src/cmd/compile/internal/gc/esc.go - go - Git at Google

 // Copyright 2011 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 gc

 import (
 	"cmd/compile/internal/types"
 	"fmt"
 )

 func escapes(all []*Node) {
 	visitBottomUp(all, escapeFuncs)
 }

 const (
 	EscFuncUnknown = 0 + iota
 	EscFuncPlanned
 	EscFuncStarted
 	EscFuncTagged
 )

 func min8(a, b int8) int8 {
 	if a < b {
 		return a
 	}
 	return b
 }

 func max8(a, b int8) int8 {
 	if a > b {
 		return a
 	}
 	return b
 }

 const (
 	EscUnknown = iota
 	EscNone    // Does not escape to heap, result, or parameters.
 	EscHeap    // Reachable from the heap
 	EscNever   // By construction will not escape.
 )

 // funcSym returns fn.Func.Nname.Sym if no nils are encountered along the way.
 func funcSym(fn *Node) *types.Sym {
 	if fn == nil || fn.Func.Nname == nil {
 		return nil
 	}
 	return fn.Func.Nname.Sym
 }

 // Mark labels that have no backjumps to them as not increasing e.loopdepth.
 // Walk hasn't generated (goto|label).Left.Sym.Label yet, so we'll cheat
 // and set it to one of the following two. Then in esc we'll clear it again.
 var (
 	looping    Node
 	nonlooping Node
 )

 func isSliceSelfAssign(dst, src *Node) bool {
 	// Detect the following special case.
 	//
 	//	func (b *Buffer) Foo() {
 	//		n, m := ...
 	//		b.buf = b.buf[n:m]
 	//	}
 	//
 	// This assignment is a no-op for escape analysis,
 	// it does not store any new pointers into b that were not already there.
 	// However, without this special case b will escape, because we assign to OIND/ODOTPTR.
 	// Here we assume that the statement will not contain calls,
 	// that is, that order will move any calls to init.
 	// Otherwise base ONAME value could change between the moments
 	// when we evaluate it for dst and for src.

 	// dst is ONAME dereference.
 	if dst.Op != ODEREF && dst.Op != ODOTPTR || dst.Left.Op != ONAME {
 		return false
 	}
 	// src is a slice operation.
 	switch src.Op {
 	case OSLICE, OSLICE3, OSLICESTR:
 		// OK.
 	case OSLICEARR, OSLICE3ARR:
 		// Since arrays are embedded into containing object,
 		// slice of non-pointer array will introduce a new pointer into b that was not already there
 		// (pointer to b itself). After such assignment, if b contents escape,
 		// b escapes as well. If we ignore such OSLICEARR, we will conclude
 		// that b does not escape when b contents do.
 		//
 		// Pointer to an array is OK since it's not stored inside b directly.
 		// For slicing an array (not pointer to array), there is an implicit OADDR.
 		// We check that to determine non-pointer array slicing.
 		if src.Left.Op == OADDR {
 			return false
 		}
 	default:
 		return false
 	}
 	// slice is applied to ONAME dereference.
 	if src.Left.Op != ODEREF && src.Left.Op != ODOTPTR || src.Left.Left.Op != ONAME {
 		return false
 	}
 	// dst and src reference the same base ONAME.
 	return dst.Left == src.Left.Left
 }

 // isSelfAssign reports whether assignment from src to dst can
 // be ignored by the escape analysis as it's effectively a self-assignment.
 func isSelfAssign(dst, src *Node) bool {
 	if isSliceSelfAssign(dst, src) {
 		return true
 	}

 	// Detect trivial assignments that assign back to the same object.
 	//
 	// It covers these cases:
 	//	val.x = val.y
 	//	val.x[i] = val.y[j]
 	//	val.x1.x2 = val.x1.y2
 	//	... etc
 	//
 	// These assignments do not change assigned object lifetime.

 	if dst == nil || src == nil || dst.Op != src.Op {
 		return false
 	}

 	switch dst.Op {
 	case ODOT, ODOTPTR:
 		// Safe trailing accessors that are permitted to differ.
 	case OINDEX:
 		if mayAffectMemory(dst.Right) || mayAffectMemory(src.Right) {
 			return false
 		}
 	default:
 		return false
 	}

 	// The expression prefix must be both "safe" and identical.
 	return samesafeexpr(dst.Left, src.Left)
 }

 // mayAffectMemory reports whether evaluation of n may affect the program's
 // memory state. If the expression can't affect memory state, then it can be
 // safely ignored by the escape analysis.
 func mayAffectMemory(n *Node) bool {
 	// We may want to use a list of "memory safe" ops instead of generally
 	// "side-effect free", which would include all calls and other ops that can
 	// allocate or change global state. For now, it's safer to start with the latter.
 	//
 	// We're ignoring things like division by zero, index out of range,
 	// and nil pointer dereference here.
 	switch n.Op {
 	case ONAME, OCLOSUREVAR, OLITERAL:
 		return false

 	// Left+Right group.
 	case OINDEX, OADD, OSUB, OOR, OXOR, OMUL, OLSH, ORSH, OAND, OANDNOT, ODIV, OMOD:
 		return mayAffectMemory(n.Left) || mayAffectMemory(n.Right)

 	// Left group.
 	case ODOT, ODOTPTR, ODEREF, OCONVNOP, OCONV, OLEN, OCAP,
 		ONOT, OBITNOT, OPLUS, ONEG, OALIGNOF, OOFFSETOF, OSIZEOF:
 		return mayAffectMemory(n.Left)

 	default:
 		return true
 	}
 }

 func mustHeapAlloc(n *Node) bool {
 	if n.Type == nil {
 		return false
 	}

 	// Parameters are always passed via the stack.
 	if n.Op == ONAME && (n.Class() == PPARAM || n.Class() == PPARAMOUT) {
 		return false
 	}

 	if n.Type.Width > maxStackVarSize {
 		return true
 	}

 	if (n.Op == ONEW || n.Op == OPTRLIT) && n.Type.Elem().Width >= maxImplicitStackVarSize {
 		return true
 	}

 	if n.Op == OCLOSURE && closureType(n).Size() >= maxImplicitStackVarSize {
 		return true
 	}
 	if n.Op == OCALLPART && partialCallType(n).Size() >= maxImplicitStackVarSize {
 		return true
 	}

 	if n.Op == OMAKESLICE && !isSmallMakeSlice(n) {
 		return true
 	}

 	return false
 }

 // addrescapes tags node n as having had its address taken
 // by "increasing" the "value" of n.Esc to EscHeap.
 // Storage is allocated as necessary to allow the address
 // to be taken.
 func addrescapes(n *Node) {
 	switch n.Op {
 	default:
 		// Unexpected Op, probably due to a previous type error. Ignore.

 	case ODEREF, ODOTPTR:
 		// Nothing to do.

 	case ONAME:
 		if n == nodfp {
 			break
 		}

 		// if this is a tmpname (PAUTO), it was tagged by tmpname as not escaping.
 		// on PPARAM it means something different.
 		if n.Class() == PAUTO && n.Esc == EscNever {
 			break
 		}

 		// If a closure reference escapes, mark the outer variable as escaping.
 		if n.Name.IsClosureVar() {
 			addrescapes(n.Name.Defn)
 			break
 		}

 		if n.Class() != PPARAM && n.Class() != PPARAMOUT && n.Class() != PAUTO {
 			break
 		}

 		// This is a plain parameter or local variable that needs to move to the heap,
 		// but possibly for the function outside the one we're compiling.
 		// That is, if we have:
 		//
 		//	func f(x int) {
 		//		func() {
 		//			global = &x
 		//		}
 		//	}
 		//
 		// then we're analyzing the inner closure but we need to move x to the
 		// heap in f, not in the inner closure. Flip over to f before calling moveToHeap.
 		oldfn := Curfn
 		Curfn = n.Name.Curfn
 		if Curfn.Func.Closure != nil && Curfn.Op == OCLOSURE {
 			Curfn = Curfn.Func.Closure
 		}
 		ln := lineno
 		lineno = Curfn.Pos
 		moveToHeap(n)
 		Curfn = oldfn
 		lineno = ln

 	// ODOTPTR has already been introduced,
 	// so these are the non-pointer ODOT and OINDEX.
 	// In &x[0], if x is a slice, then x does not
 	// escape--the pointer inside x does, but that
 	// is always a heap pointer anyway.
 	case ODOT, OINDEX, OPAREN, OCONVNOP:
 		if !n.Left.Type.IsSlice() {
 			addrescapes(n.Left)
 		}
 	}
 }

 // moveToHeap records the parameter or local variable n as moved to the heap.
 func moveToHeap(n *Node) {
 	if Debug['r'] != 0 {
 		Dump("MOVE", n)
 	}
 	if compiling_runtime {
 		yyerror("%v escapes to heap, not allowed in runtime", n)
 	}
 	if n.Class() == PAUTOHEAP {
 		Dump("n", n)
 		Fatalf("double move to heap")
 	}

 	// Allocate a local stack variable to hold the pointer to the heap copy.
 	// temp will add it to the function declaration list automatically.
 	heapaddr := temp(types.NewPtr(n.Type))
 	heapaddr.Sym = lookup("&" + n.Sym.Name)
 	heapaddr.Orig.Sym = heapaddr.Sym
 	heapaddr.Pos = n.Pos

 	// Unset AutoTemp to persist the &foo variable name through SSA to
 	// liveness analysis.
 	// TODO(mdempsky/drchase): Cleaner solution?
 	heapaddr.Name.SetAutoTemp(false)

 	// Parameters have a local stack copy used at function start/end
 	// in addition to the copy in the heap that may live longer than
 	// the function.
 	if n.Class() == PPARAM || n.Class() == PPARAMOUT {
 		if n.Xoffset == BADWIDTH {
 			Fatalf("addrescapes before param assignment")
 		}

 		// We rewrite n below to be a heap variable (indirection of heapaddr).
 		// Preserve a copy so we can still write code referring to the original,
 		// and substitute that copy into the function declaration list
 		// so that analyses of the local (on-stack) variables use it.
 		stackcopy := newname(n.Sym)
 		stackcopy.Type = n.Type
 		stackcopy.Xoffset = n.Xoffset
 		stackcopy.SetClass(n.Class())
 		stackcopy.Name.Param.Heapaddr = heapaddr
 		if n.Class() == PPARAMOUT {
 			// Make sure the pointer to the heap copy is kept live throughout the function.
 			// The function could panic at any point, and then a defer could recover.
 			// Thus, we need the pointer to the heap copy always available so the
 			// post-deferreturn code can copy the return value back to the stack.
 			// See issue 16095.
 			heapaddr.Name.SetIsOutputParamHeapAddr(true)
 		}
 		n.Name.Param.Stackcopy = stackcopy

 		// Substitute the stackcopy into the function variable list so that
 		// liveness and other analyses use the underlying stack slot
 		// and not the now-pseudo-variable n.
 		found := false
 		for i, d := range Curfn.Func.Dcl {
 			if d == n {
 				Curfn.Func.Dcl[i] = stackcopy
 				found = true
 				break
 			}
 			// Parameters are before locals, so can stop early.
 			// This limits the search even in functions with many local variables.
 			if d.Class() == PAUTO {
 				break
 			}
 		}
 		if !found {
 			Fatalf("cannot find %v in local variable list", n)
 		}
 		Curfn.Func.Dcl = append(Curfn.Func.Dcl, n)
 	}

 	// Modify n in place so that uses of n now mean indirection of the heapaddr.
 	n.SetClass(PAUTOHEAP)
 	n.Xoffset = 0
 	n.Name.Param.Heapaddr = heapaddr
 	n.Esc = EscHeap
 	if Debug['m'] != 0 {
 		Warnl(n.Pos, "moved to heap: %v", n)
 	}
 }

 // This special tag is applied to uintptr variables
 // that we believe may hold unsafe.Pointers for
 // calls into assembly functions.
 const unsafeUintptrTag = "unsafe-uintptr"

 // This special tag is applied to uintptr parameters of functions
 // marked go:uintptrescapes.
 const uintptrEscapesTag = "uintptr-escapes"

 func (e *Escape) paramTag(fn *Node, narg int, f *types.Field) string {
 	name := func() string {
 		if f.Sym != nil {
 			return f.Sym.Name
 		}
 		return fmt.Sprintf("arg#%d", narg)
 	}

 	if fn.Nbody.Len() == 0 {
 		// Assume that uintptr arguments must be held live across the call.
 		// This is most important for syscall.Syscall.
 		// See golang.org/issue/13372.
 		// This really doesn't have much to do with escape analysis per se,
 		// but we are reusing the ability to annotate an individual function
 		// argument and pass those annotations along to importing code.
 		if f.Type.Etype == TUINTPTR {
 			if Debug['m'] != 0 {
 				Warnl(f.Pos, "assuming %v is unsafe uintptr", name())
 			}
 			return unsafeUintptrTag
 		}

 		if !f.Type.HasPointers() { // don't bother tagging for scalars
 			return ""
 		}

 		var esc EscLeaks

 		// External functions are assumed unsafe, unless
 		// //go:noescape is given before the declaration.
 		if fn.Func.Pragma&Noescape != 0 {
 			if Debug['m'] != 0 && f.Sym != nil {
 				Warnl(f.Pos, "%v does not escape", name())
 			}
 		} else {
 			if Debug['m'] != 0 && f.Sym != nil {
 				Warnl(f.Pos, "leaking param: %v", name())
 			}
 			esc.AddHeap(0)
 		}

 		return esc.Encode()
 	}

 	if fn.Func.Pragma&UintptrEscapes != 0 {
 		if f.Type.Etype == TUINTPTR {
 			if Debug['m'] != 0 {
 				Warnl(f.Pos, "marking %v as escaping uintptr", name())
 			}
 			return uintptrEscapesTag
 		}
 		if f.IsDDD() && f.Type.Elem().Etype == TUINTPTR {
 			// final argument is ...uintptr.
 			if Debug['m'] != 0 {
 				Warnl(f.Pos, "marking %v as escaping ...uintptr", name())
 			}
 			return uintptrEscapesTag
 		}
 	}

 	if !f.Type.HasPointers() { // don't bother tagging for scalars
 		return ""
 	}

 	// Unnamed parameters are unused and therefore do not escape.
 	if f.Sym == nil || f.Sym.IsBlank() {
 		var esc EscLeaks
 		return esc.Encode()
 	}

 	n := asNode(f.Nname)
 	loc := e.oldLoc(n)
 	esc := loc.paramEsc
 	esc.Optimize()

 	if Debug['m'] != 0 && !loc.escapes {
 		if esc.Empty() {
 			Warnl(f.Pos, "%v does not escape", name())
 		}
 		if x := esc.Heap(); x >= 0 {
 			if x == 0 {
 				Warnl(f.Pos, "leaking param: %v", name())
 			} else {
 				// TODO(mdempsky): Mention level=x like below?
 				Warnl(f.Pos, "leaking param content: %v", name())
 			}
 		}
 		for i := 0; i < numEscResults; i++ {
 			if x := esc.Result(i); x >= 0 {
 				res := fn.Type.Results().Field(i).Sym
 				Warnl(f.Pos, "leaking param: %v to result %v level=%d", name(), res, x)
 			}
 		}
 	}

 	return esc.Encode()
 }
	// Copyright 2011 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 gc

	import (
	"cmd/compile/internal/types"
	"fmt"
	)

	func escapes(all []*Node) {
	visitBottomUp(all, escapeFuncs)
	}

	const (
	EscFuncUnknown = 0 + iota
	EscFuncPlanned
	EscFuncStarted
	EscFuncTagged
	)

	func min8(a, b int8) int8 {
	if a < b {
	return a
	}
	return b
	}

	func max8(a, b int8) int8 {
	if a > b {
	return a
	}
	return b
	}

	const (
	EscUnknown = iota
	EscNone // Does not escape to heap, result, or parameters.
	EscHeap // Reachable from the heap
	EscNever // By construction will not escape.
	)

	// funcSym returns fn.Func.Nname.Sym if no nils are encountered along the way.
	func funcSym(fn Node) types.Sym {
	if fn == nil \|\| fn.Func.Nname == nil {
	return nil
	}
	return fn.Func.Nname.Sym
	}

	// Mark labels that have no backjumps to them as not increasing e.loopdepth.
	// Walk hasn't generated (goto\|label).Left.Sym.Label yet, so we'll cheat
	// and set it to one of the following two. Then in esc we'll clear it again.
	var (
	looping Node
	nonlooping Node
	)

	func isSliceSelfAssign(dst, src *Node) bool {
	// Detect the following special case.
	//
	// func (b *Buffer) Foo() {
	// n, m := ...
	// b.buf = b.buf[n:m]
	// }
	//
	// This assignment is a no-op for escape analysis,
	// it does not store any new pointers into b that were not already there.
	// However, without this special case b will escape, because we assign to OIND/ODOTPTR.
	// Here we assume that the statement will not contain calls,
	// that is, that order will move any calls to init.
	// Otherwise base ONAME value could change between the moments
	// when we evaluate it for dst and for src.

	// dst is ONAME dereference.
	if dst.Op != ODEREF && dst.Op != ODOTPTR \|\| dst.Left.Op != ONAME {
	return false
	}
	// src is a slice operation.
	switch src.Op {
	case OSLICE, OSLICE3, OSLICESTR:
	// OK.
	case OSLICEARR, OSLICE3ARR:
	// Since arrays are embedded into containing object,
	// slice of non-pointer array will introduce a new pointer into b that was not already there
	// (pointer to b itself). After such assignment, if b contents escape,
	// b escapes as well. If we ignore such OSLICEARR, we will conclude
	// that b does not escape when b contents do.
	//
	// Pointer to an array is OK since it's not stored inside b directly.
	// For slicing an array (not pointer to array), there is an implicit OADDR.
	// We check that to determine non-pointer array slicing.
	if src.Left.Op == OADDR {
	return false
	}
	default:
	return false
	}
	// slice is applied to ONAME dereference.
	if src.Left.Op != ODEREF && src.Left.Op != ODOTPTR \|\| src.Left.Left.Op != ONAME {
	return false
	}
	// dst and src reference the same base ONAME.
	return dst.Left == src.Left.Left
	}

	// isSelfAssign reports whether assignment from src to dst can
	// be ignored by the escape analysis as it's effectively a self-assignment.
	func isSelfAssign(dst, src *Node) bool {
	if isSliceSelfAssign(dst, src) {
	return true
	}

	// Detect trivial assignments that assign back to the same object.
	//
	// It covers these cases:
	// val.x = val.y
	// val.x[i] = val.y[j]
	// val.x1.x2 = val.x1.y2
	// ... etc
	//
	// These assignments do not change assigned object lifetime.

	if dst == nil \|\| src == nil \|\| dst.Op != src.Op {
	return false
	}

	switch dst.Op {
	case ODOT, ODOTPTR:
	// Safe trailing accessors that are permitted to differ.
	case OINDEX:
	if mayAffectMemory(dst.Right) \|\| mayAffectMemory(src.Right) {
	return false
	}
	default:
	return false
	}

	// The expression prefix must be both "safe" and identical.
	return samesafeexpr(dst.Left, src.Left)
	}

	// mayAffectMemory reports whether evaluation of n may affect the program's
	// memory state. If the expression can't affect memory state, then it can be
	// safely ignored by the escape analysis.
	func mayAffectMemory(n *Node) bool {
	// We may want to use a list of "memory safe" ops instead of generally
	// "side-effect free", which would include all calls and other ops that can
	// allocate or change global state. For now, it's safer to start with the latter.
	//
	// We're ignoring things like division by zero, index out of range,
	// and nil pointer dereference here.
	switch n.Op {
	case ONAME, OCLOSUREVAR, OLITERAL:
	return false

	// Left+Right group.
	case OINDEX, OADD, OSUB, OOR, OXOR, OMUL, OLSH, ORSH, OAND, OANDNOT, ODIV, OMOD:
	return mayAffectMemory(n.Left) \|\| mayAffectMemory(n.Right)

	// Left group.
	case ODOT, ODOTPTR, ODEREF, OCONVNOP, OCONV, OLEN, OCAP,
	ONOT, OBITNOT, OPLUS, ONEG, OALIGNOF, OOFFSETOF, OSIZEOF:
	return mayAffectMemory(n.Left)

	default:
	return true
	}
	}

	func mustHeapAlloc(n *Node) bool {
	if n.Type == nil {
	return false
	}

	// Parameters are always passed via the stack.
	if n.Op == ONAME && (n.Class() == PPARAM \|\| n.Class() == PPARAMOUT) {
	return false
	}

	if n.Type.Width > maxStackVarSize {
	return true
	}

	if (n.Op == ONEW \|\| n.Op == OPTRLIT) && n.Type.Elem().Width >= maxImplicitStackVarSize {
	return true
	}

	if n.Op == OCLOSURE && closureType(n).Size() >= maxImplicitStackVarSize {
	return true
	}
	if n.Op == OCALLPART && partialCallType(n).Size() >= maxImplicitStackVarSize {
	return true
	}

	if n.Op == OMAKESLICE && !isSmallMakeSlice(n) {
	return true
	}

	return false
	}

	// addrescapes tags node n as having had its address taken
	// by "increasing" the "value" of n.Esc to EscHeap.
	// Storage is allocated as necessary to allow the address
	// to be taken.
	func addrescapes(n *Node) {
	switch n.Op {
	default:
	// Unexpected Op, probably due to a previous type error. Ignore.

	case ODEREF, ODOTPTR:
	// Nothing to do.

	case ONAME:
	if n == nodfp {
	break
	}

	// if this is a tmpname (PAUTO), it was tagged by tmpname as not escaping.
	// on PPARAM it means something different.
	if n.Class() == PAUTO && n.Esc == EscNever {
	break
	}

	// If a closure reference escapes, mark the outer variable as escaping.
	if n.Name.IsClosureVar() {
	addrescapes(n.Name.Defn)
	break
	}

	if n.Class() != PPARAM && n.Class() != PPARAMOUT && n.Class() != PAUTO {
	break
	}

	// This is a plain parameter or local variable that needs to move to the heap,
	// but possibly for the function outside the one we're compiling.
	// That is, if we have:
	//
	// func f(x int) {
	// func() {
	// global = &x
	// }
	// }
	//
	// then we're analyzing the inner closure but we need to move x to the
	// heap in f, not in the inner closure. Flip over to f before calling moveToHeap.
	oldfn := Curfn
	Curfn = n.Name.Curfn
	if Curfn.Func.Closure != nil && Curfn.Op == OCLOSURE {
	Curfn = Curfn.Func.Closure
	}
	ln := lineno
	lineno = Curfn.Pos
	moveToHeap(n)
	Curfn = oldfn
	lineno = ln

	// ODOTPTR has already been introduced,
	// so these are the non-pointer ODOT and OINDEX.
	// In &x[0], if x is a slice, then x does not
	// escape--the pointer inside x does, but that
	// is always a heap pointer anyway.
	case ODOT, OINDEX, OPAREN, OCONVNOP:
	if !n.Left.Type.IsSlice() {
	addrescapes(n.Left)
	}
	}
	}

	// moveToHeap records the parameter or local variable n as moved to the heap.
	func moveToHeap(n *Node) {
	if Debug['r'] != 0 {
	Dump("MOVE", n)
	}
	if compiling_runtime {
	yyerror("%v escapes to heap, not allowed in runtime", n)
	}
	if n.Class() == PAUTOHEAP {
	Dump("n", n)
	Fatalf("double move to heap")
	}

	// Allocate a local stack variable to hold the pointer to the heap copy.
	// temp will add it to the function declaration list automatically.
	heapaddr := temp(types.NewPtr(n.Type))
	heapaddr.Sym = lookup("&" + n.Sym.Name)
	heapaddr.Orig.Sym = heapaddr.Sym
	heapaddr.Pos = n.Pos

	// Unset AutoTemp to persist the &foo variable name through SSA to
	// liveness analysis.
	// TODO(mdempsky/drchase): Cleaner solution?
	heapaddr.Name.SetAutoTemp(false)

	// Parameters have a local stack copy used at function start/end
	// in addition to the copy in the heap that may live longer than
	// the function.
	if n.Class() == PPARAM \|\| n.Class() == PPARAMOUT {
	if n.Xoffset == BADWIDTH {
	Fatalf("addrescapes before param assignment")
	}

	// We rewrite n below to be a heap variable (indirection of heapaddr).
	// Preserve a copy so we can still write code referring to the original,
	// and substitute that copy into the function declaration list
	// so that analyses of the local (on-stack) variables use it.
	stackcopy := newname(n.Sym)
	stackcopy.Type = n.Type
	stackcopy.Xoffset = n.Xoffset
	stackcopy.SetClass(n.Class())
	stackcopy.Name.Param.Heapaddr = heapaddr
	if n.Class() == PPARAMOUT {
	// Make sure the pointer to the heap copy is kept live throughout the function.
	// The function could panic at any point, and then a defer could recover.
	// Thus, we need the pointer to the heap copy always available so the
	// post-deferreturn code can copy the return value back to the stack.
	// See issue 16095.
	heapaddr.Name.SetIsOutputParamHeapAddr(true)
	}
	n.Name.Param.Stackcopy = stackcopy

	// Substitute the stackcopy into the function variable list so that
	// liveness and other analyses use the underlying stack slot
	// and not the now-pseudo-variable n.
	found := false
	for i, d := range Curfn.Func.Dcl {
	if d == n {
	Curfn.Func.Dcl[i] = stackcopy
	found = true
	break
	}
	// Parameters are before locals, so can stop early.
	// This limits the search even in functions with many local variables.
	if d.Class() == PAUTO {
	break
	}
	}
	if !found {
	Fatalf("cannot find %v in local variable list", n)
	}
	Curfn.Func.Dcl = append(Curfn.Func.Dcl, n)
	}

	// Modify n in place so that uses of n now mean indirection of the heapaddr.
	n.SetClass(PAUTOHEAP)
	n.Xoffset = 0
	n.Name.Param.Heapaddr = heapaddr
	n.Esc = EscHeap
	if Debug['m'] != 0 {
	Warnl(n.Pos, "moved to heap: %v", n)
	}
	}

	// This special tag is applied to uintptr variables
	// that we believe may hold unsafe.Pointers for
	// calls into assembly functions.
	const unsafeUintptrTag = "unsafe-uintptr"

	// This special tag is applied to uintptr parameters of functions
	// marked go:uintptrescapes.
	const uintptrEscapesTag = "uintptr-escapes"

	func (e Escape) paramTag(fn Node, narg int, f *types.Field) string {
	name := func() string {
	if f.Sym != nil {
	return f.Sym.Name
	}
	return fmt.Sprintf("arg#%d", narg)
	}

	if fn.Nbody.Len() == 0 {
	// Assume that uintptr arguments must be held live across the call.
	// This is most important for syscall.Syscall.
	// See golang.org/issue/13372.
	// This really doesn't have much to do with escape analysis per se,
	// but we are reusing the ability to annotate an individual function
	// argument and pass those annotations along to importing code.
	if f.Type.Etype == TUINTPTR {
	if Debug['m'] != 0 {
	Warnl(f.Pos, "assuming %v is unsafe uintptr", name())
	}
	return unsafeUintptrTag
	}

	if !f.Type.HasPointers() { // don't bother tagging for scalars
	return ""
	}

	var esc EscLeaks

	// External functions are assumed unsafe, unless
	// //go:noescape is given before the declaration.
	if fn.Func.Pragma&Noescape != 0 {
	if Debug['m'] != 0 && f.Sym != nil {
	Warnl(f.Pos, "%v does not escape", name())
	}
	} else {
	if Debug['m'] != 0 && f.Sym != nil {
	Warnl(f.Pos, "leaking param: %v", name())
	}
	esc.AddHeap(0)
	}

	return esc.Encode()
	}

	if fn.Func.Pragma&UintptrEscapes != 0 {
	if f.Type.Etype == TUINTPTR {
	if Debug['m'] != 0 {
	Warnl(f.Pos, "marking %v as escaping uintptr", name())
	}
	return uintptrEscapesTag
	}
	if f.IsDDD() && f.Type.Elem().Etype == TUINTPTR {
	// final argument is ...uintptr.
	if Debug['m'] != 0 {
	Warnl(f.Pos, "marking %v as escaping ...uintptr", name())
	}
	return uintptrEscapesTag
	}
	}

	if !f.Type.HasPointers() { // don't bother tagging for scalars
	return ""
	}

	// Unnamed parameters are unused and therefore do not escape.
	if f.Sym == nil \|\| f.Sym.IsBlank() {
	var esc EscLeaks
	return esc.Encode()
	}

	n := asNode(f.Nname)
	loc := e.oldLoc(n)
	esc := loc.paramEsc
	esc.Optimize()

	if Debug['m'] != 0 && !loc.escapes {
	if esc.Empty() {
	Warnl(f.Pos, "%v does not escape", name())
	}
	if x := esc.Heap(); x >= 0 {
	if x == 0 {
	Warnl(f.Pos, "leaking param: %v", name())
	} else {
	// TODO(mdempsky): Mention level=x like below?
	Warnl(f.Pos, "leaking param content: %v", name())
	}
	}
	for i := 0; i < numEscResults; i++ {
	if x := esc.Result(i); x >= 0 {
	res := fn.Type.Results().Field(i).Sym
	Warnl(f.Pos, "leaking param: %v to result %v level=%d", name(), res, x)
	}
	}
	}

	return esc.Encode()
	}