cmd/compile: introduce LivenessMap and LivenessIndex

Currently liveness only produces a stack map index at each safe point,
so the information is summarized in a map[*ssa.Value]int. We're about
to have both a stack map index and a register map index, so replace
the int with a LivenessIndex type we can extend, and replace the map
with a LivenessMap that we can also change more easily in the future.

This also gives us an easy hook for defining the value that means "not
a safe point".

Passes toolstash -cmp.

For #24543.

Change-Id: Ic4c069839635efed4fd0f603899b80f8be3b56ec
Reviewed-on: https://go-review.googlesource.com/109347
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -120,14 +120,42 @@ type Liveness struct {
 	// Indexed sequentially by safe points in Block and Value order.
 	livevars []bvec
 
-	// stackMapIndex maps from safe points (i.e., CALLs) to their
-	// index within stackMaps.
-	stackMapIndex map[*ssa.Value]int
+	// livenessMap maps from safe points (i.e., CALLs) to their
+	// liveness map indexes.
+	//
+	// TODO(austin): Now that we have liveness at almost every PC,
+	// should this be a dense structure?
+	livenessMap LivenessMap
 	stackMaps   []bvec
 
 	cache progeffectscache
 }
 
+// LivenessMap maps from *ssa.Value to LivenessIndex.
+type LivenessMap struct {
+	m map[*ssa.Value]LivenessIndex
+}
+
+func (m LivenessMap) Get(v *ssa.Value) LivenessIndex {
+	if i, ok := m.m[v]; ok {
+		return i
+	}
+	// Not a safe point.
+	return LivenessInvalid
+}
+
+// LivenessIndex stores the liveness map index for a safe-point.
+type LivenessIndex struct {
+	stackMapIndex int
+}
+
+// LivenessInvalid indicates an unsafe point.
+var LivenessInvalid = LivenessIndex{-1}
+
+func (idx LivenessIndex) Valid() bool {
+	return idx.stackMapIndex >= 0
+}
+
 type progeffectscache struct {
 	textavarinit []int32
 	retuevar     []int32
@@ -817,10 +845,10 @@ func (lv *Liveness) clobber() {
 				before = false
 			}
 			if before {
-				clobber(lv, b, lv.stackMaps[lv.stackMapIndex[v]])
+				clobber(lv, b, lv.stackMaps[lv.livenessMap.Get(v).stackMapIndex])
 			}
 			b.Values = append(b.Values, v)
-			clobber(lv, b, lv.stackMaps[lv.stackMapIndex[v]])
+			clobber(lv, b, lv.stackMaps[lv.livenessMap.Get(v).stackMapIndex])
 		}
 	}
 }
@@ -1006,12 +1034,12 @@ Outer:
 	// These will later become PCDATA instructions.
 	lv.showlive(nil, lv.stackMaps[0])
 	pos := 1
-	lv.stackMapIndex = make(map[*ssa.Value]int)
+	lv.livenessMap = LivenessMap{make(map[*ssa.Value]LivenessIndex)}
 	for _, b := range lv.f.Blocks {
 		for _, v := range b.Values {
 			if issafepoint(v) {
 				lv.showlive(v, lv.stackMaps[remap[pos]])
-				lv.stackMapIndex[v] = remap[pos]
+				lv.livenessMap.m[v] = LivenessIndex{remap[pos]}
 				pos++
 			}
 		}
@@ -1153,8 +1181,8 @@ func (lv *Liveness) printDebug() {
 		for _, v := range b.Values {
 			fmt.Printf("(%s) %v\n", linestr(v.Pos), v.LongString())
 
-			if pos, ok := lv.stackMapIndex[v]; ok {
-				pcdata = pos
+			if pos := lv.livenessMap.Get(v); pos.Valid() {
+				pcdata = pos.stackMapIndex
 			}
 
 			pos, effect := lv.valueEffects(v)
@@ -1265,7 +1293,7 @@ func (lv *Liveness) emit(argssym, livesym *obj.LSym) {
 // pointer variables in the function and emits a runtime data
 // structure read by the garbage collector.
 // Returns a map from GC safe points to their corresponding stack map index.
-func liveness(e *ssafn, f *ssa.Func) map[*ssa.Value]int {
+func liveness(e *ssafn, f *ssa.Func) LivenessMap {
 	// Construct the global liveness state.
 	vars, idx := getvariables(e.curfn)
 	lv := newliveness(e.curfn, f, vars, idx, e.stkptrsize)
@@ -1284,5 +1312,5 @@ func liveness(e *ssafn, f *ssa.Func) map[*ssa.Value]int {
 	if ls := e.curfn.Func.lsym; ls != nil {
 		lv.emit(&ls.Func.GCArgs, &ls.Func.GCLocals)
 	}
-	return lv.stackMapIndex
+	return lv.livenessMap
 }

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

@@ -4694,9 +4694,9 @@ type SSAGenState struct {
 
 	maxarg int64 // largest frame size for arguments to calls made by the function
 
-	// Map from GC safe points to stack map index, generated by
+	// Map from GC safe points to liveness index, generated by
 	// liveness analysis.
-	stackMapIndex map[*ssa.Value]int
+	livenessMap LivenessMap
 
 	// lineRunStart records the beginning of the current run of instructions
 	// within a single block sharing the same line number
@@ -4772,7 +4772,7 @@ func genssa(f *ssa.Func, pp *Progs) {
 
 	e := f.Frontend().(*ssafn)
 
-	s.stackMapIndex = liveness(e, f)
+	s.livenessMap = liveness(e, f)
 
 	// Remember where each block starts.
 	s.bstart = make([]*obj.Prog, f.NumBlocks())
@@ -5270,13 +5270,13 @@ func (s *SSAGenState) Call(v *ssa.Value) *obj.Prog {
 // It must be called immediately before emitting the actual CALL instruction,
 // since it emits PCDATA for the stack map at the call (calls are safe points).
 func (s *SSAGenState) PrepareCall(v *ssa.Value) {
-	idx, ok := s.stackMapIndex[v]
-	if !ok {
+	idx := s.livenessMap.Get(v)
+	if !idx.Valid() {
 		Fatalf("missing stack map index for %v", v.LongString())
 	}
 	p := s.Prog(obj.APCDATA)
 	Addrconst(&p.From, objabi.PCDATA_StackMapIndex)
-	Addrconst(&p.To, int64(idx))
+	Addrconst(&p.To, int64(idx.stackMapIndex))
 
 	if sym, _ := v.Aux.(*obj.LSym); sym == Deferreturn {
 		// Deferred calls will appear to be returning to