8 files changed, 694 insertions, 22 deletions

diff --git a/src/cmd/compile/internal/ssa/cache.go b/src/cmd/compile/internal/ssa/cache.go
index f1018da497..8434084bde 100644
--- a/src/cmd/compile/internal/ssa/cache.go
+++ b/src/cmd/compile/internal/ssa/cache.go
@@ -14,6 +14,11 @@ type Cache struct {
 	blocks [200]Block
 	locs   [2000]Location
 
+	// Storage for DWARF variable locations. Lazily allocated
+	// since location lists are off by default.
+	varLocs   []VarLoc
+	curVarLoc int
+
 	// Reusable stackAllocState.
 	// See stackalloc.go's {new,put}StackAllocState.
 	stackAllocState *stackAllocState
@@ -38,4 +43,21 @@ func (c *Cache) Reset() {
 	for i := range xl {
 		xl[i] = nil
 	}
+	xvl := c.varLocs[:c.curVarLoc]
+	for i := range xvl {
+		xvl[i] = VarLoc{}
+	}
+	c.curVarLoc = 0
+}
+
+func (c *Cache) NewVarLoc() *VarLoc {
+	if c.varLocs == nil {
+		c.varLocs = make([]VarLoc, 4000)
+	}
+	if c.curVarLoc == len(c.varLocs) {
+		return &VarLoc{}
+	}
+	vl := &c.varLocs[c.curVarLoc]
+	c.curVarLoc++
+	return vl
 }
diff --git a/src/cmd/compile/internal/ssa/debug.go b/src/cmd/compile/internal/ssa/debug.go
new file mode 100644
index 0000000000..55db45b642
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/debug.go
@@ -0,0 +1,559 @@
+// Copyright 2017 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
+
+import (
+	"cmd/internal/obj"
+	"fmt"
+	"strings"
+)
+
+type SlotID int32
+
+// A FuncDebug contains all the debug information for the variables in a
+// function. Variables are identified by their LocalSlot, which may be the
+// result of decomposing a larger variable.
+type FuncDebug struct {
+	Slots     []*LocalSlot
+	Variables []VarLocList
+	Registers []Register
+}
+
+// append adds a location to the location list for slot.
+func (f *FuncDebug) append(slot SlotID, loc *VarLoc) {
+	f.Variables[slot].append(loc)
+}
+
+// lastLoc returns the last VarLoc for slot, or nil if it has none.
+func (f *FuncDebug) lastLoc(slot SlotID) *VarLoc {
+	return f.Variables[slot].last()
+}
+
+func (f *FuncDebug) String() string {
+	var vars []string
+	for slot, list := range f.Variables {
+		if len(list.Locations) == 0 {
+			continue
+		}
+		vars = append(vars, fmt.Sprintf("%v = %v", f.Slots[slot], list))
+	}
+	return fmt.Sprintf("{%v}", strings.Join(vars, ", "))
+}
+
+// A VarLocList contains the locations for a variable, in program text order.
+// It will often have gaps.
+type VarLocList struct {
+	Locations []*VarLoc
+}
+
+func (l *VarLocList) append(loc *VarLoc) {
+	l.Locations = append(l.Locations, loc)
+}
+
+// last returns the last location in the list.
+func (l *VarLocList) last() *VarLoc {
+	if l == nil || len(l.Locations) == 0 {
+		return nil
+	}
+	return l.Locations[len(l.Locations)-1]
+}
+
+// A VarLoc describes a variable's location in a single contiguous range
+// of program text. It is generated from the SSA representation, but it
+// refers to the generated machine code, so the Values referenced are better
+// understood as PCs than actual Values, and the ranges can cross blocks.
+// The range is defined first by Values, which are then mapped to Progs
+// during genssa and finally to function PCs after assembly.
+// A variable can be on the stack and in any number of registers.
+type VarLoc struct {
+	// Inclusive -- the first SSA value that the range covers. The value
+	// doesn't necessarily have anything to do with the variable; it just
+	// identifies a point in the program text.
+	Start *Value
+	// Exclusive -- the first SSA value after start that the range doesn't
+	// cover. A location with start == end is empty.
+	End *Value
+	// The prog/PCs corresponding to Start and End above. These are for the
+	// convenience of later passes, since code generation isn't done when
+	// BuildFuncDebug runs.
+	StartProg, EndProg *obj.Prog
+	StartPC, EndPC     int64
+
+	// 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
+
+	// Used only during generation. Indicates whether this location lasts
+	// past the block's end. Without this, there would be no way to distinguish
+	// between a range that ended on the last Value of a block and one that
+	// didn't end at all.
+	survivedBlock bool
+}
+
+// RegisterSet is a bitmap of registers, indexed by Register.num.
+type RegisterSet uint64
+
+func (v *VarLoc) String() string {
+	var registers []Register
+	if v.Start != nil {
+		registers = v.Start.Block.Func.Config.registers
+	}
+	loc := ""
+	if !v.OnStack && v.Registers == 0 {
+		loc = "!!!no location!!!"
+	}
+	if v.OnStack {
+		loc += "stack,"
+	}
+	var regnames []string
+	for reg := 0; reg < 64; reg++ {
+		if v.Registers&(1<<uint8(reg)) == 0 {
+			continue
+		}
+		if registers != nil {
+			regnames = append(regnames, registers[reg].Name())
+		} else {
+			regnames = append(regnames, fmt.Sprintf("reg%v", reg))
+		}
+	}
+	loc += strings.Join(regnames, ",")
+	pos := func(v *Value, p *obj.Prog, pc int64) string {
+		if v == nil {
+			return "?"
+		}
+		if p == nil {
+			return fmt.Sprintf("v%v", v.ID)
+		}
+		return fmt.Sprintf("v%v/%x", v.ID, pc)
+	}
+	surv := ""
+	if v.survivedBlock {
+		surv = "+"
+	}
+	return fmt.Sprintf("%v-%v%s@%s", pos(v.Start, v.StartProg, v.StartPC), pos(v.End, v.EndProg, v.EndPC), surv, loc)
+}
+
+// unexpected is used to indicate an inconsistency or bug in the debug info
+// generation process. These are not fixable by users. At time of writing,
+// changing this to a Fprintf(os.Stderr) and running make.bash generates
+// thousands of warnings.
+func (s *debugState) unexpected(v *Value, msg string, args ...interface{}) {
+	s.f.Logf("unexpected at "+fmt.Sprint(v.ID)+":"+msg, args...)
+}
+
+func (s *debugState) logf(msg string, args ...interface{}) {
+	s.f.Logf(msg, args...)
+}
+
+type debugState struct {
+	loggingEnabled bool
+	slots          []*LocalSlot
+	f              *Func
+	cache          *Cache
+	numRegisters   int
+
+	// working storage for BuildFuncDebug, reused between blocks.
+	registerContents [][]SlotID
+}
+
+// BuildFuncDebug returns debug information for f.
+// f must be fully processed, so that each Value is where it will be when
+// machine code is emitted.
+func BuildFuncDebug(f *Func, loggingEnabled bool) *FuncDebug {
+	if f.RegAlloc == nil {
+		f.Fatalf("BuildFuncDebug on func %v that has not been fully processed", f)
+	}
+	state := &debugState{
+		loggingEnabled:   loggingEnabled,
+		slots:            make([]*LocalSlot, len(f.Names)),
+		cache:            f.Cache,
+		f:                f,
+		numRegisters:     len(f.Config.registers),
+		registerContents: make([][]SlotID, len(f.Config.registers)),
+	}
+	// TODO: consider storing this in Cache and reusing across functions.
+	valueNames := make([][]SlotID, f.NumValues())
+
+	for i, slot := range f.Names {
+		slot := slot
+		state.slots[i] = &slot
+
+		if isSynthetic(&slot) {
+			continue
+		}
+		for _, value := range f.NamedValues[slot] {
+			valueNames[value.ID] = append(valueNames[value.ID], SlotID(i))
+		}
+	}
+
+	if state.loggingEnabled {
+		var names []string
+		for i, name := range f.Names {
+			names = append(names, fmt.Sprintf("%v = %v", i, name))
+		}
+		state.logf("Name table: %v\n", strings.Join(names, ", "))
+	}
+
+	// Build up block states, starting with the first block, then
+	// processing blocks once their predecessors have been processed.
+
+	// TODO: use a reverse post-order traversal instead of the work queue.
+
+	// Location list entries for each block.
+	blockLocs := make([]*FuncDebug, f.NumBlocks())
+
+	// Work queue of blocks to visit. Some of them may already be processed.
+	work := []*Block{f.Entry}
+
+	for len(work) > 0 {
+		b := work[0]
+		work = work[1:]
+		if blockLocs[b.ID] != nil {
+			continue // already processed
+		}
+		if !state.predecessorsDone(b, blockLocs) {
+			continue // not ready yet
+		}
+
+		for _, edge := range b.Succs {
+			if blockLocs[edge.Block().ID] != nil {
+				continue
+			}
+			work = append(work, edge.Block())
+		}
+
+		// Build the starting state for the block from the final
+		// state of its predecessors.
+		locs := state.mergePredecessors(b, blockLocs)
+		if state.loggingEnabled {
+			state.logf("Processing %v, initial locs %v, regs %v\n", b, locs, state.registerContents)
+		}
+		// Update locs/registers with the effects of each Value.
+		for _, v := range b.Values {
+			slots := valueNames[v.ID]
+
+			// Loads and stores inherit the names of their sources.
+			var source *Value
+			switch v.Op {
+			case OpStoreReg:
+				source = v.Args[0]
+			case OpLoadReg:
+				switch a := v.Args[0]; a.Op {
+				case OpArg:
+					source = a
+				case OpStoreReg:
+					source = a.Args[0]
+				default:
+					state.unexpected(v, "load with unexpected source op %v", a)
+				}
+			}
+			if source != nil {
+				slots = append(slots, valueNames[source.ID]...)
+				// As of writing, the compiler never uses a load/store as a
+				// source of another load/store, so there's no reason this should
+				// ever be consulted. Update just in case, and so that when
+				// valueNames is cached, we can reuse the memory.
+				valueNames[v.ID] = slots
+			}
+
+			if len(slots) == 0 {
+				continue
+			}
+
+			reg, _ := f.getHome(v.ID).(*Register)
+			state.processValue(locs, v, slots, reg)
+
+		}
+
+		// The block is done; end the locations for all its slots.
+		for _, locList := range locs.Variables {
+			last := locList.last()
+			if last == nil || last.End != nil {
+				continue
+			}
+			if len(b.Values) != 0 {
+				last.End = b.Values[len(b.Values)-1]
+			} else {
+				// This happens when a value survives into an empty block from its predecessor.
+				// Just carry it forward for liveness's sake.
+				last.End = last.Start
+			}
+			last.survivedBlock = true
+		}
+		if state.loggingEnabled {
+			f.Logf("Block done: locs %v, regs %v. work = %+v\n", locs, state.registerContents, work)
+		}
+		blockLocs[b.ID] = locs
+	}
+
+	// Build the complete debug info by concatenating each of the blocks'
+	// locations together.
+	info := &FuncDebug{
+		Variables: make([]VarLocList, len(state.slots)),
+		Slots:     state.slots,
+		Registers: f.Config.registers,
+	}
+	for _, b := range f.Blocks {
+		// Ignore empty blocks; there will be some records for liveness
+		// but they're all useless.
+		if len(b.Values) == 0 {
+			continue
+		}
+		if blockLocs[b.ID] == nil {
+			state.unexpected(b.Values[0], "Never processed block %v\n", b)
+			continue
+		}
+		for slot, blockLocList := range blockLocs[b.ID].Variables {
+			for _, loc := range blockLocList.Locations {
+				if !loc.OnStack && loc.Registers == 0 {
+					state.unexpected(loc.Start, "Location for %v with no storage: %+v\n", state.slots[slot], loc)
+					continue // don't confuse downstream with our bugs
+				}
+				if loc.Start == nil || loc.End == nil {
+					state.unexpected(b.Values[0], "Location for %v missing start or end: %v\n", state.slots[slot], loc)
+					continue
+				}
+				info.append(SlotID(slot), loc)
+			}
+		}
+	}
+	if state.loggingEnabled {
+		f.Logf("Final result:\n")
+		for slot, locList := range info.Variables {
+			f.Logf("\t%v => %v\n", state.slots[slot], locList)
+		}
+	}
+	return info
+}
+
+// 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.Name()[0]
+	return c == '.' || c == '~'
+}
+
+// predecessorsDone reports whether block is ready to be processed.
+func (state *debugState) predecessorsDone(b *Block, blockLocs []*FuncDebug) bool {
+	f := b.Func
+	for _, edge := range b.Preds {
+		// Ignore back branches, e.g. the continuation of a for loop.
+		// This may not work for functions with mutual gotos, which are not
+		// reducible, in which case debug information will be missing for any
+		// code after that point in the control flow.
+		if f.sdom().isAncestorEq(b, edge.b) {
+			if state.loggingEnabled {
+				f.Logf("ignoring back branch from %v to %v\n", edge.b, b)
+			}
+			continue // back branch
+		}
+		if blockLocs[edge.b.ID] == nil {
+			if state.loggingEnabled {
+				f.Logf("%v is not ready because %v isn't done\n", b, edge.b)
+			}
+			return false
+		}
+	}
+	return true
+}
+
+// mergePredecessors takes the end state of each of b's predecessors and
+// intersects them to form the starting state for b.
+// The registers slice (the second return value) will be reused for each call to mergePredecessors.
+func (state *debugState) mergePredecessors(b *Block, blockLocs []*FuncDebug) *FuncDebug {
+	live := make([]VarLocList, len(state.slots))
+
+	// Filter out back branches.
+	var preds []*Block
+	for _, pred := range b.Preds {
+		if blockLocs[pred.b.ID] != nil {
+			preds = append(preds, pred.b)
+		}
+	}
+
+	if len(preds) > 0 {
+		p := preds[0]
+		for slot, locList := range blockLocs[p.ID].Variables {
+			last := locList.last()
+			if last == nil || !last.survivedBlock {
+				continue
+			}
+			// If this block is empty, carry forward the end value for liveness.
+			// It'll be ignored later.
+			start := last.End
+			if len(b.Values) != 0 {
+				start = b.Values[0]
+			}
+			loc := state.cache.NewVarLoc()
+			loc.Start = start
+			loc.OnStack = last.OnStack
+			loc.Registers = last.Registers
+			live[slot].append(loc)
+		}
+	}
+	if state.loggingEnabled && len(b.Preds) > 1 {
+		state.logf("Starting merge with state from %v: %v\n", b.Preds[0].b, blockLocs[b.Preds[0].b.ID])
+	}
+	for i := 1; i < len(preds); i++ {
+		p := preds[i]
+		if state.loggingEnabled {
+			state.logf("Merging in state from %v: %v &= %v\n", p, live, blockLocs[p.ID])
+		}
+
+		for slot, liveVar := range live {
+			liveLoc := liveVar.last()
+			if liveLoc == nil {
+				continue
+			}
+
+			predLoc := blockLocs[p.ID].lastLoc(SlotID(slot))
+			// Clear out slots missing/dead in p.
+			if predLoc == nil || !predLoc.survivedBlock {
+				live[slot].Locations = nil
+				continue
+			}
+
+			// Unify storage locations.
+			liveLoc.OnStack = liveLoc.OnStack && predLoc.OnStack
+			liveLoc.Registers &= predLoc.Registers
+		}
+	}
+
+	// Create final result.
+	locs := &FuncDebug{Variables: live, Slots: state.slots}
+	for reg := range state.registerContents {
+		state.registerContents[reg] = state.registerContents[reg][:0]
+	}
+	for slot, locList := range live {
+		loc := locList.last()
+		if loc == nil {
+			continue
+		}
+		for reg := 0; reg < state.numRegisters; reg++ {
+			if loc.Registers&(1<<uint8(reg)) != 0 {
+				state.registerContents[reg] = append(state.registerContents[reg], SlotID(slot))
+			}
+		}
+	}
+	return locs
+}
+
+// processValue updates locs and state.registerContents to reflect v, a value with
+// the names in vSlots and homed in vReg.
+func (state *debugState) processValue(locs *FuncDebug, v *Value, vSlots []SlotID, vReg *Register) {
+	switch {
+	case v.Op == OpRegKill:
+		if state.loggingEnabled {
+			existingSlots := make([]bool, len(state.slots))
+			for _, slot := range state.registerContents[vReg.num] {
+				existingSlots[slot] = true
+			}
+			for _, slot := range vSlots {
+				if existingSlots[slot] {
+					existingSlots[slot] = false
+				} else {
+					state.unexpected(v, "regkill of unassociated name %v\n", state.slots[slot])
+				}
+			}
+			for slot, live := range existingSlots {
+				if live {
+					state.unexpected(v, "leftover register name: %v\n", state.slots[slot])
+				}
+			}
+		}
+		state.registerContents[vReg.num] = nil
+
+		for _, slot := range vSlots {
+			last := locs.lastLoc(slot)
+			if last == nil {
+				state.unexpected(v, "regkill of already dead %v, %+v\n", vReg, state.slots[slot])
+				continue
+			}
+			if state.loggingEnabled {
+				state.logf("at %v: %v regkilled out of %v\n", v.ID, state.slots[slot], vReg.Name())
+			}
+			if last.End != nil {
+				state.unexpected(v, "regkill of dead slot, died at %v\n", last.End)
+			}
+			last.End = v
+
+			regs := last.Registers &^ (1 << uint8(vReg.num))
+			if !last.OnStack && regs == 0 {
+				continue
+			}
+			loc := state.cache.NewVarLoc()
+			loc.Start = v
+			loc.OnStack = last.OnStack
+			loc.Registers = regs
+			locs.append(slot, loc)
+		}
+	case v.Op == OpArg:
+		for _, slot := range vSlots {
+			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
+			locs.append(slot, loc)
+		}
+
+	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 %v\n", vReg)
+				break
+			}
+			last.End = v
+			loc := state.cache.NewVarLoc()
+			loc.Start = v
+			loc.OnStack = true
+			loc.Registers = last.Registers
+			locs.append(slot, loc)
+		}
+
+	case vReg != nil:
+		if state.loggingEnabled {
+			newSlots := make([]bool, len(state.slots))
+			for _, slot := range vSlots {
+				newSlots[slot] = true
+			}
+
+			for _, slot := range state.registerContents[vReg.num] {
+				if !newSlots[slot] {
+					state.unexpected(v, "%v clobbered\n", state.slots[slot])
+				}
+			}
+		}
+
+		for _, slot := range vSlots {
+			if state.loggingEnabled {
+				state.logf("at %v: %v now in %v\n", v.ID, state.slots[slot], vReg.Name())
+			}
+			last := locs.lastLoc(slot)
+			if last != nil && last.End == nil {
+				last.End = v
+			}
+			state.registerContents[vReg.num] = append(state.registerContents[vReg.num], slot)
+			loc := state.cache.NewVarLoc()
+			loc.Start = v
+			if last != nil {
+				loc.OnStack = last.OnStack
+				loc.Registers = last.Registers
+			}
+			loc.Registers |= 1 << uint8(vReg.num)
+			locs.append(slot, loc)
+		}
+	default:
+		state.unexpected(v, "named value with no reg\n")
+	}
+
+}
diff --git a/src/cmd/compile/internal/ssa/gen/genericOps.go b/src/cmd/compile/internal/ssa/gen/genericOps.go
index d962e4a193..63bb9a8b61 100644
--- a/src/cmd/compile/internal/ssa/gen/genericOps.go
+++ b/src/cmd/compile/internal/ssa/gen/genericOps.go
@@ -417,6 +417,7 @@ var genericOps = []opData{
 	{name: "VarKill", argLength: 1, aux: "Sym", symEffect: "None"},            // aux is a *gc.Node of a variable that is known to be dead.  arg0=mem, returns mem
 	{name: "VarLive", argLength: 1, aux: "Sym", symEffect: "None"},            // aux is a *gc.Node of a variable that must be kept live.  arg0=mem, returns mem
 	{name: "KeepAlive", argLength: 2, typ: "Mem"},                             // arg[0] is a value that must be kept alive until this mark.  arg[1]=mem, returns mem
+	{name: "RegKill"},                                                         // regalloc has determined that the value in this register is dead
 
 	// Ops for breaking 64-bit operations on 32-bit architectures
 	{name: "Int64Make", argLength: 2, typ: "UInt64"}, // arg0=hi, arg1=lo
diff --git a/src/cmd/compile/internal/ssa/html.go b/src/cmd/compile/internal/ssa/html.go
index d554907beb..6efe93e74f 100644
--- a/src/cmd/compile/internal/ssa/html.go
+++ b/src/cmd/compile/internal/ssa/html.go
@@ -11,6 +11,7 @@ import (
 	"html"
 	"io"
 	"os"
+	"strings"
 )
 
 type HTMLWriter struct {
@@ -362,6 +363,18 @@ func (v *Value) LongHTML() string {
 	if int(v.ID) < len(r) && r[v.ID] != nil {
 		s += " : " + html.EscapeString(r[v.ID].Name())
 	}
+	var names []string
+	for name, values := range v.Block.Func.NamedValues {
+		for _, value := range values {
+			if value == v {
+				names = append(names, name.Name())
+				break // drop duplicates.
+			}
+		}
+	}
+	if len(names) != 0 {
+		s += " (" + strings.Join(names, ", ") + ")"
+	}
 	s += "</span>"
 	return s
 }
diff --git a/src/cmd/compile/internal/ssa/location.go b/src/cmd/compile/internal/ssa/location.go
index 70afa47e9d..dc01bd4235 100644
--- a/src/cmd/compile/internal/ssa/location.go
+++ b/src/cmd/compile/internal/ssa/location.go
@@ -26,6 +26,12 @@ func (r *Register) Name() string {
 	return r.name
 }
 
+// ObjNum returns the register number from cmd/internal/obj/$ARCH that
+// corresponds to this register.
+func (r *Register) ObjNum() int16 {
+	return r.objNum
+}
+
 // A LocalSlot is a location in the stack frame, which identifies and stores
 // part or all of a PPARAM, PPARAMOUT, or PAUTO ONAME node.
 // It can represent a whole variable, part of a larger stack slot, or part of a
diff --git a/src/cmd/compile/internal/ssa/opGen.go b/src/cmd/compile/internal/ssa/opGen.go
index ae2dd5f550..87a2ea0656 100644
--- a/src/cmd/compile/internal/ssa/opGen.go
+++ b/src/cmd/compile/internal/ssa/opGen.go
@@ -1897,6 +1897,7 @@ const (
 	OpVarKill
 	OpVarLive
 	OpKeepAlive
+	OpRegKill
 	OpInt64Make
 	OpInt64Hi
 	OpInt64Lo
@@ -22498,6 +22499,11 @@ var opcodeTable = [...]opInfo{
 		generic: true,
 	},
 	{
+		name:    "RegKill",
+		argLen:  0,
+		generic: true,
+	},
+	{
 		name:    "Int64Make",
 		argLen:  2,
 		generic: true,
diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index e297e6bce7..0abaeaeeb5 100644
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -242,6 +242,9 @@ type regAllocState struct {
 	// current state of each (preregalloc) Value
 	values []valState
 
+	// names associated with each Value
+	valueNames [][]LocalSlot
+
 	// ID of SP, SB values
 	sp, sb ID
 
@@ -300,6 +303,13 @@ type startReg struct {
 
 // freeReg frees up register r. Any current user of r is kicked out.
 func (s *regAllocState) freeReg(r register) {
+	s.freeOrResetReg(r, false)
+}
+
+// freeOrResetReg frees up register r. Any current user of r is kicked out.
+// resetting indicates that the operation is only for bookkeeping,
+// e.g. when clearing out state upon entry to a new block.
+func (s *regAllocState) freeOrResetReg(r register, resetting bool) {
 	v := s.regs[r].v
 	if v == nil {
 		s.f.Fatalf("tried to free an already free register %d\n", r)
@@ -309,6 +319,16 @@ func (s *regAllocState) freeReg(r register) {
 	if s.f.pass.debug > regDebug {
 		fmt.Printf("freeReg %s (dump %s/%s)\n", s.registers[r].Name(), v, s.regs[r].c)
 	}
+	if !resetting && s.f.Config.ctxt.Flag_locationlists && len(s.valueNames[v.ID]) != 0 {
+		kill := s.curBlock.NewValue0(src.NoXPos, OpRegKill, types.TypeVoid)
+		for int(kill.ID) >= len(s.orig) {
+			s.orig = append(s.orig, nil)
+		}
+		for _, name := range s.valueNames[v.ID] {
+			s.f.NamedValues[name] = append(s.f.NamedValues[name], kill)
+		}
+		s.f.setHome(kill, &s.registers[r])
+	}
 	s.regs[r] = regState{}
 	s.values[v.ID].regs &^= regMask(1) << r
 	s.used &^= regMask(1) << r
@@ -599,6 +619,17 @@ func (s *regAllocState) init(f *Func) {
 	s.values = make([]valState, f.NumValues())
 	s.orig = make([]*Value, f.NumValues())
 	s.copies = make(map[*Value]bool)
+	if s.f.Config.ctxt.Flag_locationlists {
+		s.valueNames = make([][]LocalSlot, f.NumValues())
+		for slot, values := range f.NamedValues {
+			if isSynthetic(&slot) {
+				continue
+			}
+			for _, value := range values {
+				s.valueNames[value.ID] = append(s.valueNames[value.ID], slot)
+			}
+		}
+	}
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if !v.Type.IsMemory() && !v.Type.IsVoid() && !v.Type.IsFlags() && !v.Type.IsTuple() {
@@ -692,7 +723,9 @@ func (s *regAllocState) liveAfterCurrentInstruction(v *Value) bool {
 
 // Sets the state of the registers to that encoded in regs.
 func (s *regAllocState) setState(regs []endReg) {
-	s.freeRegs(s.used)
+	for s.used != 0 {
+		s.freeOrResetReg(pickReg(s.used), true)
+	}
 	for _, x := range regs {
 		s.assignReg(x.r, x.v, x.c)
 	}
@@ -735,6 +768,9 @@ func (s *regAllocState) regalloc(f *Func) {
 	}
 
 	for _, b := range f.Blocks {
+		if s.f.pass.debug > regDebug {
+			fmt.Printf("Begin processing block %v\n", b)
+		}
 		s.curBlock = b
 
 		// Initialize regValLiveSet and uses fields for this block.
@@ -830,9 +866,6 @@ func (s *regAllocState) regalloc(f *Func) {
 			// This is the complicated case. We have more than one predecessor,
 			// which means we may have Phi ops.
 
-			// Copy phi ops into new schedule.
-			b.Values = append(b.Values, phis...)
-
 			// Start with the final register state of the primary predecessor
 			idx := s.primary[b.ID]
 			if idx < 0 {
@@ -910,6 +943,9 @@ func (s *regAllocState) regalloc(f *Func) {
 				}
 			}
 
+			// Copy phi ops into new schedule.
+			b.Values = append(b.Values, phis...)
+
 			// Third pass - pick registers for phis whose inputs
 			// were not in a register.
 			for i, v := range phis {
@@ -1005,7 +1041,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			pidx := e.i
 			for _, v := range succ.Values {
 				if v.Op != OpPhi {
-					break
+					continue
 				}
 				if !s.values[v.ID].needReg {
 					continue
@@ -1565,6 +1601,9 @@ func (s *regAllocState) placeSpills() {
 	for _, b := range f.Blocks {
 		var m regMask
 		for _, v := range b.Values {
+			if v.Op == OpRegKill {
+				continue
+			}
 			if v.Op != OpPhi {
 				break
 			}
@@ -1675,7 +1714,7 @@ func (s *regAllocState) placeSpills() {
 	for _, b := range f.Blocks {
 		nphi := 0
 		for _, v := range b.Values {
-			if v.Op != OpPhi {
+			if v.Op != OpRegKill && v.Op != OpPhi {
 				break
 			}
 			nphi++
@@ -1800,6 +1839,9 @@ func (e *edgeState) setup(idx int, srcReg []endReg, dstReg []startReg, stacklive
 	}
 	// Phis need their args to end up in a specific location.
 	for _, v := range e.b.Values {
+		if v.Op == OpRegKill {
+			continue
+		}
 		if v.Op != OpPhi {
 			break
 		}
@@ -1878,6 +1920,7 @@ func (e *edgeState) process() {
 		if e.s.f.pass.debug > regDebug {
 			fmt.Printf("breaking cycle with v%d in %s:%s\n", vid, loc.Name(), c)
 		}
+		e.erase(r)
 		if _, isReg := loc.(*Register); isReg {
 			c = e.p.NewValue1(d.pos, OpCopy, c.Type, c)
 		} else {
@@ -1943,6 +1986,18 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 		}
 	}
 	_, dstReg := loc.(*Register)
+
+	// Pre-clobber destination. This avoids the
+	// following situation:
+	//   - v is currently held in R0 and stacktmp0.
+	//   - We want to copy stacktmp1 to stacktmp0.
+	//   - We choose R0 as the temporary register.
+	// During the copy, both R0 and stacktmp0 are
+	// clobbered, losing both copies of v. Oops!
+	// Erasing the destination early means R0 will not
+	// be chosen as the temp register, as it will then
+	// be the last copy of v.
+	e.erase(loc)
 	var x *Value
 	if c == nil {
 		if !e.s.values[vid].rematerializeable {
@@ -1953,8 +2008,8 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 		} else {
 			// Rematerialize into stack slot. Need a free
 			// register to accomplish this.
-			e.erase(loc) // see pre-clobber comment below
 			r := e.findRegFor(v.Type)
+			e.erase(r)
 			x = v.copyIntoNoXPos(e.p)
 			e.set(r, vid, x, false, pos)
 			// Make sure we spill with the size of the slot, not the
@@ -1976,20 +2031,8 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 				x = e.p.NewValue1(pos, OpLoadReg, c.Type, c)
 			} else {
 				// mem->mem. Use temp register.
-
-				// Pre-clobber destination. This avoids the
-				// following situation:
-				//   - v is currently held in R0 and stacktmp0.
-				//   - We want to copy stacktmp1 to stacktmp0.
-				//   - We choose R0 as the temporary register.
-				// During the copy, both R0 and stacktmp0 are
-				// clobbered, losing both copies of v. Oops!
-				// Erasing the destination early means R0 will not
-				// be chosen as the temp register, as it will then
-				// be the last copy of v.
-				e.erase(loc)
-
 				r := e.findRegFor(c.Type)
+				e.erase(r)
 				t := e.p.NewValue1(pos, OpLoadReg, c.Type, c)
 				e.set(r, vid, t, false, pos)
 				x = e.p.NewValue1(pos, OpStoreReg, loc.(LocalSlot).Type, t)
@@ -2008,7 +2051,6 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 // set changes the contents of location loc to hold the given value and its cached representative.
 func (e *edgeState) set(loc Location, vid ID, c *Value, final bool, pos src.XPos) {
 	e.s.f.setHome(c, loc)
-	e.erase(loc)
 	e.contents[loc] = contentRecord{vid, c, final, pos}
 	a := e.cache[vid]
 	if len(a) == 0 {
@@ -2059,6 +2101,16 @@ func (e *edgeState) erase(loc Location) {
 				fmt.Printf("v%d no longer available in %s:%s\n", vid, loc.Name(), c)
 			}
 			a[i], a = a[len(a)-1], a[:len(a)-1]
+			if e.s.f.Config.ctxt.Flag_locationlists {
+				if _, isReg := loc.(*Register); isReg && int(c.ID) < len(e.s.valueNames) && len(e.s.valueNames[c.ID]) != 0 {
+					kill := e.p.NewValue0(src.NoXPos, OpRegKill, types.TypeVoid)
+					e.s.f.setHome(kill, loc)
+					for _, name := range e.s.valueNames[c.ID] {
+						e.s.f.NamedValues[name] = append(e.s.f.NamedValues[name], kill)
+					}
+				}
+			}
+
 			break
 		}
 	}
diff --git a/src/cmd/compile/internal/ssa/value.go b/src/cmd/compile/internal/ssa/value.go
index 7edc71be52..6df535153a 100644
--- a/src/cmd/compile/internal/ssa/value.go
+++ b/src/cmd/compile/internal/ssa/value.go
@@ -10,6 +10,7 @@ import (
 	"cmd/internal/src"
 	"fmt"
 	"math"
+	"strings"
 )
 
 // A Value represents a value in the SSA representation of the program.
@@ -98,7 +99,7 @@ func (v *Value) AuxValAndOff() ValAndOff {
 	return ValAndOff(v.AuxInt)
 }
 
-// long form print.  v# = opcode <type> [aux] args [: reg]
+// long form print.  v# = opcode <type> [aux] args [: reg] (names)
 func (v *Value) LongString() string {
 	s := fmt.Sprintf("v%d = %s", v.ID, v.Op)
 	s += " <" + v.Type.String() + ">"
@@ -110,6 +111,18 @@ func (v *Value) LongString() string {
 	if int(v.ID) < len(r) && r[v.ID] != nil {
 		s += " : " + r[v.ID].Name()
 	}
+	var names []string
+	for name, values := range v.Block.Func.NamedValues {
+		for _, value := range values {
+			if value == v {
+				names = append(names, name.Name())
+				break // drop duplicates.
+			}
+		}
+	}
+	if len(names) != 0 {
+		s += " (" + strings.Join(names, ", ") + ")"
+	}
 	return s
 }