cmd/compile/internal: use sparseSet, optimize isSynthetic

changedVars was functionally a set, but couldn't be iterated over
efficiently. In functions with many variables, the wasted iteration was
costly. Use a sparseSet instead.

(*gc.Node).String() is very expensive: it calls Sprintf, which does
reflection, etc, etc. Instead, just look at .Sym.Name, which is all we
care about.

Change-Id: Ib61cd7b5c796e1813b8859135e85da5bfe2ac686
Reviewed-on: https://go-review.googlesource.com/92402Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/syntax.go

@@ -59,6 +59,11 @@ type Node struct {
 	Etype types.EType // op for OASOP, etype for OTYPE, exclam for export, 6g saved reg, ChanDir for OTCHAN, for OINDEXMAP 1=LHS,0=RHS
 }
 
+func (n *Node) IsSynthetic() bool {
+	name := n.Sym.Name
+	return name[0] == '.' || name[0] == '~'
+}
+
 // IsAutoTmp indicates if n was created by the compiler as a temporary,
 // based on the setting of the .AutoTemp flag in n's Name.
 func (n *Node) IsAutoTmp() bool {

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

@@ -142,6 +142,7 @@ type Frontend interface {
 type GCNode interface {
 	Typ() *types.Type
 	String() string
+	IsSynthetic() bool
 	StorageClass() StorageClass
 }
 

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

@@ -183,7 +183,7 @@ type debugState struct {
 	// The current state of whatever analysis is running.
 	currentState stateAtPC
 	liveCount    []int
-	changedVars  []bool
+	changedVars  *sparseSet
 }
 
 func (state *debugState) initializeCache() {
@@ -226,7 +226,7 @@ func (state *debugState) initializeCache() {
 	state.liveCount = make([]int, len(state.slots))
 
 	// A relatively small slice, but used many times as the return from processValue.
-	state.changedVars = make([]bool, len(state.vars))
+	state.changedVars = newSparseSet(len(state.vars))
 
 	// A pending entry per user variable, with space to track each of its pieces.
 	nPieces := 0
@@ -310,7 +310,7 @@ func BuildFuncDebug(ctxt *obj.Link, f *Func, loggingEnabled bool, stackOffset fu
 	for i, slot := range f.Names {
 		slot := slot
 		state.slots[i] = &slot
-		if isSynthetic(&slot) {
+		if slot.N.IsSynthetic() {
 			continue
 		}
 
@@ -338,7 +338,7 @@ func BuildFuncDebug(ctxt *obj.Link, f *Func, loggingEnabled bool, stackOffset fu
 
 	state.initializeCache()
 	for i, slot := range f.Names {
-		if isSynthetic(&slot) {
+		if slot.N.IsSynthetic() {
 			continue
 		}
 		for _, value := range f.NamedValues[slot] {
@@ -357,13 +357,6 @@ func BuildFuncDebug(ctxt *obj.Link, f *Func, loggingEnabled bool, stackOffset fu
 	}
 }
 
-// isSynthetic reports whether if slot represents a compiler-inserted variable,
-// e.g. an autotmp or an anonymous return value that needed a stack slot.
-func isSynthetic(slot *LocalSlot) bool {
-	c := slot.N.String()[0]
-	return c == '.' || c == '~'
-}
-
 // liveness walks the function in control flow order, calculating the start
 // and end state of each block.
 func (state *debugState) liveness() []*BlockDebug {
@@ -519,16 +512,15 @@ func (state *debugState) mergePredecessors(b *Block, blockLocs []*BlockDebug) ([
 
 	// A slot is live if it was seen in all predecessors, and they all had
 	// some storage in common.
-	for slotID, slotLoc := range slotLocs {
-		// Not seen in any predecessor.
-		if slotLoc.absent() {
-			continue
-		}
-		// Seen in only some predecessors. Clear it out.
+	for slotID := range p0 {
+		slotLoc := slotLocs[slotID]
+
 		if state.liveCount[slotID] != len(preds) {
+			// Seen in only some predecessors. Clear it out.
 			slotLocs[slotID] = VarLoc{}
 			continue
 		}
+
 		// Present in all predecessors.
 		mask := uint64(slotLoc.Registers)
 		for {
@@ -553,7 +545,7 @@ func (state *debugState) processValue(v *Value, vSlots []SlotID, vReg *Register)
 	changed := false
 	setSlot := func(slot SlotID, loc VarLoc) {
 		changed = true
-		state.changedVars[state.slotVars[slot]] = true
+		state.changedVars.add(ID(state.slotVars[slot]))
 		state.currentState.slots[slot] = loc
 	}
 
@@ -860,13 +852,10 @@ func (state *debugState) buildLocationLists(Ctxt *obj.Link, blockLocs []*BlockDe
 			}
 
 			phisPending = false
-			for varID := range state.changedVars {
-				if !state.changedVars[varID] {
-					continue
-				}
-				state.changedVars[varID] = false
+			for _, varID := range state.changedVars.contents() {
 				updateVar(VarID(varID), v, state.currentState.slots)
 			}
+			state.changedVars.clear()
 		}
 
 	}

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

@@ -79,6 +79,10 @@ func (d *DummyAuto) StorageClass() StorageClass {
 	return ClassAuto
 }
 
+func (d *DummyAuto) IsSynthetic() bool {
+	return false
+}
+
 func (DummyFrontend) StringData(s string) interface{} {
 	return nil
 }