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

// Copyright 2013 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.

// Garbage collector liveness bitmap generation.

// The command line flag -live causes this code to print debug information.
// The levels are:
//
//	-live (aka -live=1): print liveness lists as code warnings at safe points
//	-live=2: print an assembly listing with liveness annotations
//
// Each level includes the earlier output as well.

package liveness

import (
	"crypto/sha1"
	"fmt"
	"os"
	"sort"
	"strings"

	"cmd/compile/internal/abi"
	"cmd/compile/internal/base"
	"cmd/compile/internal/bitvec"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/objw"
	"cmd/compile/internal/reflectdata"
	"cmd/compile/internal/ssa"
	"cmd/compile/internal/typebits"
	"cmd/compile/internal/types"
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"cmd/internal/src"
)

// OpVarDef is an annotation for the liveness analysis, marking a place
// where a complete initialization (definition) of a variable begins.
// Since the liveness analysis can see initialization of single-word
// variables quite easy, OpVarDef is only needed for multi-word
// variables satisfying isfat(n.Type). For simplicity though, buildssa
// emits OpVarDef regardless of variable width.
//
// An 'OpVarDef x' annotation in the instruction stream tells the liveness
// analysis to behave as though the variable x is being initialized at that
// point in the instruction stream. The OpVarDef must appear before the
// actual (multi-instruction) initialization, and it must also appear after
// any uses of the previous value, if any. For example, if compiling:
//
//	x = x[1:]
//
// it is important to generate code like:
//
//	base, len, cap = pieces of x[1:]
//	OpVarDef x
//	x = {base, len, cap}
//
// If instead the generated code looked like:
//
//	OpVarDef x
//	base, len, cap = pieces of x[1:]
//	x = {base, len, cap}
//
// then the liveness analysis would decide the previous value of x was
// unnecessary even though it is about to be used by the x[1:] computation.
// Similarly, if the generated code looked like:
//
//	base, len, cap = pieces of x[1:]
//	x = {base, len, cap}
//	OpVarDef x
//
// then the liveness analysis will not preserve the new value of x, because
// the OpVarDef appears to have "overwritten" it.
//
// OpVarDef is a bit of a kludge to work around the fact that the instruction
// stream is working on single-word values but the liveness analysis
// wants to work on individual variables, which might be multi-word
// aggregates. It might make sense at some point to look into letting
// the liveness analysis work on single-word values as well, although
// there are complications around interface values, slices, and strings,
// all of which cannot be treated as individual words.
//
// OpVarKill is the opposite of OpVarDef: it marks a value as no longer needed,
// even if its address has been taken. That is, an OpVarKill annotation asserts
// that its argument is certainly dead, for use when the liveness analysis
// would not otherwise be able to deduce that fact.

// TODO: get rid of OpVarKill here. It's useful for stack frame allocation
// so the compiler can allocate two temps to the same location. Here it's now
// useless, since the implementation of stack objects.

// blockEffects summarizes the liveness effects on an SSA block.
type blockEffects struct {
	// Computed during Liveness.prologue using only the content of
	// individual blocks:
	//
	//	uevar: upward exposed variables (used before set in block)
	//	varkill: killed variables (set in block)
	uevar   bitvec.BitVec
	varkill bitvec.BitVec

	// Computed during Liveness.solve using control flow information:
	//
	//	livein: variables live at block entry
	//	liveout: variables live at block exit
	livein  bitvec.BitVec
	liveout bitvec.BitVec
}

// A collection of global state used by liveness analysis.
type liveness struct {
	fn         *ir.Func
	f          *ssa.Func
	vars       []*ir.Name
	idx        map[*ir.Name]int32
	stkptrsize int64

	be []blockEffects

	// allUnsafe indicates that all points in this function are
	// unsafe-points.
	allUnsafe bool
	// unsafePoints bit i is set if Value ID i is an unsafe-point
	// (preemption is not allowed). Only valid if !allUnsafe.
	unsafePoints bitvec.BitVec

	// An array with a bit vector for each safe point in the
	// current Block during liveness.epilogue. Indexed in Value
	// order for that block. Additionally, for the entry block
	// livevars[0] is the entry bitmap. liveness.compact moves
	// these to stackMaps.
	livevars []bitvec.BitVec

	// livenessMap maps from safe points (i.e., CALLs) to their
	// liveness map indexes.
	livenessMap Map
	stackMapSet bvecSet
	stackMaps   []bitvec.BitVec

	cache progeffectscache

	// partLiveArgs includes input arguments (PPARAM) that may
	// be partially live. That is, it is considered live because
	// a part of it is used, but we may not initialize all parts.
	partLiveArgs map[*ir.Name]bool

	doClobber     bool // Whether to clobber dead stack slots in this function.
	noClobberArgs bool // Do not clobber function arguments
}

// Map maps from *ssa.Value to LivenessIndex.
type Map struct {
	Vals map[ssa.ID]objw.LivenessIndex
	// The set of live, pointer-containing variables at the DeferReturn
	// call (only set when open-coded defers are used).
	DeferReturn objw.LivenessIndex
}

func (m *Map) reset() {
	if m.Vals == nil {
		m.Vals = make(map[ssa.ID]objw.LivenessIndex)
	} else {
		for k := range m.Vals {
			delete(m.Vals, k)
		}
	}
	m.DeferReturn = objw.LivenessDontCare
}

func (m *Map) set(v *ssa.Value, i objw.LivenessIndex) {
	m.Vals[v.ID] = i
}

func (m Map) Get(v *ssa.Value) objw.LivenessIndex {
	// If v isn't in the map, then it's a "don't care" and not an
	// unsafe-point.
	if idx, ok := m.Vals[v.ID]; ok {
		return idx
	}
	return objw.LivenessIndex{StackMapIndex: objw.StackMapDontCare, IsUnsafePoint: false}
}

type progeffectscache struct {
	retuevar    []int32
	tailuevar   []int32
	initialized bool
}

// shouldTrack reports whether the liveness analysis
// should track the variable n.
// We don't care about variables that have no pointers,
// nor do we care about non-local variables,
// nor do we care about empty structs (handled by the pointer check),
// nor do we care about the fake PAUTOHEAP variables.
func shouldTrack(n *ir.Name) bool {
	return (n.Class == ir.PAUTO && n.Esc() != ir.EscHeap || n.Class == ir.PPARAM || n.Class == ir.PPARAMOUT) && n.Type().HasPointers()
}

// getvariables returns the list of on-stack variables that we need to track
// and a map for looking up indices by *Node.
func getvariables(fn *ir.Func) ([]*ir.Name, map[*ir.Name]int32) {
	var vars []*ir.Name
	for _, n := range fn.Dcl {
		if shouldTrack(n) {
			vars = append(vars, n)
		}
	}
	idx := make(map[*ir.Name]int32, len(vars))
	for i, n := range vars {
		idx[n] = int32(i)
	}
	return vars, idx
}

func (lv *liveness) initcache() {
	if lv.cache.initialized {
		base.Fatalf("liveness cache initialized twice")
		return
	}
	lv.cache.initialized = true

	for i, node := range lv.vars {
		switch node.Class {
		case ir.PPARAM:
			// A return instruction with a p.to is a tail return, which brings
			// the stack pointer back up (if it ever went down) and then jumps
			// to a new function entirely. That form of instruction must read
			// all the parameters for correctness, and similarly it must not
			// read the out arguments - they won't be set until the new
			// function runs.
			lv.cache.tailuevar = append(lv.cache.tailuevar, int32(i))

		case ir.PPARAMOUT:
			// All results are live at every return point.
			// Note that this point is after escaping return values
			// are copied back to the stack using their PAUTOHEAP references.
			lv.cache.retuevar = append(lv.cache.retuevar, int32(i))
		}
	}
}

// A liveEffect is a set of flags that describe an instruction's
// liveness effects on a variable.
//
// The possible flags are:
//	uevar - used by the instruction
//	varkill - killed by the instruction (set)
// A kill happens after the use (for an instruction that updates a value, for example).
type liveEffect int

const (
	uevar liveEffect = 1 << iota
	varkill
)

// valueEffects returns the index of a variable in lv.vars and the
// liveness effects v has on that variable.
// If v does not affect any tracked variables, it returns -1, 0.
func (lv *liveness) valueEffects(v *ssa.Value) (int32, liveEffect) {
	n, e := affectedVar(v)
	if e == 0 || n == nil { // cheapest checks first
		return -1, 0
	}
	// AllocFrame has dropped unused variables from
	// lv.fn.Func.Dcl, but they might still be referenced by
	// OpVarFoo pseudo-ops. Ignore them to prevent "lost track of
	// variable" ICEs (issue 19632).
	switch v.Op {
	case ssa.OpVarDef, ssa.OpVarKill, ssa.OpVarLive, ssa.OpKeepAlive:
		if !n.Used() {
			return -1, 0
		}
	}

	if n.Class == ir.PPARAM && !n.Addrtaken() && n.Type().Size() > int64(types.PtrSize) {
		// Only aggregate-typed arguments that are not address-taken can be
		// partially live.
		lv.partLiveArgs[n] = true
	}

	var effect liveEffect
	// Read is a read, obviously.
	//
	// Addr is a read also, as any subsequent holder of the pointer must be able
	// to see all the values (including initialization) written so far.
	// This also prevents a variable from "coming back from the dead" and presenting
	// stale pointers to the garbage collector. See issue 28445.
	if e&(ssa.SymRead|ssa.SymAddr) != 0 {
		effect |= uevar
	}
	if e&ssa.SymWrite != 0 && (!isfat(n.Type()) || v.Op == ssa.OpVarDef) {
		effect |= varkill
	}

	if effect == 0 {
		return -1, 0
	}

	if pos, ok := lv.idx[n]; ok {
		return pos, effect
	}
	return -1, 0
}

// affectedVar returns the *ir.Name node affected by v
func affectedVar(v *ssa.Value) (*ir.Name, ssa.SymEffect) {
	// Special cases.
	switch v.Op {
	case ssa.OpLoadReg:
		n, _ := ssa.AutoVar(v.Args[0])
		return n, ssa.SymRead
	case ssa.OpStoreReg:
		n, _ := ssa.AutoVar(v)
		return n, ssa.SymWrite

	case ssa.OpArgIntReg:
		// This forces the spill slot for the register to be live at function entry.
		// one of the following holds for a function F with pointer-valued register arg X:
		//  0. No GC (so an uninitialized spill slot is okay)
		//  1. GC at entry of F.  GC is precise, but the spills around morestack initialize X's spill slot
		//  2. Stack growth at entry of F.  Same as GC.
		//  3. GC occurs within F itself.  This has to be from preemption, and thus GC is conservative.
		//     a. X is in a register -- then X is seen, and the spill slot is also scanned conservatively.
		//     b. X is spilled -- the spill slot is initialized, and scanned conservatively
		//     c. X is not live -- the spill slot is scanned conservatively, and it may contain X from an earlier spill.
		//  4. GC within G, transitively called from F
		//    a. X is live at call site, therefore is spilled, to its spill slot (which is live because of subsequent LoadReg).
		//    b. X is not live at call site -- but neither is its spill slot.
		n, _ := ssa.AutoVar(v)
		return n, ssa.SymRead

	case ssa.OpVarLive:
		return v.Aux.(*ir.Name), ssa.SymRead
	case ssa.OpVarDef, ssa.OpVarKill:
		return v.Aux.(*ir.Name), ssa.SymWrite
	case ssa.OpKeepAlive:
		n, _ := ssa.AutoVar(v.Args[0])
		return n, ssa.SymRead
	}

	e := v.Op.SymEffect()
	if e == 0 {
		return nil, 0
	}

	switch a := v.Aux.(type) {
	case nil, *obj.LSym:
		// ok, but no node
		return nil, e
	case *ir.Name:
		return a, e
	default:
		base.Fatalf("weird aux: %s", v.LongString())
		return nil, e
	}
}

type livenessFuncCache struct {
	be          []blockEffects
	livenessMap Map
}

// Constructs a new liveness structure used to hold the global state of the
// liveness computation. The cfg argument is a slice of *BasicBlocks and the
// vars argument is a slice of *Nodes.
func newliveness(fn *ir.Func, f *ssa.Func, vars []*ir.Name, idx map[*ir.Name]int32, stkptrsize int64) *liveness {
	lv := &liveness{
		fn:         fn,
		f:          f,
		vars:       vars,
		idx:        idx,
		stkptrsize: stkptrsize,
	}

	// Significant sources of allocation are kept in the ssa.Cache
	// and reused. Surprisingly, the bit vectors themselves aren't
	// a major source of allocation, but the liveness maps are.
	if lc, _ := f.Cache.Liveness.(*livenessFuncCache); lc == nil {
		// Prep the cache so liveness can fill it later.
		f.Cache.Liveness = new(livenessFuncCache)
	} else {
		if cap(lc.be) >= f.NumBlocks() {
			lv.be = lc.be[:f.NumBlocks()]
		}
		lv.livenessMap = Map{Vals: lc.livenessMap.Vals, DeferReturn: objw.LivenessDontCare}
		lc.livenessMap.Vals = nil
	}
	if lv.be == nil {
		lv.be = make([]blockEffects, f.NumBlocks())
	}

	nblocks := int32(len(f.Blocks))
	nvars := int32(len(vars))
	bulk := bitvec.NewBulk(nvars, nblocks*7)
	for _, b := range f.Blocks {
		be := lv.blockEffects(b)

		be.uevar = bulk.Next()
		be.varkill = bulk.Next()
		be.livein = bulk.Next()
		be.liveout = bulk.Next()
	}
	lv.livenessMap.reset()

	lv.markUnsafePoints()

	lv.partLiveArgs = make(map[*ir.Name]bool)

	lv.enableClobber()

	return lv
}

func (lv *liveness) blockEffects(b *ssa.Block) *blockEffects {
	return &lv.be[b.ID]
}

// Generates live pointer value maps for arguments and local variables. The
// this argument and the in arguments are always assumed live. The vars
// argument is a slice of *Nodes.
func (lv *liveness) pointerMap(liveout bitvec.BitVec, vars []*ir.Name, args, locals bitvec.BitVec) {
	for i := int32(0); ; i++ {
		i = liveout.Next(i)
		if i < 0 {
			break
		}
		node := vars[i]
		switch node.Class {
		case ir.PPARAM, ir.PPARAMOUT:
			if !node.IsOutputParamInRegisters() {
				if node.FrameOffset() < 0 {
					lv.f.Fatalf("Node %v has frameoffset %d\n", node.Sym().Name, node.FrameOffset())
				}
				typebits.Set(node.Type(), node.FrameOffset(), args)
				break
			}
			fallthrough // PPARAMOUT in registers acts memory-allocates like an AUTO
		case ir.PAUTO:
			typebits.Set(node.Type(), node.FrameOffset()+lv.stkptrsize, locals)
		}
	}
}

// IsUnsafe indicates that all points in this function are
// unsafe-points.
func IsUnsafe(f *ssa.Func) bool {
	// The runtime assumes the only safe-points are function
	// prologues (because that's how it used to be). We could and
	// should improve that, but for now keep consider all points
	// in the runtime unsafe. obj will add prologues and their
	// safe-points.
	//
	// go:nosplit functions are similar. Since safe points used to
	// be coupled with stack checks, go:nosplit often actually
	// means "no safe points in this function".
	return base.Flag.CompilingRuntime || f.NoSplit
}

// markUnsafePoints finds unsafe points and computes lv.unsafePoints.
func (lv *liveness) markUnsafePoints() {
	if IsUnsafe(lv.f) {
		// No complex analysis necessary.
		lv.allUnsafe = true
		return
	}

	lv.unsafePoints = bitvec.New(int32(lv.f.NumValues()))

	// Mark architecture-specific unsafe points.
	for _, b := range lv.f.Blocks {
		for _, v := range b.Values {
			if v.Op.UnsafePoint() {
				lv.unsafePoints.Set(int32(v.ID))
			}
		}
	}

	// Mark write barrier unsafe points.
	for _, wbBlock := range lv.f.WBLoads {
		if wbBlock.Kind == ssa.BlockPlain && len(wbBlock.Values) == 0 {
			// The write barrier block was optimized away
			// but we haven't done dead block elimination.
			// (This can happen in -N mode.)
			continue
		}
		// Check that we have the expected diamond shape.
		if len(wbBlock.Succs) != 2 {
			lv.f.Fatalf("expected branch at write barrier block %v", wbBlock)
		}
		s0, s1 := wbBlock.Succs[0].Block(), wbBlock.Succs[1].Block()
		if s0 == s1 {
			// There's no difference between write barrier on and off.
			// Thus there's no unsafe locations. See issue 26024.
			continue
		}
		if s0.Kind != ssa.BlockPlain || s1.Kind != ssa.BlockPlain {
			lv.f.Fatalf("expected successors of write barrier block %v to be plain", wbBlock)
		}
		if s0.Succs[0].Block() != s1.Succs[0].Block() {
			lv.f.Fatalf("expected successors of write barrier block %v to converge", wbBlock)
		}

		// Flow backwards from the control value to find the
		// flag load. We don't know what lowered ops we're
		// looking for, but all current arches produce a
		// single op that does the memory load from the flag
		// address, so we look for that.
		var load *ssa.Value
		v := wbBlock.Controls[0]
		for {
			if sym, ok := v.Aux.(*obj.LSym); ok && sym == ir.Syms.WriteBarrier {
				load = v
				break
			}
			switch v.Op {
			case ssa.Op386TESTL:
				// 386 lowers Neq32 to (TESTL cond cond),
				if v.Args[0] == v.Args[1] {
					v = v.Args[0]
					continue
				}
			case ssa.Op386MOVLload, ssa.OpARM64MOVWUload, ssa.OpPPC64MOVWZload, ssa.OpWasmI64Load32U:
				// Args[0] is the address of the write
				// barrier control. Ignore Args[1],
				// which is the mem operand.
				// TODO: Just ignore mem operands?
				v = v.Args[0]
				continue
			}
			// Common case: just flow backwards.
			if len(v.Args) != 1 {
				v.Fatalf("write barrier control value has more than one argument: %s", v.LongString())
			}
			v = v.Args[0]
		}

		// Mark everything after the load unsafe.
		found := false
		for _, v := range wbBlock.Values {
			found = found || v == load
			if found {
				lv.unsafePoints.Set(int32(v.ID))
			}
		}

		// Mark the two successor blocks unsafe. These come
		// back together immediately after the direct write in
		// one successor and the last write barrier call in
		// the other, so there's no need to be more precise.
		for _, succ := range wbBlock.Succs {
			for _, v := range succ.Block().Values {
				lv.unsafePoints.Set(int32(v.ID))
			}
		}
	}

	// Find uintptr -> unsafe.Pointer conversions and flood
	// unsafeness back to a call (which is always a safe point).
	//
	// Looking for the uintptr -> unsafe.Pointer conversion has a
	// few advantages over looking for unsafe.Pointer -> uintptr
	// conversions:
	//
	// 1. We avoid needlessly blocking safe-points for
	// unsafe.Pointer -> uintptr conversions that never go back to
	// a Pointer.
	//
	// 2. We don't have to detect calls to reflect.Value.Pointer,
	// reflect.Value.UnsafeAddr, and reflect.Value.InterfaceData,
	// which are implicit unsafe.Pointer -> uintptr conversions.
	// We can't even reliably detect this if there's an indirect
	// call to one of these methods.
	//
	// TODO: For trivial unsafe.Pointer arithmetic, it would be
	// nice to only flood as far as the unsafe.Pointer -> uintptr
	// conversion, but it's hard to know which argument of an Add
	// or Sub to follow.
	var flooded bitvec.BitVec
	var flood func(b *ssa.Block, vi int)
	flood = func(b *ssa.Block, vi int) {
		if flooded.N == 0 {
			flooded = bitvec.New(int32(lv.f.NumBlocks()))
		}
		if flooded.Get(int32(b.ID)) {
			return
		}
		for i := vi - 1; i >= 0; i-- {
			v := b.Values[i]
			if v.Op.IsCall() {
				// Uintptrs must not contain live
				// pointers across calls, so stop
				// flooding.
				return
			}
			lv.unsafePoints.Set(int32(v.ID))
		}
		if vi == len(b.Values) {
			// We marked all values in this block, so no
			// need to flood this block again.
			flooded.Set(int32(b.ID))
		}
		for _, pred := range b.Preds {
			flood(pred.Block(), len(pred.Block().Values))
		}
	}
	for _, b := range lv.f.Blocks {
		for i, v := range b.Values {
			if !(v.Op == ssa.OpConvert && v.Type.IsPtrShaped()) {
				continue
			}
			// Flood the unsafe-ness of this backwards
			// until we hit a call.
			flood(b, i+1)
		}
	}
}

// Returns true for instructions that must have a stack map.
//
// This does not necessarily mean the instruction is a safe-point. In
// particular, call Values can have a stack map in case the callee
// grows the stack, but not themselves be a safe-point.
func (lv *liveness) hasStackMap(v *ssa.Value) bool {
	if !v.Op.IsCall() {
		return false
	}
	// typedmemclr and typedmemmove are write barriers and
	// deeply non-preemptible. They are unsafe points and
	// hence should not have liveness maps.
	if sym, ok := v.Aux.(*ssa.AuxCall); ok && (sym.Fn == ir.Syms.Typedmemclr || sym.Fn == ir.Syms.Typedmemmove) {
		return false
	}
	return true
}

// Initializes the sets for solving the live variables. Visits all the
// instructions in each basic block to summarizes the information at each basic
// block
func (lv *liveness) prologue() {
	lv.initcache()

	for _, b := range lv.f.Blocks {
		be := lv.blockEffects(b)

		// Walk the block instructions backward and update the block
		// effects with the each prog effects.
		for j := len(b.Values) - 1; j >= 0; j-- {
			pos, e := lv.valueEffects(b.Values[j])
			if e&varkill != 0 {
				be.varkill.Set(pos)
				be.uevar.Unset(pos)
			}
			if e&uevar != 0 {
				be.uevar.Set(pos)
			}
		}
	}
}

// Solve the liveness dataflow equations.
func (lv *liveness) solve() {
	// These temporary bitvectors exist to avoid successive allocations and
	// frees within the loop.
	nvars := int32(len(lv.vars))
	newlivein := bitvec.New(nvars)
	newliveout := bitvec.New(nvars)

	// Walk blocks in postorder ordering. This improves convergence.
	po := lv.f.Postorder()

	// Iterate through the blocks in reverse round-robin fashion. A work
	// queue might be slightly faster. As is, the number of iterations is
	// so low that it hardly seems to be worth the complexity.

	for change := true; change; {
		change = false
		for _, b := range po {
			be := lv.blockEffects(b)

			newliveout.Clear()
			switch b.Kind {
			case ssa.BlockRet:
				for _, pos := range lv.cache.retuevar {
					newliveout.Set(pos)
				}
			case ssa.BlockRetJmp:
				for _, pos := range lv.cache.tailuevar {
					newliveout.Set(pos)
				}
			case ssa.BlockExit:
				// panic exit - nothing to do
			default:
				// A variable is live on output from this block
				// if it is live on input to some successor.
				//
				// out[b] = \bigcup_{s \in succ[b]} in[s]
				newliveout.Copy(lv.blockEffects(b.Succs[0].Block()).livein)
				for _, succ := range b.Succs[1:] {
					newliveout.Or(newliveout, lv.blockEffects(succ.Block()).livein)
				}
			}

			if !be.liveout.Eq(newliveout) {
				change = true
				be.liveout.Copy(newliveout)
			}

			// A variable is live on input to this block
			// if it is used by this block, or live on output from this block and
			// not set by the code in this block.
			//
			// in[b] = uevar[b] \cup (out[b] \setminus varkill[b])
			newlivein.AndNot(be.liveout, be.varkill)
			be.livein.Or(newlivein, be.uevar)
		}
	}
}

// Visits all instructions in a basic block and computes a bit vector of live
// variables at each safe point locations.
func (lv *liveness) epilogue() {
	nvars := int32(len(lv.vars))
	liveout := bitvec.New(nvars)
	livedefer := bitvec.New(nvars) // always-live variables

	// If there is a defer (that could recover), then all output
	// parameters are live all the time.  In addition, any locals
	// that are pointers to heap-allocated output parameters are
	// also always live (post-deferreturn code needs these
	// pointers to copy values back to the stack).
	// TODO: if the output parameter is heap-allocated, then we
	// don't need to keep the stack copy live?
	if lv.fn.HasDefer() {
		for i, n := range lv.vars {
			if n.Class == ir.PPARAMOUT {
				if n.IsOutputParamHeapAddr() {
					// Just to be paranoid.  Heap addresses are PAUTOs.
					base.Fatalf("variable %v both output param and heap output param", n)
				}
				if n.Heapaddr != nil {
					// If this variable moved to the heap, then
					// its stack copy is not live.
					continue
				}
				// Note: zeroing is handled by zeroResults in walk.go.
				livedefer.Set(int32(i))
			}
			if n.IsOutputParamHeapAddr() {
				// This variable will be overwritten early in the function
				// prologue (from the result of a mallocgc) but we need to
				// zero it in case that malloc causes a stack scan.
				n.SetNeedzero(true)
				livedefer.Set(int32(i))
			}
			if n.OpenDeferSlot() {
				// Open-coded defer args slots must be live
				// everywhere in a function, since a panic can
				// occur (almost) anywhere. Because it is live
				// everywhere, it must be zeroed on entry.
				livedefer.Set(int32(i))
				// It was already marked as Needzero when created.
				if !n.Needzero() {
					base.Fatalf("all pointer-containing defer arg slots should have Needzero set")
				}
			}
		}
	}

	// We must analyze the entry block first. The runtime assumes
	// the function entry map is index 0. Conveniently, layout
	// already ensured that the entry block is first.
	if lv.f.Entry != lv.f.Blocks[0] {
		lv.f.Fatalf("entry block must be first")
	}

	{
		// Reserve an entry for function entry.
		live := bitvec.New(nvars)
		lv.livevars = append(lv.livevars, live)
	}

	for _, b := range lv.f.Blocks {
		be := lv.blockEffects(b)

		// Walk forward through the basic block instructions and
		// allocate liveness maps for those instructions that need them.
		for _, v := range b.Values {
			if !lv.hasStackMap(v) {
				continue
			}

			live := bitvec.New(nvars)
			lv.livevars = append(lv.livevars, live)
		}

		// walk backward, construct maps at each safe point
		index := int32(len(lv.livevars) - 1)

		liveout.Copy(be.liveout)
		for i := len(b.Values) - 1; i >= 0; i-- {
			v := b.Values[i]

			if lv.hasStackMap(v) {
				// Found an interesting instruction, record the
				// corresponding liveness information.

				live := &lv.livevars[index]
				live.Or(*live, liveout)
				live.Or(*live, livedefer) // only for non-entry safe points
				index--
			}

			// Update liveness information.
			pos, e := lv.valueEffects(v)
			if e&varkill != 0 {
				liveout.Unset(pos)
			}
			if e&uevar != 0 {
				liveout.Set(pos)
			}
		}

		if b == lv.f.Entry {
			if index != 0 {
				base.Fatalf("bad index for entry point: %v", index)
			}

			// Check to make sure only input variables are live.
			for i, n := range lv.vars {
				if !liveout.Get(int32(i)) {
					continue
				}
				if n.Class == ir.PPARAM {
					continue // ok
				}
				base.FatalfAt(n.Pos(), "bad live variable at entry of %v: %L", lv.fn.Nname, n)
			}

			// Record live variables.
			live := &lv.livevars[index]
			live.Or(*live, liveout)
		}

		if lv.doClobber {
			lv.clobber(b)
		}

		// The liveness maps for this block are now complete. Compact them.
		lv.compact(b)
	}

	// If we have an open-coded deferreturn call, make a liveness map for it.
	if lv.fn.OpenCodedDeferDisallowed() {
		lv.livenessMap.DeferReturn = objw.LivenessDontCare
	} else {
		lv.livenessMap.DeferReturn = objw.LivenessIndex{
			StackMapIndex: lv.stackMapSet.add(livedefer),
			IsUnsafePoint: false,
		}
	}

	// Done compacting. Throw out the stack map set.
	lv.stackMaps = lv.stackMapSet.extractUnique()
	lv.stackMapSet = bvecSet{}

	// Useful sanity check: on entry to the function,
	// the only things that can possibly be live are the
	// input parameters.
	for j, n := range lv.vars {
		if n.Class != ir.PPARAM && lv.stackMaps[0].Get(int32(j)) {
			lv.f.Fatalf("%v %L recorded as live on entry", lv.fn.Nname, n)
		}
	}
}

// Compact coalesces identical bitmaps from lv.livevars into the sets
// lv.stackMapSet.
//
// Compact clears lv.livevars.
//
// There are actually two lists of bitmaps, one list for the local variables and one
// list for the function arguments. Both lists are indexed by the same PCDATA
// index, so the corresponding pairs must be considered together when
// merging duplicates. The argument bitmaps change much less often during
// function execution than the local variable bitmaps, so it is possible that
// we could introduce a separate PCDATA index for arguments vs locals and
// then compact the set of argument bitmaps separately from the set of
// local variable bitmaps. As of 2014-04-02, doing this to the godoc binary
// is actually a net loss: we save about 50k of argument bitmaps but the new
// PCDATA tables cost about 100k. So for now we keep using a single index for
// both bitmap lists.
func (lv *liveness) compact(b *ssa.Block) {
	pos := 0
	if b == lv.f.Entry {
		// Handle entry stack map.
		lv.stackMapSet.add(lv.livevars[0])
		pos++
	}
	for _, v := range b.Values {
		hasStackMap := lv.hasStackMap(v)
		isUnsafePoint := lv.allUnsafe || v.Op != ssa.OpClobber && lv.unsafePoints.Get(int32(v.ID))
		idx := objw.LivenessIndex{StackMapIndex: objw.StackMapDontCare, IsUnsafePoint: isUnsafePoint}
		if hasStackMap {
			idx.StackMapIndex = lv.stackMapSet.add(lv.livevars[pos])
			pos++
		}
		if hasStackMap || isUnsafePoint {
			lv.livenessMap.set(v, idx)
		}
	}

	// Reset livevars.
	lv.livevars = lv.livevars[:0]
}

func (lv *liveness) enableClobber() {
	// The clobberdead experiment inserts code to clobber pointer slots in all
	// the dead variables (locals and args) at every synchronous safepoint.
	if !base.Flag.ClobberDead {
		return
	}
	if lv.fn.Pragma&ir.CgoUnsafeArgs != 0 {
		// C or assembly code uses the exact frame layout. Don't clobber.
		return
	}
	if len(lv.vars) > 10000 || len(lv.f.Blocks) > 10000 {
		// Be careful to avoid doing too much work.
		// Bail if >10000 variables or >10000 blocks.
		// Otherwise, giant functions make this experiment generate too much code.
		return
	}
	if lv.f.Name == "forkAndExecInChild" {
		// forkAndExecInChild calls vfork on some platforms.
		// The code we add here clobbers parts of the stack in the child.
		// When the parent resumes, it is using the same stack frame. But the
		// child has clobbered stack variables that the parent needs. Boom!
		// In particular, the sys argument gets clobbered.
		return
	}
	if lv.f.Name == "wbBufFlush" ||
		((lv.f.Name == "callReflect" || lv.f.Name == "callMethod") && lv.fn.ABIWrapper()) {
		// runtime.wbBufFlush must not modify its arguments. See the comments
		// in runtime/mwbbuf.go:wbBufFlush.
		//
		// reflect.callReflect and reflect.callMethod are called from special
		// functions makeFuncStub and methodValueCall. The runtime expects
		// that it can find the first argument (ctxt) at 0(SP) in makeFuncStub
		// and methodValueCall's frame (see runtime/traceback.go:getArgInfo).
		// Normally callReflect and callMethod already do not modify the
		// argument, and keep it alive. But the compiler-generated ABI wrappers
		// don't do that. Special case the wrappers to not clobber its arguments.
		lv.noClobberArgs = true
	}
	if h := os.Getenv("GOCLOBBERDEADHASH"); h != "" {
		// Clobber only functions where the hash of the function name matches a pattern.
		// Useful for binary searching for a miscompiled function.
		hstr := ""
		for _, b := range sha1.Sum([]byte(lv.f.Name)) {
			hstr += fmt.Sprintf("%08b", b)
		}
		if !strings.HasSuffix(hstr, h) {
			return
		}
		fmt.Printf("\t\t\tCLOBBERDEAD %s\n", lv.f.Name)
	}
	lv.doClobber = true
}

// Inserts code to clobber pointer slots in all the dead variables (locals and args)
// at every synchronous safepoint in b.
func (lv *liveness) clobber(b *ssa.Block) {
	// Copy block's values to a temporary.
	oldSched := append([]*ssa.Value{}, b.Values...)
	b.Values = b.Values[:0]
	idx := 0

	// Clobber pointer slots in all dead variables at entry.
	if b == lv.f.Entry {
		for len(oldSched) > 0 && len(oldSched[0].Args) == 0 {
			// Skip argless ops. We need to skip at least
			// the lowered ClosurePtr op, because it
			// really wants to be first. This will also
			// skip ops like InitMem and SP, which are ok.
			b.Values = append(b.Values, oldSched[0])
			oldSched = oldSched[1:]
		}
		clobber(lv, b, lv.livevars[0])
		idx++
	}

	// Copy values into schedule, adding clobbering around safepoints.
	for _, v := range oldSched {
		if !lv.hasStackMap(v) {
			b.Values = append(b.Values, v)
			continue
		}
		clobber(lv, b, lv.livevars[idx])
		b.Values = append(b.Values, v)
		idx++
	}
}

// clobber generates code to clobber pointer slots in all dead variables
// (those not marked in live). Clobbering instructions are added to the end
// of b.Values.
func clobber(lv *liveness, b *ssa.Block, live bitvec.BitVec) {
	for i, n := range lv.vars {
		if !live.Get(int32(i)) && !n.Addrtaken() && !n.OpenDeferSlot() && !n.IsOutputParamHeapAddr() {
			// Don't clobber stack objects (address-taken). They are
			// tracked dynamically.
			// Also don't clobber slots that are live for defers (see
			// the code setting livedefer in epilogue).
			if lv.noClobberArgs && n.Class == ir.PPARAM {
				continue
			}
			clobberVar(b, n)
		}
	}
}

// clobberVar generates code to trash the pointers in v.
// Clobbering instructions are added to the end of b.Values.
func clobberVar(b *ssa.Block, v *ir.Name) {
	clobberWalk(b, v, 0, v.Type())
}

// b = block to which we append instructions
// v = variable
// offset = offset of (sub-portion of) variable to clobber (in bytes)
// t = type of sub-portion of v.
func clobberWalk(b *ssa.Block, v *ir.Name, offset int64, t *types.Type) {
	if !t.HasPointers() {
		return
	}
	switch t.Kind() {
	case types.TPTR,
		types.TUNSAFEPTR,
		types.TFUNC,
		types.TCHAN,
		types.TMAP:
		clobberPtr(b, v, offset)

	case types.TSTRING:
		// struct { byte *str; int len; }
		clobberPtr(b, v, offset)

	case types.TINTER:
		// struct { Itab *tab; void *data; }
		// or, when isnilinter(t)==true:
		// struct { Type *type; void *data; }
		clobberPtr(b, v, offset)
		clobberPtr(b, v, offset+int64(types.PtrSize))

	case types.TSLICE:
		// struct { byte *array; int len; int cap; }
		clobberPtr(b, v, offset)

	case types.TARRAY:
		for i := int64(0); i < t.NumElem(); i++ {
			clobberWalk(b, v, offset+i*t.Elem().Size(), t.Elem())
		}

	case types.TSTRUCT:
		for _, t1 := range t.Fields().Slice() {
			clobberWalk(b, v, offset+t1.Offset, t1.Type)
		}

	default:
		base.Fatalf("clobberWalk: unexpected type, %v", t)
	}
}

// clobberPtr generates a clobber of the pointer at offset offset in v.
// The clobber instruction is added at the end of b.
func clobberPtr(b *ssa.Block, v *ir.Name, offset int64) {
	b.NewValue0IA(src.NoXPos, ssa.OpClobber, types.TypeVoid, offset, v)
}

func (lv *liveness) showlive(v *ssa.Value, live bitvec.BitVec) {
	if base.Flag.Live == 0 || ir.FuncName(lv.fn) == "init" || strings.HasPrefix(ir.FuncName(lv.fn), ".") {
		return
	}
	if lv.fn.Wrapper() || lv.fn.Dupok() {
		// Skip reporting liveness information for compiler-generated wrappers.
		return
	}
	if !(v == nil || v.Op.IsCall()) {
		// Historically we only printed this information at
		// calls. Keep doing so.
		return
	}
	if live.IsEmpty() {
		return
	}

	pos := lv.fn.Nname.Pos()
	if v != nil {
		pos = v.Pos
	}

	s := "live at "
	if v == nil {
		s += fmt.Sprintf("entry to %s:", ir.FuncName(lv.fn))
	} else if sym, ok := v.Aux.(*ssa.AuxCall); ok && sym.Fn != nil {
		fn := sym.Fn.Name
		if pos := strings.Index(fn, "."); pos >= 0 {
			fn = fn[pos+1:]
		}
		s += fmt.Sprintf("call to %s:", fn)
	} else {
		s += "indirect call:"
	}

	for j, n := range lv.vars {
		if live.Get(int32(j)) {
			s += fmt.Sprintf(" %v", n)
		}
	}

	base.WarnfAt(pos, s)
}

func (lv *liveness) printbvec(printed bool, name string, live bitvec.BitVec) bool {
	if live.IsEmpty() {
		return printed
	}

	if !printed {
		fmt.Printf("\t")
	} else {
		fmt.Printf(" ")
	}
	fmt.Printf("%s=", name)

	comma := ""
	for i, n := range lv.vars {
		if !live.Get(int32(i)) {
			continue
		}
		fmt.Printf("%s%s", comma, n.Sym().Name)
		comma = ","
	}
	return true
}

// printeffect is like printbvec, but for valueEffects.
func (lv *liveness) printeffect(printed bool, name string, pos int32, x bool) bool {
	if !x {
		return printed
	}
	if !printed {
		fmt.Printf("\t")
	} else {
		fmt.Printf(" ")
	}
	fmt.Printf("%s=", name)
	if x {
		fmt.Printf("%s", lv.vars[pos].Sym().Name)
	}

	return true
}

// Prints the computed liveness information and inputs, for debugging.
// This format synthesizes the information used during the multiple passes
// into a single presentation.
func (lv *liveness) printDebug() {
	fmt.Printf("liveness: %s\n", ir.FuncName(lv.fn))

	for i, b := range lv.f.Blocks {
		if i > 0 {
			fmt.Printf("\n")
		}

		// bb#0 pred=1,2 succ=3,4
		fmt.Printf("bb#%d pred=", b.ID)
		for j, pred := range b.Preds {
			if j > 0 {
				fmt.Printf(",")
			}
			fmt.Printf("%d", pred.Block().ID)
		}
		fmt.Printf(" succ=")
		for j, succ := range b.Succs {
			if j > 0 {
				fmt.Printf(",")
			}
			fmt.Printf("%d", succ.Block().ID)
		}
		fmt.Printf("\n")

		be := lv.blockEffects(b)

		// initial settings
		printed := false
		printed = lv.printbvec(printed, "uevar", be.uevar)
		printed = lv.printbvec(printed, "livein", be.livein)
		if printed {
			fmt.Printf("\n")
		}

		// program listing, with individual effects listed

		if b == lv.f.Entry {
			live := lv.stackMaps[0]
			fmt.Printf("(%s) function entry\n", base.FmtPos(lv.fn.Nname.Pos()))
			fmt.Printf("\tlive=")
			printed = false
			for j, n := range lv.vars {
				if !live.Get(int32(j)) {
					continue
				}
				if printed {
					fmt.Printf(",")
				}
				fmt.Printf("%v", n)
				printed = true
			}
			fmt.Printf("\n")
		}

		for _, v := range b.Values {
			fmt.Printf("(%s) %v\n", base.FmtPos(v.Pos), v.LongString())

			pcdata := lv.livenessMap.Get(v)

			pos, effect := lv.valueEffects(v)
			printed = false
			printed = lv.printeffect(printed, "uevar", pos, effect&uevar != 0)
			printed = lv.printeffect(printed, "varkill", pos, effect&varkill != 0)
			if printed {
				fmt.Printf("\n")
			}

			if pcdata.StackMapValid() {
				fmt.Printf("\tlive=")
				printed = false
				if pcdata.StackMapValid() {
					live := lv.stackMaps[pcdata.StackMapIndex]
					for j, n := range lv.vars {
						if !live.Get(int32(j)) {
							continue
						}
						if printed {
							fmt.Printf(",")
						}
						fmt.Printf("%v", n)
						printed = true
					}
				}
				fmt.Printf("\n")
			}

			if pcdata.IsUnsafePoint {
				fmt.Printf("\tunsafe-point\n")
			}
		}

		// bb bitsets
		fmt.Printf("end\n")
		printed = false
		printed = lv.printbvec(printed, "varkill", be.varkill)
		printed = lv.printbvec(printed, "liveout", be.liveout)
		if printed {
			fmt.Printf("\n")
		}
	}

	fmt.Printf("\n")
}

// Dumps a slice of bitmaps to a symbol as a sequence of uint32 values. The
// first word dumped is the total number of bitmaps. The second word is the
// length of the bitmaps. All bitmaps are assumed to be of equal length. The
// remaining bytes are the raw bitmaps.
func (lv *liveness) emit() (argsSym, liveSym *obj.LSym) {
	// Size args bitmaps to be just large enough to hold the largest pointer.
	// First, find the largest Xoffset node we care about.
	// (Nodes without pointers aren't in lv.vars; see ShouldTrack.)
	var maxArgNode *ir.Name
	for _, n := range lv.vars {
		switch n.Class {
		case ir.PPARAM, ir.PPARAMOUT:
			if !n.IsOutputParamInRegisters() {
				if maxArgNode == nil || n.FrameOffset() > maxArgNode.FrameOffset() {
					maxArgNode = n
				}
			}
		}
	}
	// Next, find the offset of the largest pointer in the largest node.
	var maxArgs int64
	if maxArgNode != nil {
		maxArgs = maxArgNode.FrameOffset() + types.PtrDataSize(maxArgNode.Type())
	}

	// Size locals bitmaps to be stkptrsize sized.
	// We cannot shrink them to only hold the largest pointer,
	// because their size is used to calculate the beginning
	// of the local variables frame.
	// Further discussion in https://golang.org/cl/104175.
	// TODO: consider trimming leading zeros.
	// This would require shifting all bitmaps.
	maxLocals := lv.stkptrsize

	// Temporary symbols for encoding bitmaps.
	var argsSymTmp, liveSymTmp obj.LSym

	args := bitvec.New(int32(maxArgs / int64(types.PtrSize)))
	aoff := objw.Uint32(&argsSymTmp, 0, uint32(len(lv.stackMaps))) // number of bitmaps
	aoff = objw.Uint32(&argsSymTmp, aoff, uint32(args.N))          // number of bits in each bitmap

	locals := bitvec.New(int32(maxLocals / int64(types.PtrSize)))
	loff := objw.Uint32(&liveSymTmp, 0, uint32(len(lv.stackMaps))) // number of bitmaps
	loff = objw.Uint32(&liveSymTmp, loff, uint32(locals.N))        // number of bits in each bitmap

	for _, live := range lv.stackMaps {
		args.Clear()
		locals.Clear()

		lv.pointerMap(live, lv.vars, args, locals)

		aoff = objw.BitVec(&argsSymTmp, aoff, args)
		loff = objw.BitVec(&liveSymTmp, loff, locals)
	}

	// These symbols will be added to Ctxt.Data by addGCLocals
	// after parallel compilation is done.
	return base.Ctxt.GCLocalsSym(argsSymTmp.P), base.Ctxt.GCLocalsSym(liveSymTmp.P)
}

// Entry pointer for Compute analysis. Solves for the Compute of
// 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,
// and a map that contains all input parameters that may be partially live.
func Compute(curfn *ir.Func, f *ssa.Func, stkptrsize int64, pp *objw.Progs) (Map, map[*ir.Name]bool) {
	// Construct the global liveness state.
	vars, idx := getvariables(curfn)
	lv := newliveness(curfn, f, vars, idx, stkptrsize)

	// Run the dataflow framework.
	lv.prologue()
	lv.solve()
	lv.epilogue()
	if base.Flag.Live > 0 {
		lv.showlive(nil, lv.stackMaps[0])
		for _, b := range f.Blocks {
			for _, val := range b.Values {
				if idx := lv.livenessMap.Get(val); idx.StackMapValid() {
					lv.showlive(val, lv.stackMaps[idx.StackMapIndex])
				}
			}
		}
	}
	if base.Flag.Live >= 2 {
		lv.printDebug()
	}

	// Update the function cache.
	{
		cache := f.Cache.Liveness.(*livenessFuncCache)
		if cap(lv.be) < 2000 { // Threshold from ssa.Cache slices.
			for i := range lv.be {
				lv.be[i] = blockEffects{}
			}
			cache.be = lv.be
		}
		if len(lv.livenessMap.Vals) < 2000 {
			cache.livenessMap = lv.livenessMap
		}
	}

	// Emit the live pointer map data structures
	ls := curfn.LSym
	fninfo := ls.Func()
	fninfo.GCArgs, fninfo.GCLocals = lv.emit()

	p := pp.Prog(obj.AFUNCDATA)
	p.From.SetConst(objabi.FUNCDATA_ArgsPointerMaps)
	p.To.Type = obj.TYPE_MEM
	p.To.Name = obj.NAME_EXTERN
	p.To.Sym = fninfo.GCArgs

	p = pp.Prog(obj.AFUNCDATA)
	p.From.SetConst(objabi.FUNCDATA_LocalsPointerMaps)
	p.To.Type = obj.TYPE_MEM
	p.To.Name = obj.NAME_EXTERN
	p.To.Sym = fninfo.GCLocals

	if x := lv.emitStackObjects(); x != nil {
		p := pp.Prog(obj.AFUNCDATA)
		p.From.SetConst(objabi.FUNCDATA_StackObjects)
		p.To.Type = obj.TYPE_MEM
		p.To.Name = obj.NAME_EXTERN
		p.To.Sym = x
	}

	return lv.livenessMap, lv.partLiveArgs
}

func (lv *liveness) emitStackObjects() *obj.LSym {
	var vars []*ir.Name
	for _, n := range lv.fn.Dcl {
		if shouldTrack(n) && n.Addrtaken() && n.Esc() != ir.EscHeap {
			vars = append(vars, n)
		}
	}
	if len(vars) == 0 {
		return nil
	}

	// Sort variables from lowest to highest address.
	sort.Slice(vars, func(i, j int) bool { return vars[i].FrameOffset() < vars[j].FrameOffset() })

	// Populate the stack object data.
	// Format must match runtime/stack.go:stackObjectRecord.
	x := base.Ctxt.Lookup(lv.fn.LSym.Name + ".stkobj")
	x.Set(obj.AttrContentAddressable, true)
	lv.fn.LSym.Func().StackObjects = x
	off := 0
	off = objw.Uintptr(x, off, uint64(len(vars)))
	for _, v := range vars {
		// Note: arguments and return values have non-negative Xoffset,
		// in which case the offset is relative to argp.
		// Locals have a negative Xoffset, in which case the offset is relative to varp.
		// We already limit the frame size, so the offset and the object size
		// should not be too big.
		frameOffset := v.FrameOffset()
		if frameOffset != int64(int32(frameOffset)) {
			base.Fatalf("frame offset too big: %v %d", v, frameOffset)
		}
		off = objw.Uint32(x, off, uint32(frameOffset))

		t := v.Type()
		sz := t.Size()
		if sz != int64(int32(sz)) {
			base.Fatalf("stack object too big: %v of type %v, size %d", v, t, sz)
		}
		lsym, useGCProg, ptrdata := reflectdata.GCSym(t)
		if useGCProg {
			ptrdata = -ptrdata
		}
		off = objw.Uint32(x, off, uint32(sz))
		off = objw.Uint32(x, off, uint32(ptrdata))
		off = objw.SymPtrOff(x, off, lsym)
	}

	if base.Flag.Live != 0 {
		for _, v := range vars {
			base.WarnfAt(v.Pos(), "stack object %v %v", v, v.Type())
		}
	}

	return x
}

// isfat reports whether a variable of type t needs multiple assignments to initialize.
// For example:
//
// 	type T struct { x, y int }
// 	x := T{x: 0, y: 1}
//
// Then we need:
//
// 	var t T
// 	t.x = 0
// 	t.y = 1
//
// to fully initialize t.
func isfat(t *types.Type) bool {
	if t != nil {
		switch t.Kind() {
		case types.TSLICE, types.TSTRING,
			types.TINTER: // maybe remove later
			return true
		case types.TARRAY:
			// Array of 1 element, check if element is fat
			if t.NumElem() == 1 {
				return isfat(t.Elem())
			}
			return true
		case types.TSTRUCT:
			// Struct with 1 field, check if field is fat
			if t.NumFields() == 1 {
				return isfat(t.Field(0).Type)
			}
			return true
		}
	}

	return false
}

// WriteFuncMap writes the pointer bitmaps for bodyless function fn's
// inputs and outputs as the value of symbol <fn>.args_stackmap.
// If fn has outputs, two bitmaps are written, otherwise just one.
func WriteFuncMap(fn *ir.Func, abiInfo *abi.ABIParamResultInfo) {
	if ir.FuncName(fn) == "_" || fn.Sym().Linkname != "" {
		return
	}
	nptr := int(abiInfo.ArgWidth() / int64(types.PtrSize))
	bv := bitvec.New(int32(nptr) * 2)

	for _, p := range abiInfo.InParams() {
		typebits.Set(p.Type, p.FrameOffset(abiInfo), bv)
	}

	nbitmap := 1
	if fn.Type().NumResults() > 0 {
		nbitmap = 2
	}
	lsym := base.Ctxt.Lookup(fn.LSym.Name + ".args_stackmap")
	off := objw.Uint32(lsym, 0, uint32(nbitmap))
	off = objw.Uint32(lsym, off, uint32(bv.N))
	off = objw.BitVec(lsym, off, bv)

	if fn.Type().NumResults() > 0 {
		for _, p := range abiInfo.OutParams() {
			if len(p.Registers) == 0 {
				typebits.Set(p.Type, p.FrameOffset(abiInfo), bv)
			}
		}
		off = objw.BitVec(lsym, off, bv)
	}

	objw.Global(lsym, int32(off), obj.RODATA|obj.LOCAL)
}