[dev.ssa] cmd/compile: better register allocation
Use a more precise computation of next use. It properly
detects lifetime holes and deallocates values during those holes.
It also uses a more precise version of distance to next use which
affects which values get spilled.
Change-Id: I49eb3ebe2d2cb64842ecdaa7fb4f3792f8afb90b
Reviewed-on: https://go-review.googlesource.com/16760
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index a751d66..535885a 100644
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -202,41 +202,43 @@
}
}
-// A use is a record of a position (2*pc for value uses, odd numbers for other uses)
-// and a value ID that is used at that position.
type use struct {
- idx int32
- vid ID
+ dist int32 // distance from start of the block to a use of a value
+ next *use // linked list of uses of a value in nondecreasing dist order
}
type valState struct {
regs regMask // the set of registers holding a Value (usually just one)
- uses []int32 // sorted list of places where Value is used
- usestorage [2]int32
- spill *Value // spilled copy of the Value
- spill2 *Value // special alternate spill location used for phi resolution
+ uses *use // list of uses in this block
+ spill *Value // spilled copy of the Value
+ spill2 *Value // special alternate spill location used for phi resolution
spillUsed bool
spill2used bool
}
type regState struct {
v *Value // Original (preregalloc) Value stored in this register.
- c *Value // A Value equal to v which is currently in register. Might be v or a copy of it.
+ c *Value // A Value equal to v which is currently in a register. Might be v or a copy of it.
// If a register is unused, v==c==nil
}
type regAllocState struct {
f *Func
+ // For each value, whether it needs a register or not.
+ // Cached value of !v.Type.IsMemory() && !v.Type.IsVoid().
+ needReg []bool
+
// for each block, its primary predecessor.
// A predecessor of b is primary if it is the closest
// predecessor that appears before b in the layout order.
// We record the index in the Preds list where the primary predecessor sits.
primary []int32
- // live values on each edge. live[b.ID][idx] is a list of value IDs
- // which are live on b's idx'th successor edge.
- live [][][]ID
+ // live values at the end of each block. live[b.ID] is a list of value IDs
+ // which are live at the end of b, together with a count of how many instructions
+ // forward to the next use.
+ live [][]liveInfo
// current state of each (preregalloc) Value
values []valState
@@ -254,14 +256,14 @@
// mask of registers currently in use
used regMask
- // An ordered list (by idx) of all uses in the function
- uses []use
-
// Home locations (registers) for Values
home []Location
// current block we're working on
curBlock *Block
+
+ // cache of use records
+ freeUseRecords *use
}
// freeReg frees up register r. Any current user of r is kicked out.
@@ -350,18 +352,25 @@
// farthest-in-the-future use.
// TODO: Prefer registers with already spilled Values?
// TODO: Modify preference using affinity graph.
+ // TODO: if a single value is in multiple registers, spill one of them
+ // before spilling a value in just a single register.
// SP and SB are allocated specially. No regular value should
// be allocated to them.
mask &^= 1<<4 | 1<<32
+ // Find a register to spill. We spill the register containing the value
+ // whose next use is as far in the future as possible.
+ // https://en.wikipedia.org/wiki/Page_replacement_algorithm#The_theoretically_optimal_page_replacement_algorithm
maxuse := int32(-1)
for t := register(0); t < numRegs; t++ {
if mask>>t&1 == 0 {
continue
}
v := s.regs[t].v
- if len(s.values[v.ID].uses) == 0 {
+
+ if s.values[v.ID].uses == nil {
+ // No subsequent use.
// This can happen when fixing up merge blocks at the end.
// We've already run through the use lists so they are empty.
// Any register would be ok at this point.
@@ -369,7 +378,9 @@
maxuse = 0
break
}
- if n := s.values[v.ID].uses[0]; n > maxuse {
+ if n := s.values[v.ID].uses.dist; n > maxuse {
+ // v's next use is farther in the future than any value
+ // we've seen so far. A new best spill candidate.
r = t
maxuse = n
}
@@ -402,7 +413,12 @@
return s.regs[r].c
}
- mask &^= 1<<4 | 1<<32 // don't spill SP or SB
+ if v.Op != OpSP {
+ mask &^= 1 << 4 // dont' spill SP
+ }
+ if v.Op != OpSB {
+ mask &^= 1 << 32 // don't spill SB
+ }
mask &^= s.reserved()
// Allocate a register.
@@ -484,18 +500,20 @@
}
s.f = f
+ s.needReg = make([]bool, f.NumValues())
s.regs = make([]regState, numRegs)
s.values = make([]valState, f.NumValues())
- for i := range s.values {
- s.values[i].uses = s.values[i].usestorage[:0]
- }
s.orig = make([]*Value, f.NumValues())
for _, b := range f.Blocks {
for _, v := range b.Values {
+ if v.Type.IsMemory() || v.Type.IsVoid() {
+ continue
+ }
+ s.needReg[v.ID] = true
s.orig[v.ID] = v
}
}
- s.live = f.live()
+ s.computeLive()
// Compute block order. This array allows us to distinguish forward edges
// from backward edges and compute how far they go.
@@ -518,63 +536,41 @@
}
s.primary[b.ID] = int32(best)
}
+}
- // Compute uses. We assign a PC to each Value in the program, in f.Blocks
- // and then b.Values order. Uses are recorded using this numbering.
- // Uses by Values are recorded as 2*PC. Special uses (block control values,
- // pseudo-uses for backedges) are recorded as 2*(last PC in block)+1.
- var pc int32
- for _, b := range f.Blocks {
- // uses in regular Values
- for _, v := range b.Values {
- for _, a := range v.Args {
- s.values[a.ID].uses = append(s.values[a.ID].uses, pc*2)
- s.uses = append(s.uses, use{pc * 2, a.ID})
- }
- pc++
- }
- // use as a block control value
- endIdx := pc*2 - 1
- if b.Control != nil {
- s.values[b.Control.ID].uses = append(s.values[b.Control.ID].uses, endIdx)
- s.uses = append(s.uses, use{endIdx, b.Control.ID})
- }
- // uses by backedges
- // Backedges are treated as uses so that the uses span the entire live
- // range of the value.
- for i, c := range b.Succs {
- if blockOrder[c.ID] > blockOrder[b.ID] {
- continue // forward edge
- }
- for _, vid := range s.live[b.ID][i] {
- s.values[vid].uses = append(s.values[vid].uses, endIdx)
- s.uses = append(s.uses, use{endIdx, vid})
- }
- }
+// Adds a use record for id at distance dist from the start of the block.
+// All calls to addUse must happen with nonincreasing dist.
+func (s *regAllocState) addUse(id ID, dist int32) {
+ r := s.freeUseRecords
+ if r != nil {
+ s.freeUseRecords = r.next
+ } else {
+ r = &use{}
}
- if pc*2 < 0 {
- f.Fatalf("pc too large: function too big")
+ r.dist = dist
+ r.next = s.values[id].uses
+ s.values[id].uses = r
+ if r.next != nil && dist > r.next.dist {
+ s.f.Fatalf("uses added in wrong order")
}
}
-// clearUses drops any uses <= useIdx. Any values which have no future
-// uses are dropped from registers.
-func (s *regAllocState) clearUses(useIdx int32) {
- for len(s.uses) > 0 && s.uses[0].idx <= useIdx {
- idx := s.uses[0].idx
- vid := s.uses[0].vid
- s.uses = s.uses[1:]
-
- vi := &s.values[vid]
- if vi.uses[0] != idx {
- s.f.Fatalf("use mismatch for v%d\n", vid)
- }
- vi.uses = vi.uses[1:]
- if len(vi.uses) != 0 {
+// advanceUses advances the uses of v's args from the state before v to the state after v.
+// Any values which have no more uses are deallocated from registers.
+func (s *regAllocState) advanceUses(v *Value) {
+ for _, a := range v.Args {
+ if !s.needReg[a.ID] {
continue
}
- // Value is dead, free all registers that hold it (except SP & SB).
- s.freeRegs(vi.regs &^ (1<<4 | 1<<32))
+ ai := &s.values[a.ID]
+ r := ai.uses
+ ai.uses = r.next
+ if r.next == nil {
+ // Value is dead, free all registers that hold it.
+ s.freeRegs(ai.regs)
+ }
+ r.next = s.freeUseRecords
+ s.freeUseRecords = r
}
}
@@ -601,28 +597,69 @@
}
func (s *regAllocState) regalloc(f *Func) {
- liveset := newSparseSet(f.NumValues())
+ liveSet := newSparseSet(f.NumValues())
argset := newSparseSet(f.NumValues())
var oldSched []*Value
var phis []*Value
var stackPhis []*Value
var regPhis []*Value
+ var phiRegs []register
+ var args []*Value
if f.Entry != f.Blocks[0] {
f.Fatalf("entry block must be first")
}
- var phiRegs []register
-
// For each merge block, we record the starting register state (after phi ops)
// for that merge block. Indexed by blockid/regnum.
startRegs := make([][]*Value, f.NumBlocks())
// end state of registers for each block, idexed by blockid/regnum.
endRegs := make([][]regState, f.NumBlocks())
- var pc int32
for _, b := range f.Blocks {
s.curBlock = b
+ // Initialize liveSet and uses fields for this block.
+ // Walk backwards through the block doing liveness analysis.
+ liveSet.clear()
+ for _, e := range s.live[b.ID] {
+ s.addUse(e.ID, int32(len(b.Values))+e.dist) // pseudo-uses from beyond end of block
+ liveSet.add(e.ID)
+ }
+ if c := b.Control; c != nil && s.needReg[c.ID] {
+ s.addUse(c.ID, int32(len(b.Values))) // psuedo-use by control value
+ liveSet.add(c.ID)
+ }
+ for i := len(b.Values) - 1; i >= 0; i-- {
+ v := b.Values[i]
+ if v.Op == OpPhi {
+ break // Don't process phi ops.
+ }
+ liveSet.remove(v.ID)
+ for _, a := range v.Args {
+ if !s.needReg[a.ID] {
+ continue
+ }
+ s.addUse(a.ID, int32(i))
+ liveSet.add(a.ID)
+ }
+ }
+ if regDebug {
+ fmt.Printf("uses for %s:%s\n", s.f.Name, b)
+ for i := range s.values {
+ vi := &s.values[i]
+ u := vi.uses
+ if u == nil {
+ continue
+ }
+ fmt.Printf("v%d:", i)
+ for u != nil {
+ fmt.Printf(" %d", u.dist)
+ u = u.next
+ }
+ fmt.Println()
+ }
+ }
+
// Make a copy of the block schedule so we can generate a new one in place.
// We make a separate copy for phis and regular values.
nphi := 0
@@ -648,6 +685,15 @@
if nphi > 0 {
f.Fatalf("phis in single-predecessor block")
}
+ // Drop any values which are no longer live.
+ // This may happen because at the end of p, a value may be
+ // live but only used by some other successor of p.
+ for r := register(0); r < numRegs; r++ {
+ v := s.regs[r].v
+ if v != nil && !liveSet.contains(v.ID) {
+ s.freeReg(r)
+ }
+ }
} else {
// This is the complicated case. We have more than one predecessor,
// which means we may have Phi ops.
@@ -663,25 +709,6 @@
p := b.Preds[idx]
s.setState(endRegs[p.ID])
- // Drop anything not live on the c->b edge.
- var idx2 int
- for idx2 = 0; idx2 < len(p.Succs); idx2++ {
- if p.Succs[idx2] == b {
- break
- }
- }
- liveset.clear()
- liveset.addAll(s.live[p.ID][idx2])
- for r := register(0); r < numRegs; r++ {
- v := s.regs[r].v
- if v == nil {
- continue
- }
- if !liveset.contains(v.ID) {
- s.freeReg(r)
- }
- }
-
// Decide on registers for phi ops. Use the registers determined
// by the primary predecessor if we can.
// TODO: pick best of (already processed) predecessors?
@@ -742,21 +769,20 @@
}
// Process all the non-phi values.
- pc += int32(nphi)
- for _, v := range oldSched {
+ for idx, v := range oldSched {
if v.Op == OpPhi {
f.Fatalf("phi %s not at start of block", v)
}
if v.Op == OpSP {
s.assignReg(4, v, v) // TODO: arch-dependent
b.Values = append(b.Values, v)
- pc++
+ s.advanceUses(v)
continue
}
if v.Op == OpSB {
s.assignReg(32, v, v) // TODO: arch-dependent
b.Values = append(b.Values, v)
- pc++
+ s.advanceUses(v)
continue
}
if v.Op == OpArg {
@@ -766,19 +792,17 @@
s.values[v.ID].spill = v
s.values[v.ID].spillUsed = true // use is guaranteed
b.Values = append(b.Values, v)
- pc++
+ s.advanceUses(v)
continue
}
- s.clearUses(pc*2 - 1)
regspec := opcodeTable[v.Op].reg
if regDebug {
- fmt.Printf("%d: working on %s %s %v\n", pc, v, v.LongString(), regspec)
+ fmt.Printf("%d: working on %s %s %v\n", idx, v, v.LongString(), regspec)
}
if len(regspec.inputs) == 0 && len(regspec.outputs) == 0 {
// No register allocation required (or none specified yet)
s.freeRegs(regspec.clobbers)
b.Values = append(b.Values, v)
- pc++
continue
}
@@ -786,22 +810,23 @@
// Value is rematerializeable, don't issue it here.
// It will get issued just before each use (see
// allocValueToReg).
- pc++
+ s.advanceUses(v)
continue
}
- // Move arguments to registers
+ // Move arguments to registers. Process in an ordering defined
+ // by the register specification (most constrained first).
+ args = append(args[:0], v.Args...)
for _, i := range regspec.inputs {
- a := v.Args[i.idx]
- v.Args[i.idx] = s.allocValToReg(a, i.regs, true)
+ args[i.idx] = s.allocValToReg(v.Args[i.idx], i.regs, true)
}
// Now that all args are in regs, we're ready to issue the value itself.
- // Before we pick a register for the value, allow input registers
+ // Before we pick a register for the output value, allow input registers
// to be deallocated. We do this here so that the output can use the
// same register as a dying input.
s.nospill = 0
- s.clearUses(pc * 2)
+ s.advanceUses(v) // frees any registers holding args that are no longer live
// Dump any registers which will be clobbered
s.freeRegs(regspec.clobbers)
@@ -818,34 +843,60 @@
}
// Issue the Value itself.
+ for i, a := range args {
+ v.Args[i] = a // use register version of arguments
+ }
b.Values = append(b.Values, v)
// Issue a spill for this value. We issue spills unconditionally,
// then at the end of regalloc delete the ones we never use.
+ // TODO: schedule the spill at a point that dominates all restores.
+ // The restore may be off in an unlikely branch somewhere and it
+ // would be better to have the spill in that unlikely branch as well.
+ // v := ...
+ // if unlikely {
+ // f()
+ // }
+ // It would be good to have both spill and restore inside the IF.
if !v.Type.IsFlags() {
spill := b.NewValue1(v.Line, OpStoreReg, v.Type, v)
s.setOrig(spill, v)
s.values[v.ID].spill = spill
s.values[v.ID].spillUsed = false
}
-
- // Increment pc for next Value.
- pc++
}
- // Load control value into reg
- if b.Control != nil && !b.Control.Type.IsMemory() && !b.Control.Type.IsVoid() {
+ if c := b.Control; c != nil && s.needReg[c.ID] {
+ // Load control value into reg.
// TODO: regspec for block control values, instead of using
// register set from the control op's output.
- s.allocValToReg(b.Control, opcodeTable[b.Control.Op].reg.outputs[0], false)
+ s.allocValToReg(c, opcodeTable[c.Op].reg.outputs[0], false)
+ // Remove this use from the uses list.
+ u := s.values[c.ID].uses
+ s.values[c.ID].uses = u.next
+ u.next = s.freeUseRecords
+ s.freeUseRecords = u
}
// Record endRegs
endRegs[b.ID] = make([]regState, numRegs)
copy(endRegs[b.ID], s.regs)
- // Allow control Values and Values live only on backedges to be dropped.
- s.clearUses(pc*2 - 1)
+ // Clear any final uses.
+ // All that is left should be the pseudo-uses added for values which
+ // are live at the end of b.
+ for _, e := range s.live[b.ID] {
+ u := s.values[e.ID].uses
+ if u == nil {
+ f.Fatalf("live at end, no uses v%d", e.ID)
+ }
+ if u.next != nil {
+ f.Fatalf("live at end, too many uses v%d", e.ID)
+ }
+ s.values[e.ID].uses = nil
+ u.next = s.freeUseRecords
+ s.freeUseRecords = u
+ }
}
// Process merge block input edges. They are the tricky ones.
@@ -1034,20 +1085,24 @@
return false
}
-// live returns a map from block ID and successor edge index to a list
-// of value IDs live on that edge.
+type liveInfo struct {
+ ID ID // ID of variable
+ dist int32 // # of instructions before next use
+}
+
+// computeLive computes a map from block ID to a list of value IDs live at the end
+// of that block. Together with the value ID is a count of how many instructions
+// to the next use of that value. The resulting map is stored at s.live.
// 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 (f *Func) live() [][][]ID {
- live := make([][][]ID, f.NumBlocks())
- for _, b := range f.Blocks {
- live[b.ID] = make([][]ID, len(b.Succs))
- }
+func (s *regAllocState) computeLive() {
+ f := s.f
+ s.live = make([][]liveInfo, f.NumBlocks())
var phis []*Value
- s := newSparseSet(f.NumValues())
- t := newSparseSet(f.NumValues())
+ live := newSparseMap(f.NumValues())
+ t := newSparseMap(f.NumValues())
// Instead of iterating over f.Blocks, iterate over their postordering.
// Liveness information flows backward, so starting at the end
@@ -1061,20 +1116,22 @@
po := postorder(f)
for {
for _, b := range po {
- f.Logf("live %s %v\n", b, live[b.ID])
+ f.Logf("live %s %v\n", b, s.live[b.ID])
}
changed := false
for _, b := range po {
- // Start with known live values at the end of the block
- s.clear()
- for i := 0; i < len(b.Succs); i++ {
- s.addAll(live[b.ID][i])
+ // Start with known live values at the end of the block.
+ // Add len(b.Values) to adjust from end-of-block distance
+ // to beginning-of-block distance.
+ live.clear()
+ for _, e := range s.live[b.ID] {
+ live.set(e.ID, e.dist+int32(len(b.Values)))
}
// Mark control value as live
- if b.Control != nil {
- s.add(b.Control.ID)
+ if b.Control != nil && s.needReg[b.Control.ID] {
+ live.set(b.Control.ID, int32(len(b.Values)))
}
// Propagate backwards to the start of the block
@@ -1082,36 +1139,75 @@
phis := phis[:0]
for i := len(b.Values) - 1; i >= 0; i-- {
v := b.Values[i]
- s.remove(v.ID)
+ live.remove(v.ID)
if v.Op == OpPhi {
// save phi ops for later
phis = append(phis, v)
continue
}
- s.addAllValues(v.Args)
- }
-
- // 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, p := range b.Preds {
- // Find index of b in p's successors.
- var j int
- for j = 0; j < len(p.Succs); j++ {
- if p.Succs[j] == b {
- break
+ for _, a := range v.Args {
+ if s.needReg[a.ID] {
+ live.set(a.ID, int32(i))
}
}
- t.clear()
- t.addAll(live[p.ID][j])
- t.addAll(s.contents())
- for _, v := range phis {
- t.add(v.Args[i].ID)
+ }
+
+ // For each predecessor of b, expand its list of live-at-end values.
+ // invariant: live contains the values live at the start of b (excluding phi inputs)
+ for i, p := range b.Preds {
+ // Compute additional distance for the edge.
+ const normalEdge = 10
+ const likelyEdge = 1
+ const unlikelyEdge = 100
+ // Note: delta must be at least 1 to distinguish the control
+ // value use from the first user in a successor block.
+ delta := int32(normalEdge)
+ if len(p.Succs) == 2 {
+ if p.Succs[0] == b && p.Likely == BranchLikely ||
+ p.Succs[1] == b && p.Likely == BranchUnlikely {
+ delta = likelyEdge
+ }
+ if p.Succs[0] == b && p.Likely == BranchUnlikely ||
+ p.Succs[1] == b && p.Likely == BranchLikely {
+ delta = unlikelyEdge
+ }
}
- if t.size() == len(live[p.ID][j]) {
+
+ // Start t off with the previously known live values at the end of p.
+ t.clear()
+ for _, e := range s.live[p.ID] {
+ t.set(e.ID, e.dist)
+ }
+ update := false
+
+ // Add new live values from scanning this block.
+ for _, e := range live.contents() {
+ d := e.val + delta
+ if !t.contains(e.key) || d < t.get(e.key) {
+ update = true
+ t.set(e.key, d)
+ }
+ }
+ // Also add the correct arg from the saved phi values.
+ // All phis are at distance delta (we consider them
+ // simultaneously happening at the start of the block).
+ for _, v := range phis {
+ id := v.Args[i].ID
+ if s.needReg[id] && !t.contains(id) || delta < t.get(id) {
+ update = true
+ t.set(id, delta)
+ }
+ }
+
+ if !update {
continue
}
- // grow p's live set
- live[p.ID][j] = append(live[p.ID][j][:0], t.contents()...)
+ // The live set has changed, update it.
+ l := s.live[p.ID][:0]
+ for _, e := range t.contents() {
+ l = append(l, liveInfo{e.key, e.val})
+ }
+ s.live[p.ID] = l
changed = true
}
}
@@ -1120,35 +1216,6 @@
break
}
}
-
- // Make sure that there is only one live memory variable in each set.
- // Ideally we should check this at every instructiom, but at every
- // edge seems good enough for now.
- isMem := make([]bool, f.NumValues())
- for _, b := range f.Blocks {
- for _, v := range b.Values {
- isMem[v.ID] = v.Type.IsMemory()
- }
- }
- for _, b := range f.Blocks {
- for i, c := range b.Succs {
- nmem := 0
- for _, id := range live[b.ID][i] {
- if isMem[id] {
- nmem++
- }
- }
- if nmem > 1 {
- f.Fatalf("more than one mem live on edge %v->%v: %v", b, c, live[b.ID][i])
- }
- // TODO: figure out why we get nmem==0 occasionally.
- //if nmem == 0 {
- // f.Fatalf("no mem live on edge %v->%v: %v", b, c, live[b.ID][i])
- //}
- }
- }
-
- return live
}
// reserved returns a mask of reserved registers.
diff --git a/src/cmd/compile/internal/ssa/sparsemap.go b/src/cmd/compile/internal/ssa/sparsemap.go
new file mode 100644
index 0000000..6c0043b
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/sparsemap.go
@@ -0,0 +1,69 @@
+// 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
+
+// from http://research.swtch.com/sparse
+// in turn, from Briggs and Torczon
+
+type sparseEntry struct {
+ key ID
+ val int32
+}
+
+type sparseMap struct {
+ dense []sparseEntry
+ sparse []int
+}
+
+// newSparseMap returns a sparseMap that can map
+// integers between 0 and n-1 to int32s.
+func newSparseMap(n int) *sparseMap {
+ return &sparseMap{nil, make([]int, n)}
+}
+
+func (s *sparseMap) size() int {
+ return len(s.dense)
+}
+
+func (s *sparseMap) contains(k ID) bool {
+ i := s.sparse[k]
+ return i < len(s.dense) && s.dense[i].key == k
+}
+
+func (s *sparseMap) get(k ID) int32 {
+ i := s.sparse[k]
+ if i < len(s.dense) && s.dense[i].key == k {
+ return s.dense[i].val
+ }
+ return -1
+}
+
+func (s *sparseMap) set(k ID, v int32) {
+ i := s.sparse[k]
+ if i < len(s.dense) && s.dense[i].key == k {
+ s.dense[i].val = v
+ return
+ }
+ s.dense = append(s.dense, sparseEntry{k, v})
+ s.sparse[k] = len(s.dense) - 1
+}
+
+func (s *sparseMap) remove(k ID) {
+ i := s.sparse[k]
+ if i < len(s.dense) && s.dense[i].key == k {
+ y := s.dense[len(s.dense)-1]
+ s.dense[i] = y
+ s.sparse[y.key] = i
+ s.dense = s.dense[:len(s.dense)-1]
+ }
+}
+
+func (s *sparseMap) clear() {
+ s.dense = s.dense[:0]
+}
+
+func (s *sparseMap) contents() []sparseEntry {
+ return s.dense
+}
