v
 		}
 	}

 	// We assign a store number to each value. Store number is the
 	// index of the latest store that this value transitively depends.
 	// The i-th store in the current block gets store number 3*i. A nil
 	// check that depends on the i-th store gets store number 3*i+1.
 	// Other values that depends on the i-th store gets store number 3*i+2.
 	// Special case: 0 -- unassigned, 1 or 2 -- the latest store it depends
 	// is in the previous block (or no store at all, e.g. value is Const).
 	// First we assign the number to all stores by walking back the store chain,
 	// then assign the number to other values in DFS order.
 	count := make([]int32, 3*(len(stores)+1))
 	sset.clear() // reuse sparse set to ensure that a value is pushed to stack only once
 	for n, w := len(stores), last; n > 0; n-- {
 		storeNumber[w.ID] = int32(3 * n)
 		count[3*n]++
 		sset.add(w.ID)
 		if w.Op == OpInitMem || w.Op == OpPhi {
 			if n != 1 {
 				f.Fatalf("store order is wrong: there are stores before %v", w)
 			}
 			break
 		}
 		w = w.MemoryArg()
 	}
 	var stack []*Value
 	for _, v := range values {
 		if sset.contains(v.ID) {
 			// in sset means v is a store, or already pushed to stack, or already assigned a store number
 			continue
 		}
 		stack = append(stack, v)
 		sset.add(v.ID)

 		for len(stack) > 0 {
 			w := stack[len(stack)-1]
 			if storeNumber[w.ID] != 0 {
 				stack = stack[:len(stack)-1]
 				continue
 			}
 			if w.Op == OpPhi {
 				// Phi value doesn't depend on store in the current block.
 				// Do this early to avoid dependency cycle.
 				storeNumber[w.ID] = 2
 				count[2]++
 				stack = stack[:len(stack)-1]
 				continue
 			}

 			max := int32(0) // latest store dependency
 			argsdone := true
 			for _, a := range w.Args {
 				if a.Block != w.Block {
 					continue
 				}
 				if !sset.contains(a.ID) {
 					stack = append(stack, a)
 					sset.add(a.ID)
 					argsdone = false
 					break
 				}
 				if storeNumber[a.ID]/3 > max {
 					max = storeNumber[a.ID] / 3
 				}
 			}
 			if !argsdone {
 				continue
 			}

 			n := 3*max + 2
 			if w.Op == OpNilCheck {
 				n = 3*max + 1
 			}
 			storeNumber[w.ID] = n
 			count[n]++
 			stack = stack[:len(stack)-1]
 		}
 	}

 	// convert count to prefix sum of counts: count'[i] = sum_{j<=i} count[i]
 	for i := range count {
 		if i == 0 {
 			continue
 		}
 		count[i] += count[i-1]
 	}
 	if count[len(count)-1] != int32(len(values)) {
 		f.Fatalf("storeOrder: value is missing, total count = %d, values = %v", count[len(count)-1], values)
 	}

 	// place values in count-indexed bins, which are in the desired store order
 	order := make([]*Value, len(values))
 	for _, v := range values {
 		s := storeNumber[v.ID]
 		order[count[s-1]] = v
 		count[s-1]++
 	}

 	// Order nil checks in source order. We want the first in source order to trigger.
 	// If two are on the same line, we don't really care which happens first.
 	// See issue 18169.
 	if hasNilCheck {
 		start := -1
 		for i, v := range order {
 			if v.Op == OpNilCheck {
 				if start == -1 {
 					start = i
 				}
 			} else {
 				if start != -1 {
 					sort.Sort(bySourcePos(order[start:i]))
 					start = -1
 				}
 			}
 		}
 		if start != -1 {
 			sort.Sort(bySourcePos(order[start:]))
 		}
 	}

 	return order
 }

 // isFlagOp reports if v is an OP with the flag type.
 func (v *Value) isFlagOp() bool {
 	if v.Type.IsFlags() || v.Type.IsTuple() && v.Type.FieldType(1).IsFlags() {
 		return true
 	}
 	// PPC64 carry generators put their carry in a non-flag-typed register
 	// in their output.
 	switch v.Op {
 	case OpPPC64SUBC, OpPPC64ADDC, OpPPC64SUBCconst, OpPPC64ADDCconst:
 		return true
 	}
 	return false
 }

 // hasFlagInput reports whether v has a flag value as any of its inputs.
 func (v *Value) hasFlagInput() bool {
 	for _, a := range v.Args {
 		if a.isFlagOp() {
 			return true
 		}
 	}
 	// PPC64 carry dependencies are conveyed through their final argument,
 	// so we treat those operations as taking flags as well.
 	switch v.Op {
 	case OpPPC64SUBE, OpPPC64ADDE, OpPPC64SUBZEzero, OpPPC64ADDZE, OpPPC64ADDZEzero:
 		return true
 	}
 	return false
 }

 type bySourcePos []*Value

 func (s bySourcePos) Len() int           { return len(s) }
 func (s bySourcePos) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 func (s bySourcePos) Less(i, j int) bool { return s[i].Pos.Before(s[j].Pos) }
	// 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.

	package ssa

	import (
	"cmd/compile/internal/base"
	"cmd/compile/internal/types"
	"container/heap"
	"sort"
	)

	const (
	ScorePhi = iota // towards top of block
	ScoreArg // must occur at the top of the entry block
	ScoreInitMem // after the args - used as mark by debug info generation
	ScoreReadTuple // must occur immediately after tuple-generating insn (or call)
	ScoreNilCheck
	ScoreMemory
	ScoreReadFlags
	ScoreDefault
	ScoreFlags
	ScoreControl // towards bottom of block
	)

	type ValHeap struct {
	a []*Value
	score []int8
	inBlockUses []bool
	}

	func (h ValHeap) Len() int { return len(h.a) }
	func (h ValHeap) Swap(i, j int) { a := h.a; a[i], a[j] = a[j], a[i] }

	func (h *ValHeap) Push(x interface{}) {
	// Push and Pop use pointer receivers because they modify the slice's length,
	// not just its contents.
	v := x.(*Value)
	h.a = append(h.a, v)
	}
	func (h *ValHeap) Pop() interface{} {
	old := h.a
	n := len(old)
	x := old[n-1]
	h.a = old[0 : n-1]
	return x
	}
	func (h ValHeap) Less(i, j int) bool {
	x := h.a[i]
	y := h.a[j]
	sx := h.score[x.ID]
	sy := h.score[y.ID]
	if c := sx - sy; c != 0 {
	return c < 0 // lower scores come earlier.
	}
	// Note: only scores are required for correct scheduling.
	// Everything else is just heuristics.

	ix := h.inBlockUses[x.ID]
	iy := h.inBlockUses[y.ID]
	if ix != iy {
	return ix // values with in-block uses come earlier
	}

	if x.Pos != y.Pos { // Favor in-order line stepping
	return x.Pos.Before(y.Pos)
	}
	if x.Op != OpPhi {
	if c := len(x.Args) - len(y.Args); c != 0 {
	return c > 0 // smaller args come later
	}
	}
	if c := x.Uses - y.Uses; c != 0 {
	return c > 0 // smaller uses come later
	}
	// These comparisons are fairly arbitrary.
	// The goal here is stability in the face
	// of unrelated changes elsewhere in the compiler.
	if c := x.AuxInt - y.AuxInt; c != 0 {
	return c < 0
	}
	if cmp := x.Type.Compare(y.Type); cmp != types.CMPeq {
	return cmp == types.CMPlt
	}
	return x.ID < y.ID
	}

	func (op Op) isLoweredGetClosurePtr() bool {
	switch op {
	case OpAMD64LoweredGetClosurePtr, OpPPC64LoweredGetClosurePtr, OpARMLoweredGetClosurePtr, OpARM64LoweredGetClosurePtr,
	Op386LoweredGetClosurePtr, OpMIPS64LoweredGetClosurePtr, OpLOONG64LoweredGetClosurePtr, OpS390XLoweredGetClosurePtr, OpMIPSLoweredGetClosurePtr,
	OpRISCV64LoweredGetClosurePtr, OpWasmLoweredGetClosurePtr:
	return true
	}
	return false
	}

	// Schedule the Values in each Block. After this phase returns, the
	// order of b.Values matters and is the order in which those values
	// will appear in the assembly output. For now it generates a
	// reasonable valid schedule using a priority queue. TODO(khr):
	// schedule smarter.
	func schedule(f *Func) {
	// reusable priority queue
	priq := new(ValHeap)

	// "priority" for a value
	score := f.Cache.allocInt8Slice(f.NumValues())
	defer f.Cache.freeInt8Slice(score)

	// maps mem values to the next live memory value
	nextMem := f.Cache.allocValueSlice(f.NumValues())
	defer f.Cache.freeValueSlice(nextMem)

	// inBlockUses records whether a value is used in the block
	// in which it lives. (block control values don't count as uses.)
	inBlockUses := f.Cache.allocBoolSlice(f.NumValues())
	defer f.Cache.freeBoolSlice(inBlockUses)
	if f.Config.optimize {
	for _, b := range f.Blocks {
	for _, v := range b.Values {
	for _, a := range v.Args {
	if a.Block == b {
	inBlockUses[a.ID] = true
	}
	}
	}
	}
	}
	priq.inBlockUses = inBlockUses

	for _, b := range f.Blocks {
	// Compute score. Larger numbers are scheduled closer to the end of the block.
	for _, v := range b.Values {
	switch {
	case v.Op.isLoweredGetClosurePtr():
	// We also score GetLoweredClosurePtr as early as possible to ensure that the
	// context register is not stomped. GetLoweredClosurePtr should only appear
	// in the entry block where there are no phi functions, so there is no
	// conflict or ambiguity here.
	if b != f.Entry {
	f.Fatalf("LoweredGetClosurePtr appeared outside of entry block, b=%s", b.String())
	}
	score[v.ID] = ScorePhi
	case opcodeTable[v.Op].nilCheck:
	// Nil checks must come before loads from the same address.
	score[v.ID] = ScoreNilCheck
	case v.Op == OpPhi:
	// We want all the phis first.
	score[v.ID] = ScorePhi
	case v.Op == OpArgIntReg \|\| v.Op == OpArgFloatReg:
	// In-register args must be scheduled as early as possible to ensure that they
	// are not stomped (similar to the closure pointer above).
	// In particular, they need to come before regular OpArg operations because
	// of how regalloc places spill code (see regalloc.go:placeSpills:mustBeFirst).
	if b != f.Entry {
	f.Fatalf("%s appeared outside of entry block, b=%s", v.Op, b.String())
	}
	score[v.ID] = ScorePhi
	case v.Op == OpArg \|\| v.Op == OpSP \|\| v.Op == OpSB:
	// We want all the args as early as possible, for better debugging.
	score[v.ID] = ScoreArg
	case v.Op == OpInitMem:
	// Early, but after args. See debug.go:buildLocationLists
	score[v.ID] = ScoreInitMem
	case v.Type.IsMemory():
	// Schedule stores as early as possible. This tends to
	// reduce register pressure.
	score[v.ID] = ScoreMemory
	case v.Op == OpSelect0 \|\| v.Op == OpSelect1 \|\| v.Op == OpSelectN:
	// Tuple selectors need to appear immediately after the instruction
	// that generates the tuple.
	score[v.ID] = ScoreReadTuple
	case v.hasFlagInput():
	// Schedule flag-reading ops earlier, to minimize the lifetime
	// of flag values.
	score[v.ID] = ScoreReadFlags
	case v.isFlagOp():
	// Schedule flag register generation as late as possible.
	// This makes sure that we only have one live flags
	// value at a time.
	// Note that this case is after the case above, so values
	// which both read and generate flags are given ScoreReadFlags.
	score[v.ID] = ScoreFlags
	default:
	score[v.ID] = ScoreDefault
	// If we're reading flags, schedule earlier to keep flag lifetime short.
	for _, a := range v.Args {
	if a.isFlagOp() {
	score[v.ID] = ScoreReadFlags
	}
	}
	}
	}
	for _, c := range b.ControlValues() {
	// Force the control values to be scheduled at the end,
	// unless they have other special priority.
	if c.Block != b \|\| score[c.ID] < ScoreReadTuple {
	continue
	}
	if score[c.ID] == ScoreReadTuple {
	score[c.Args[0].ID] = ScoreControl
	continue
	}
	score[c.ID] = ScoreControl
	}
	}
	priq.score = score

	// An edge represents a scheduling constraint that x must appear before y in the schedule.
	type edge struct {
	x, y *Value
	}
	edges := make([]edge, 0, 64)

	// inEdges is the number of scheduling edges incoming from values that haven't been scheduled yet.
	// i.e. inEdges[y.ID] = \|e in edges where e.y == y and e.x is not in the schedule yet\|.
	inEdges := f.Cache.allocInt32Slice(f.NumValues())
	defer f.Cache.freeInt32Slice(inEdges)

	for _, b := range f.Blocks {
	edges = edges[:0]
	// Standard edges: from the argument of a value to that value.
	for _, v := range b.Values {
	if v.Op == OpPhi {
	// If a value is used by a phi, it does not induce
	// a scheduling edge because that use is from the
	// previous iteration.
	continue
	}
	for _, a := range v.Args {
	if a.Block == b {
	edges = append(edges, edge{a, v})
	}
	}
	}

	// Find store chain for block.
	// Store chains for different blocks overwrite each other, so
	// the calculated store chain is good only for this block.
	for _, v := range b.Values {
	if v.Op != OpPhi && v.Op != OpInitMem && v.Type.IsMemory() {
	nextMem[v.MemoryArg().ID] = v
	}
	}

	// Add edges to enforce that any load must come before the following store.
	for _, v := range b.Values {
	if v.Op == OpPhi \|\| v.Type.IsMemory() {
	continue
	}
	w := v.MemoryArg()
	if w == nil {
	continue
	}
	if s := nextMem[w.ID]; s != nil && s.Block == b {
	edges = append(edges, edge{v, s})
	}
	}

	// Sort all the edges by source Value ID.
	sort.Slice(edges, func(i, j int) bool {
	return edges[i].x.ID < edges[j].x.ID
	})
	// Compute inEdges for values in this block.
	for _, e := range edges {
	inEdges[e.y.ID]++
	}

	// Initialize priority queue with schedulable values.
	priq.a = priq.a[:0]
	for _, v := range b.Values {
	if inEdges[v.ID] == 0 {
	heap.Push(priq, v)
	}
	}

	// Produce the schedule. Pick the highest priority scheduleable value,
	// add it to the schedule, add any of its uses that are now scheduleable
	// to the queue, and repeat.
	nv := len(b.Values)
	b.Values = b.Values[:0]
	for priq.Len() > 0 {
	// Schedule the next schedulable value in priority order.
	v := heap.Pop(priq).(*Value)
	b.Values = append(b.Values, v)

	// Find all the scheduling edges out from this value.
	i := sort.Search(len(edges), func(i int) bool {
	return edges[i].x.ID >= v.ID
	})
	j := sort.Search(len(edges), func(i int) bool {
	return edges[i].x.ID > v.ID
	})
	// Decrement inEdges for each target of edges from v.
	for _, e := range edges[i:j] {
	inEdges[e.y.ID]--
	if inEdges[e.y.ID] == 0 {
	heap.Push(priq, e.y)
	}
	}
	}
	if len(b.Values) != nv {
	f.Fatalf("schedule does not include all values in block %s", b)
	}
	}

	// Remove SPanchored now that we've scheduled.
	// Also unlink nil checks now that ordering is assured
	// between the nil check and the uses of the nil-checked pointer.
	for _, b := range f.Blocks {
	for _, v := range b.Values {
	for i, a := range v.Args {
	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 {
	base.Fatalf("SPAnchored still has %d uses", v.Uses)
	}
	v.resetArgs()
	f.freeValue(v)
	} else {
	if opcodeTable[v.Op].nilCheck {
	if v.Uses != 0 {
	base.Fatalf("nilcheck still has %d uses", v.Uses)
	}
	// We can't delete the nil check, but we mark
	// it as having void type so regalloc won't
	// try to allocate a register for it.
	v.Type = types.TypeVoid
	}
	b.Values[i] = v
	i++
	}
	}
	b.truncateValues(i)
	}

	f.scheduled = true
	}

	// storeOrder orders values with respect to stores. That is,
	// if v transitively depends on store s, v is ordered after s,
	// otherwise v is ordered before s.
	// Specifically, values are ordered like
	//
	// store1
	// NilCheck that depends on store1
	// other values that depends on store1
	// store2
	// NilCheck that depends on store2
	// other values that depends on store2
	// ...
	//
	// The order of non-store and non-NilCheck values are undefined
	// (not necessarily dependency order). This should be cheaper
	// than a full scheduling as done above.
	// Note that simple dependency order won't work: there is no
	// dependency between NilChecks and values like IsNonNil.
	// Auxiliary data structures are passed in as arguments, so
	// that they can be allocated in the caller and be reused.
	// This function takes care of reset them.
	func storeOrder(values []Value, sset sparseSet, storeNumber []int32) []*Value {
	if len(values) == 0 {
	return values
	}

	f := values[0].Block.Func

	// find all stores

	// Members of values that are store values.
	// A constant bound allows this to be stack-allocated. 64 is
	// enough to cover almost every storeOrder call.
	stores := make([]*Value, 0, 64)
	hasNilCheck := false
	sset.clear() // sset is the set of stores that are used in other values
	for _, v := range values {
	if v.Type.IsMemory() {
	stores = append(stores, v)
	if v.Op == OpInitMem \|\| v.Op == OpPhi {
	continue
	}
	sset.add(v.MemoryArg().ID) // record that v's memory arg is used
	}
	if v.Op == OpNilCheck {
	hasNilCheck = true
	}
	}
	if len(stores) == 0 \|\| !hasNilCheck && f.pass.name == "nilcheckelim" {
	// there is no store, the order does not matter
	return values
	}

	// find last store, which is the one that is not used by other stores
	var last *Value
	for _, v := range stores {
	if !sset.contains(v.ID) {
	if last != nil {
	f.Fatalf("two stores live simultaneously: %v and %v", v, last)
	}
	last = v
	}
	}

	// We assign a store number to each value. Store number is the
	// index of the latest store that this value transitively depends.
	// The i-th store in the current block gets store number 3*i. A nil
	// check that depends on the i-th store gets store number 3*i+1.
	// Other values that depends on the i-th store gets store number 3*i+2.
	// Special case: 0 -- unassigned, 1 or 2 -- the latest store it depends
	// is in the previous block (or no store at all, e.g. value is Const).
	// First we assign the number to all stores by walking back the store chain,
	// then assign the number to other values in DFS order.
	count := make([]int32, 3*(len(stores)+1))
	sset.clear() // reuse sparse set to ensure that a value is pushed to stack only once
	for n, w := len(stores), last; n > 0; n-- {
	storeNumber[w.ID] = int32(3 * n)
	count[3*n]++
	sset.add(w.ID)
	if w.Op == OpInitMem \|\| w.Op == OpPhi {
	if n != 1 {
	f.Fatalf("store order is wrong: there are stores before %v", w)
	}
	break
	}
	w = w.MemoryArg()
	}
	var stack []*Value
	for _, v := range values {
	if sset.contains(v.ID) {
	// in sset means v is a store, or already pushed to stack, or already assigned a store number
	continue
	}
	stack = append(stack, v)
	sset.add(v.ID)

	for len(stack) > 0 {
	w := stack[len(stack)-1]
	if storeNumber[w.ID] != 0 {
	stack = stack[:len(stack)-1]
	continue
	}
	if w.Op == OpPhi {
	// Phi value doesn't depend on store in the current block.
	// Do this early to avoid dependency cycle.
	storeNumber[w.ID] = 2
	count[2]++
	stack = stack[:len(stack)-1]
	continue
	}

	max := int32(0) // latest store dependency
	argsdone := true
	for _, a := range w.Args {
	if a.Block != w.Block {
	continue
	}
	if !sset.contains(a.ID) {
	stack = append(stack, a)
	sset.add(a.ID)
	argsdone = false
	break
	}
	if storeNumber[a.ID]/3 > max {
	max = storeNumber[a.ID] / 3
	}
	}
	if !argsdone {
	continue
	}

	n := 3*max + 2
	if w.Op == OpNilCheck {
	n = 3*max + 1
	}
	storeNumber[w.ID] = n
	count[n]++
	stack = stack[:len(stack)-1]
	}
	}

	// convert count to prefix sum of counts: count'[i] = sum_{j<=i} count[i]
	for i := range count {
	if i == 0 {
	continue
	}
	count[i] += count[i-1]
	}
	if count[len(count)-1] != int32(len(values)) {
	f.Fatalf("storeOrder: value is missing, total count = %d, values = %v", count[len(count)-1], values)
	}

	// place values in count-indexed bins, which are in the desired store order
	order := make([]*Value, len(values))
	for _, v := range values {
	s := storeNumber[v.ID]
	order[count[s-1]] = v
	count[s-1]++
	}

	// Order nil checks in source order. We want the first in source order to trigger.
	// If two are on the same line, we don't really care which happens first.
	// See issue 18169.
	if hasNilCheck {
	start := -1
	for i, v := range order {
	if v.Op == OpNilCheck {
	if start == -1 {
	start = i
	}
	} else {
	if start != -1 {
	sort.Sort(bySourcePos(order[start:i]))
	start = -1
	}
	}
	}
	if start != -1 {
	sort.Sort(bySourcePos(order[start:]))
	}
	}

	return order
	}

	// isFlagOp reports if v is an OP with the flag type.
	func (v *Value) isFlagOp() bool {
	if v.Type.IsFlags() \|\| v.Type.IsTuple() && v.Type.FieldType(1).IsFlags() {
	return true
	}
	// PPC64 carry generators put their carry in a non-flag-typed register
	// in their output.
	switch v.Op {
	case OpPPC64SUBC, OpPPC64ADDC, OpPPC64SUBCconst, OpPPC64ADDCconst:
	return true
	}
	return false
	}

	// hasFlagInput reports whether v has a flag value as any of its inputs.
	func (v *Value) hasFlagInput() bool {
	for _, a := range v.Args {
	if a.isFlagOp() {
	return true
	}
	}
	// PPC64 carry dependencies are conveyed through their final argument,
	// so we treat those operations as taking flags as well.
	switch v.Op {
	case OpPPC64SUBE, OpPPC64ADDE, OpPPC64SUBZEzero, OpPPC64ADDZE, OpPPC64ADDZEzero:
	return true
	}
	return false
	}

	type bySourcePos []*Value

	func (s bySourcePos) Len() int { return len(s) }
	func (s bySourcePos) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
	func (s bySourcePos) Less(i, j int) bool { return s[i].Pos.Before(s[j].Pos) }