cmd/compile: use correct stack slots in location lists

When variables need to be spilled to the stack, they usually get their
own stack slot. Local variables have a slot allocated if they need one,
and arguments start out on the stack. Before this CL, the debug
information made the assumption that this was always the case, and so
didn't bother storing an actual stack offset during SSA analysis.

There's at least one case where this isn't true: variables that alias
arguments. Since the argument is the source of the variable, the
variable will begin its life on the stack in the argument's stack slot,
not its own. Therefore the debug info needs to track the actual stack
slot for each location entry.

No detectable performance change, despite the O(N) loop in getHomeSlot.

Change-Id: I2701adb7eddee17d4524336cb7aa6786e8f32b46
Reviewed-on: https://go-review.googlesource.com/67231Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -437,7 +437,7 @@ func createComplexVars(fnsym *obj.LSym, debugInfo *ssa.FuncDebug) ([]*Node, []*d
 
 	// Group SSA variables by the user variable they were decomposed from.
 	varParts := map[*Node][]varPart{}
-	for slotID, slot := range debugInfo.Slots {
+	for slotID, slot := range debugInfo.VarSlots {
 		for slot.SplitOf != nil {
 			slot = slot.SplitOf
 		}
@@ -450,7 +450,7 @@ func createComplexVars(fnsym *obj.LSym, debugInfo *ssa.FuncDebug) ([]*Node, []*d
 	// createComplexVar has side effects. Instead, go by slot.
 	var decls []*Node
 	var vars []*dwarf.Var
-	for _, slot := range debugInfo.Slots {
+	for _, slot := range debugInfo.VarSlots {
 		for slot.SplitOf != nil {
 			slot = slot.SplitOf
 		}
@@ -490,6 +490,26 @@ func (a partsByVarOffset) Len() int           { return len(a) }
 func (a partsByVarOffset) Less(i, j int) bool { return a[i].varOffset < a[j].varOffset }
 func (a partsByVarOffset) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
 
+// stackOffset returns the stack location of a LocalSlot relative to the
+// stack pointer, suitable for use in a DWARF location entry. This has nothing
+// to do with its offset in the user variable.
+func stackOffset(slot *ssa.LocalSlot) int32 {
+	n := slot.N.(*Node)
+	var base int64
+	switch n.Class() {
+	case PAUTO:
+		if Ctxt.FixedFrameSize() == 0 {
+			base -= int64(Widthptr)
+		}
+		if objabi.Framepointer_enabled(objabi.GOOS, objabi.GOARCH) {
+			base -= int64(Widthptr)
+		}
+	case PPARAM, PPARAMOUT:
+		base += Ctxt.FixedFrameSize()
+	}
+	return int32(base + n.Xoffset + slot.Off)
+}
+
 // createComplexVar builds a DWARF variable entry and location list representing n.
 func createComplexVar(debugInfo *ssa.FuncDebug, n *Node, parts []varPart) *dwarf.Var {
 	slots := debugInfo.Slots
@@ -514,14 +534,15 @@ func createComplexVar(debugInfo *ssa.FuncDebug, n *Node, parts []varPart) *dwarf
 
 	gotype := ngotype(n).Linksym()
 	typename := dwarf.InfoPrefix + gotype.Name[len("type."):]
-	// The stack offset is used as a sorting key, so for decomposed
-	// variables just give it the lowest one. It's not used otherwise.
-	stackOffset := debugInfo.Slots[parts[0].slot].N.(*Node).Xoffset + offs
 	dvar := &dwarf.Var{
-		Name:        n.Sym.Name,
-		Abbrev:      abbrev,
-		Type:        Ctxt.Lookup(typename),
-		StackOffset: int32(stackOffset),
+		Name:   n.Sym.Name,
+		Abbrev: abbrev,
+		Type:   Ctxt.Lookup(typename),
+		// The stack offset is used as a sorting key, so for decomposed
+		// variables just give it the lowest one. It's not used otherwise.
+		// This won't work well if the first slot hasn't been assigned a stack
+		// location, but it's not obvious how to do better.
+		StackOffset: int32(stackOffset(slots[parts[0].slot])),
 		DeclLine:    n.Pos.Line(),
 	}
 
@@ -666,7 +687,7 @@ func createComplexVar(debugInfo *ssa.FuncDebug, n *Node, parts []varPart) *dwarf
 			}
 			if loc.OnStack {
 				dpiece.OnStack = true
-				dpiece.StackOffset = int32(offs + slots[part.slot].Off + slots[part.slot].N.(*Node).Xoffset)
+				dpiece.StackOffset = stackOffset(slots[loc.StackLocation])
 			} else {
 				for reg := 0; reg < len(debugInfo.Registers); reg++ {
 					if loc.Registers&(1<<uint8(reg)) != 0 {

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

@@ -15,9 +15,12 @@ type SlotID int32
 // function. Variables are identified by their LocalSlot, which may be the
 // result of decomposing a larger variable.
 type FuncDebug struct {
-	// Slots are all the slots in the function, indexed by their SlotID as
-	// used in various functions and parallel to BlockDebug.Variables.
+	// Slots is all the slots used in the debug info, indexed by their SlotID.
+	// Use this when getting a LocalSlot from a SlotID.
 	Slots []*LocalSlot
+	// VarSlots is the slots that represent part of user variables.
+	// Use this when iterating over all the slots to generate debug information.
+	VarSlots []*LocalSlot
 	// The blocks in the function, in program text order.
 	Blocks []*BlockDebug
 	// The registers of the current architecture, indexed by Register.num.
@@ -27,11 +30,11 @@ type FuncDebug struct {
 func (f *FuncDebug) BlockString(b *BlockDebug) string {
 	var vars []string
 
-	for slot, list := range b.Variables {
-		if len(list.Locations) == 0 {
+	for slot := range f.VarSlots {
+		if len(b.Variables[slot].Locations) == 0 {
 			continue
 		}
-		vars = append(vars, fmt.Sprintf("%v = %v", f.Slots[slot], list))
+		vars = append(vars, fmt.Sprintf("%v = %v", f.Slots[slot], b.Variables[slot]))
 	}
 	return fmt.Sprintf("{%v}", strings.Join(vars, ", "))
 }
@@ -159,9 +162,10 @@ type VarLoc struct {
 	// The registers this variable is available in. There can be more than
 	// one in various situations, e.g. it's being moved between registers.
 	Registers RegisterSet
-	// Indicates whether the variable is on the stack. The stack position is
-	// stored in the associated gc.Node.
-	OnStack bool
+	// OnStack indicates that the variable is on the stack in the LocalSlot
+	// identified by StackLocation.
+	OnStack       bool
+	StackLocation SlotID
 }
 
 var BlockStart = &Value{
@@ -194,6 +198,7 @@ func (s *debugState) logf(msg string, args ...interface{}) {
 type debugState struct {
 	loggingEnabled bool
 	slots          []*LocalSlot
+	varSlots       []*LocalSlot
 	f              *Func
 	cache          *Cache
 	numRegisters   int
@@ -202,9 +207,24 @@ type debugState struct {
 	registerContents [][]SlotID
 }
 
+// getHomeSlot returns the SlotID of the home slot for v, adding to s.slots
+// if necessary.
+func (s *debugState) getHomeSlot(v *Value) SlotID {
+	home := s.f.getHome(v.ID).(LocalSlot)
+	for id, slot := range s.slots {
+		if *slot == home {
+			return SlotID(id)
+		}
+	}
+	// This slot wasn't in the NamedValue table so it needs to be added.
+	s.slots = append(s.slots, &home)
+	return SlotID(len(s.slots) - 1)
+}
+
 func (s *debugState) BlockString(b *BlockDebug) string {
 	f := &FuncDebug{
 		Slots:     s.slots,
+		VarSlots:  s.varSlots,
 		Registers: s.f.Config.registers,
 	}
 	return f.BlockString(b)
@@ -239,6 +259,9 @@ func BuildFuncDebug(f *Func, loggingEnabled bool) *FuncDebug {
 			valueNames[value.ID] = append(valueNames[value.ID], SlotID(i))
 		}
 	}
+	// state.varSlots is never changed, and state.slots is only appended to,
+	// so aliasing is safe.
+	state.varSlots = state.slots
 
 	if state.loggingEnabled {
 		var names []string
@@ -335,6 +358,7 @@ func BuildFuncDebug(f *Func, loggingEnabled bool) *FuncDebug {
 
 	info := &FuncDebug{
 		Slots:     state.slots,
+		VarSlots:  state.varSlots,
 		Registers: f.Config.registers,
 	}
 	// Consumers want the information in textual order, not by block ID.
@@ -344,7 +368,7 @@ func BuildFuncDebug(f *Func, loggingEnabled bool) *FuncDebug {
 
 	if state.loggingEnabled {
 		f.Logf("Final result:\n")
-		for slot := range info.Slots {
+		for slot := range info.VarSlots {
 			f.Logf("\t%v => %v\n", info.Slots[slot], info.SlotLocsString(SlotID(slot)))
 		}
 	}
@@ -406,6 +430,7 @@ func (state *debugState) mergePredecessors(b *Block, blockLocs []*BlockDebug) *B
 			loc := state.cache.NewVarLoc()
 			loc.Start = BlockStart
 			loc.OnStack = last.OnStack
+			loc.StackLocation = last.StackLocation
 			loc.Registers = last.Registers
 			live[slot].append(loc)
 		}
@@ -433,7 +458,10 @@ func (state *debugState) mergePredecessors(b *Block, blockLocs []*BlockDebug) *B
 			}
 
 			// Unify storage locations.
-			liveLoc.OnStack = liveLoc.OnStack && predLoc.OnStack
+			if !liveLoc.OnStack || !predLoc.OnStack || liveLoc.StackLocation != predLoc.StackLocation {
+				liveLoc.OnStack = false
+				liveLoc.StackLocation = 0
+			}
 			liveLoc.Registers &= predLoc.Registers
 		}
 	}
@@ -506,6 +534,7 @@ func (state *debugState) processValue(locs *BlockDebug, v *Value, vSlots []SlotI
 			loc := state.cache.NewVarLoc()
 			loc.Start = v
 			loc.OnStack = last.OnStack
+			loc.StackLocation = last.StackLocation
 			loc.Registers = regs
 			locs.append(slot, loc)
 		}
@@ -515,20 +544,18 @@ func (state *debugState) processValue(locs *BlockDebug, v *Value, vSlots []SlotI
 				state.unexpected(v, "Arg op on already-live slot %v", state.slots[slot])
 				last.End = v
 			}
-			if state.loggingEnabled {
-				state.logf("at %v: %v now on stack from arg\n", v.ID, state.slots[slot])
-			}
 			loc := state.cache.NewVarLoc()
 			loc.Start = v
 			loc.OnStack = true
+			loc.StackLocation = state.getHomeSlot(v)
 			locs.append(slot, loc)
+			if state.loggingEnabled {
+				state.logf("at %v: arg %v now on stack in location %v\n", v.ID, state.slots[slot], state.slots[loc.StackLocation])
+			}
 		}
 
 	case v.Op == OpStoreReg:
 		for _, slot := range vSlots {
-			if state.loggingEnabled {
-				state.logf("at %v: %v spilled to stack\n", v.ID, state.slots[slot])
-			}
 			last := locs.lastLoc(slot)
 			if last == nil {
 				state.unexpected(v, "spill of unnamed register %s\n", vReg)
@@ -538,8 +565,13 @@ func (state *debugState) processValue(locs *BlockDebug, v *Value, vSlots []SlotI
 			loc := state.cache.NewVarLoc()
 			loc.Start = v
 			loc.OnStack = true
+			loc.StackLocation = state.getHomeSlot(v)
 			loc.Registers = last.Registers
 			locs.append(slot, loc)
+			if state.loggingEnabled {
+				state.logf("at %v: %v spilled to stack location %v\n", v.ID, state.slots[slot], state.slots[loc.StackLocation])
+			}
+
 		}
 
 	case vReg != nil:
@@ -569,6 +601,7 @@ func (state *debugState) processValue(locs *BlockDebug, v *Value, vSlots []SlotI
 			loc.Start = v
 			if last != nil {
 				loc.OnStack = last.OnStack
+				loc.StackLocation = last.StackLocation
 				loc.Registers = last.Registers
 			}
 			loc.Registers |= 1 << uint8(vReg.num)