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// 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"
"cmd/internal/src"
"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
if x.Block == x.Block.Func.Entry && x.Pos.IsStmt() != y.Pos.IsStmt() {
// In the entry block, put statement-marked instructions earlier.
return x.Pos.IsStmt() == src.PosIsStmt && y.Pos.IsStmt() != src.PosIsStmt
}
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.
for _, b := range f.Blocks {
for _, v := range b.Values {
for i, a := range v.Args {
if a.Op == OpSPanchored {
v.SetArg(i, a.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 {
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, 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) }