cmd/compile/internal/ssa: only store relevant slots in pendingEntries

For functions with many local variables, keeping track of every
LocalSlot for every variable is very expensive. Only track the slots
that are actually used by a given variable.

Change-Id: Iaafbce030a782b8b8c4a0eb7cf025e59af899ea4
Reviewed-on: https://go-review.googlesource.com/92400Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/debug.go

@@ -214,10 +214,14 @@ func (state *debugState) initializeCache() {
 	state.changedVars = make([]bool, len(state.vars))
 
 	// A pending entry per user variable, with space to track each of its pieces.
-	if want := len(state.vars) * len(state.slots); cap(state.cache.pendingSlotLocs) < want {
-		state.cache.pendingSlotLocs = make([]VarLoc, want)
+	nPieces := 0
+	for i := range state.varSlots {
+		nPieces += len(state.varSlots[i])
 	}
-	psl := state.cache.pendingSlotLocs[:len(state.vars)*len(state.slots)]
+	if cap(state.cache.pendingSlotLocs) < nPieces {
+		state.cache.pendingSlotLocs = make([]VarLoc, nPieces)
+	}
+	psl := state.cache.pendingSlotLocs[:nPieces]
 	for i := range psl {
 		psl[i] = VarLoc{}
 	}
@@ -225,10 +229,12 @@ func (state *debugState) initializeCache() {
 		state.cache.pendingEntries = make([]pendingEntry, len(state.vars))
 	}
 	pe := state.cache.pendingEntries[:len(state.vars)]
-	for varID := range pe {
+	freePieceIdx := 0
+	for varID, slots := range state.varSlots {
 		pe[varID] = pendingEntry{
-			pieces: state.cache.pendingSlotLocs[varID*len(state.slots) : (varID+1)*len(state.slots)],
+			pieces: state.cache.pendingSlotLocs[freePieceIdx : freePieceIdx+len(slots)],
 		}
+		freePieceIdx += len(slots)
 	}
 }
 
@@ -657,9 +663,8 @@ func (a partsByVarOffset) Swap(i, j int) { a.slotIDs[i], a.slotIDs[j] = a.slotID
 type pendingEntry struct {
 	present                bool
 	startBlock, startValue ID
-	// The location of each piece of the variable, indexed by *SlotID*,
-	// even though only a few slots are used in each entry. This could be
-	// improved by only storing the relevant slots.
+	// The location of each piece of the variable, in the same order as the
+	// SlotIDs in varParts.
 	pieces []VarLoc
 }
 
@@ -736,14 +741,14 @@ func (state *debugState) buildLocationLists(Ctxt *obj.Link, blockLocs []*BlockDe
 
 		if state.loggingEnabled {
 			var partStrs []string
-			for _, slot := range state.varSlots[varID] {
-				partStrs = append(partStrs, fmt.Sprintf("%v@%v", state.slots[slot], state.LocString(pending.pieces[slot])))
+			for i, slot := range state.varSlots[varID] {
+				partStrs = append(partStrs, fmt.Sprintf("%v@%v", state.slots[slot], state.LocString(pending.pieces[i])))
 			}
 			state.logf("Add entry for %v: \tb%vv%v-b%vv%v = \t%v\n", state.vars[varID], pending.startBlock, pending.startValue, endBlock, endValue, strings.Join(partStrs, " "))
 		}
 
-		for _, slotID := range state.varSlots[varID] {
-			loc := pending.pieces[slotID]
+		for i, slotID := range state.varSlots[varID] {
+			loc := pending.pieces[i]
 			slot := state.slots[slotID]
 
 			if !loc.absent() {
@@ -795,8 +800,8 @@ func (state *debugState) buildLocationLists(Ctxt *obj.Link, blockLocs []*BlockDe
 		// Extend the previous entry if possible.
 		if pending.present {
 			merge := true
-			for _, slotID := range state.varSlots[varID] {
-				if !canMerge(pending.pieces[slotID], curLoc[slotID]) {
+			for i, slotID := range state.varSlots[varID] {
+				if !canMerge(pending.pieces[i], curLoc[slotID]) {
 					merge = false
 					break
 				}
@@ -810,7 +815,9 @@ func (state *debugState) buildLocationLists(Ctxt *obj.Link, blockLocs []*BlockDe
 		pending.present = true
 		pending.startBlock = v.Block.ID
 		pending.startValue = v.ID
-		copy(pending.pieces, curLoc)
+		for i, slot := range state.varSlots[varID] {
+			pending.pieces[i] = curLoc[slot]
+		}
 		return
 
 	}