[dev.debug] cmd/compile: better DWARF with optimizations on

Debuggers use DWARF information to find local variables on the
stack and in registers. Prior to this CL, the DWARF information for
functions claimed that all variables were on the stack at all times.
That's incorrect when optimizations are enabled, and results in
debuggers showing data that is out of date or complete gibberish.

After this CL, the compiler is capable of representing variable
locations more accurately, and attempts to do so. Due to limitations of
the SSA backend, it's not possible to be completely correct.

There are a number of problems in the current design. One of the easier
to understand is that variable names currently must be attached to an
SSA value, but not all assignments in the source code actually result
in machine code. For example:

  type myint int
  var a int
  b := myint(int)
and
  b := (*uint64)(unsafe.Pointer(a))

don't generate machine code because the underlying representation is the
same, so the correct value of b will not be set when the user would
expect.

Generating the more precise debug information is behind a flag,
dwarflocationlists. Because of the issues described above, setting the
flag may not make the debugging experience much better, and may actually
make it worse in cases where the variable actually is on the stack and
the more complicated analysis doesn't realize it.

A number of changes are included:
- Add a new pseudo-instruction, RegKill, which indicates that the value
in the register has been clobbered.
- Adjust regalloc to emit RegKills in the right places. Significantly,
this means that phis are mixed with StoreReg and RegKills after
regalloc.
- Track variable decomposition in ssa.LocalSlots.
- After the SSA backend is done, analyze the result and build location
lists for each LocalSlot.
- After assembly is done, update the location lists with the assembled
PC offsets, recompose variables, and build DWARF location lists. Emit the
list as a new linker symbol, one per function.
- In the linker, aggregate the location lists into a .debug_loc section.

TODO:
- currently disabled for non-X86/AMD64 because there are no data tables.

go build -toolexec 'toolstash -cmp' -a std succeeds.

With -dwarflocationlists false:
before: f02812195637909ff675782c0b46836a8ff01976
after:  06f61e8112a42ac34fb80e0c818b3cdb84a5e7ec
benchstat -geomean  /tmp/220352263 /tmp/621364410
completed   15 of   15, estimated time remaining 0s (eta 3:52PM)
name        old time/op       new time/op       delta
Template          199ms ± 3%        198ms ± 2%     ~     (p=0.400 n=15+14)
Unicode          96.6ms ± 5%       96.4ms ± 5%     ~     (p=0.838 n=15+15)
GoTypes           653ms ± 2%        647ms ± 2%     ~     (p=0.102 n=15+14)
Flate             133ms ± 6%        129ms ± 3%   -2.62%  (p=0.041 n=15+15)
GoParser          164ms ± 5%        159ms ± 3%   -3.05%  (p=0.000 n=15+15)
Reflect           428ms ± 4%        422ms ± 3%     ~     (p=0.156 n=15+13)
Tar               123ms ±10%        124ms ± 8%     ~     (p=0.461 n=15+15)
XML               228ms ± 3%        224ms ± 3%   -1.57%  (p=0.045 n=15+15)
[Geo mean]        206ms             377ms       +82.86%

name        old user-time/op  new user-time/op  delta
Template          292ms ±10%        301ms ±12%     ~     (p=0.189 n=15+15)
Unicode           166ms ±37%        158ms ±14%     ~     (p=0.418 n=15+14)
GoTypes           962ms ± 6%        963ms ± 7%     ~     (p=0.976 n=15+15)
Flate             207ms ±19%        200ms ±14%     ~     (p=0.345 n=14+15)
GoParser          246ms ±22%        240ms ±15%     ~     (p=0.587 n=15+15)
Reflect           611ms ±13%        587ms ±14%     ~     (p=0.085 n=15+13)
Tar               211ms ±12%        217ms ±14%     ~     (p=0.355 n=14+15)
XML               335ms ±15%        320ms ±18%     ~     (p=0.169 n=15+15)
[Geo mean]        317ms             583ms       +83.72%

name        old alloc/op      new alloc/op      delta
Template         40.2MB ± 0%       40.2MB ± 0%   -0.15%  (p=0.000 n=14+15)
Unicode          29.2MB ± 0%       29.3MB ± 0%     ~     (p=0.624 n=15+15)
GoTypes           114MB ± 0%        114MB ± 0%   -0.15%  (p=0.000 n=15+14)
Flate            25.7MB ± 0%       25.6MB ± 0%   -0.18%  (p=0.000 n=13+15)
GoParser         32.2MB ± 0%       32.2MB ± 0%   -0.14%  (p=0.003 n=15+15)
Reflect          77.8MB ± 0%       77.9MB ± 0%     ~     (p=0.061 n=15+15)
Tar              27.1MB ± 0%       27.0MB ± 0%   -0.11%  (p=0.029 n=15+15)
XML              42.7MB ± 0%       42.5MB ± 0%   -0.29%  (p=0.000 n=15+15)
[Geo mean]       42.1MB            75.0MB       +78.05%

name        old allocs/op     new allocs/op     delta
Template           402k ± 1%         398k ± 0%   -0.91%  (p=0.000 n=15+15)
Unicode            344k ± 1%         344k ± 0%     ~     (p=0.715 n=15+14)
GoTypes           1.18M ± 0%        1.17M ± 0%   -0.91%  (p=0.000 n=15+14)
Flate              243k ± 0%         240k ± 1%   -1.05%  (p=0.000 n=13+15)
GoParser           327k ± 1%         324k ± 1%   -0.96%  (p=0.000 n=15+15)
Reflect            984k ± 1%         982k ± 0%     ~     (p=0.050 n=15+15)
Tar                261k ± 1%         259k ± 1%   -0.77%  (p=0.000 n=15+15)
XML                411k ± 0%         404k ± 1%   -1.55%  (p=0.000 n=15+15)
[Geo mean]         439k              755k       +72.01%

name        old text-bytes    new text-bytes    delta
HelloSize         694kB ± 0%        694kB ± 0%   -0.00%  (p=0.000 n=15+15)

name        old data-bytes    new data-bytes    delta
HelloSize        5.55kB ± 0%       5.55kB ± 0%     ~     (all equal)

name        old bss-bytes     new bss-bytes     delta
HelloSize         133kB ± 0%        133kB ± 0%     ~     (all equal)

name        old exe-bytes     new exe-bytes     delta
HelloSize        1.04MB ± 0%       1.04MB ± 0%     ~     (all equal)

Change-Id: I991fc553ef175db46bb23b2128317bbd48de70d8
Reviewed-on: https://go-review.googlesource.com/41770
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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")
+	}
+
+}