cmd/compile: fix liveness computation for heap-escaped parameters

The liveness computation of parameters generally was never
correct, but forcing all parameters to be live throughout the
function covered up that problem. The new SSA back end is
too clever: even though it currently keeps the parameter values live
throughout the function, it may find optimizations that mean
the current values are not written back to the original parameter
stack slots immediately or ever (for example if a parameter is set
to nil, SSA constant propagation may replace all later uses of the
parameter with a constant nil, eliminating the need to write the nil
value back to the stack slot), so the liveness code must now
track the actual operations on the stack slots, exposing these
problems.

One small problem in the handling of arguments is that nodarg
can return ONAME PPARAM nodes with adjusted offsets, so that
there are actually multiple *Node pointers for the same parameter
in the instruction stream. This might be possible to correct, but
not in this CL. For now, we fix this by using n.Orig instead of n
when considering PPARAM and PPARAMOUT nodes.

The major problem in the handling of arguments is general
confusion in the liveness code about the meaning of PPARAM|PHEAP
and PPARAMOUT|PHEAP nodes, especially as contrasted with PAUTO|PHEAP.
The difference between these two is that when a local variable "moves"
to the heap, it's really just allocated there to start with; in contrast,
when an argument moves to the heap, the actual data has to be copied
there from the stack at the beginning of the function, and when a
result "moves" to the heap the value in the heap has to be copied
back to the stack when the function returns
This general confusion is also present in the SSA back end.

The PHEAP bit worked decently when I first introduced it 7 years ago (!)
in 391425ae. The back end did nothing sophisticated, and in particular
there was no analysis at all: no escape analysis, no liveness analysis,
and certainly no SSA back end. But the complications caused in the
various downstream consumers suggest that this should be a detail
kept mainly in the front end.

This CL therefore eliminates both the PHEAP bit and even the idea of
"heap variables" from the back ends.

First, it replaces the PPARAM|PHEAP, PPARAMOUT|PHEAP, and PAUTO|PHEAP
variable classes with the single PAUTOHEAP, a pseudo-class indicating
a variable maintained on the heap and available by indirecting a
local variable kept on the stack (a plain PAUTO).

Second, walkexpr replaces all references to PAUTOHEAP variables
with indirections of the corresponding PAUTO variable.
The back ends and the liveness code now just see plain indirected
variables. This may actually produce better code, but the real goal
here is to eliminate these little-used and somewhat suspect code
paths in the back end analyses.

The OPARAM node type goes away too.

A followup CL will do the same to PPARAMREF. I'm not sure that
the back ends (SSA in particular) are handling those right either,
and with the framework established in this CL that change is trivial
and the result clearly more correct.

Fixes #15747.

Change-Id: I2770b1ce3cbc93981bfc7166be66a9da12013d74
Reviewed-on: https://go-review.googlesource.com/23393
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/amd64/gsubr.go

@@ -116,7 +116,7 @@ func ginscmp(op gc.Op, t *gc.Type, n1, n2 *gc.Node, likely int) *obj.Prog {
 		base = n1.Left
 	}
 
-	if base.Op == gc.ONAME && base.Class&gc.PHEAP == 0 || n1.Op == gc.OINDREG {
+	if base.Op == gc.ONAME && base.Class != gc.PAUTOHEAP || n1.Op == gc.OINDREG {
 		r1 = *n1
 	} else {
 		gc.Regalloc(&r1, t, n1)
@@ -229,6 +229,8 @@ func gmove(f *gc.Node, t *gc.Node) {
 
 	switch uint32(ft)<<16 | uint32(tt) {
 	default:
+		gc.Dump("f", f)
+		gc.Dump("t", t)
 		gc.Fatalf("gmove %v -> %v", gc.Tconv(f.Type, gc.FmtLong), gc.Tconv(t.Type, gc.FmtLong))
 
 		/*

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

@@ -55,12 +55,15 @@ func widstruct(errtype *Type, t *Type, o int64, flag int) int64 {
 		}
 		f.Offset = o
 		if f.Nname != nil {
-			// this same stackparam logic is in addrescapes
+			// addrescapes has similar code to update these offsets.
-			// in typecheck.go.  usually addrescapes runs after
+			// Usually addrescapes runs after widstruct,
-			// widstruct, in which case we could drop this,
+			// in which case we could drop this,
 			// but function closure functions are the exception.
-			if f.Nname.Name.Param.Stackparam != nil {
+			// NOTE(rsc): This comment may be stale.
-				f.Nname.Name.Param.Stackparam.Xoffset = o
+			// It's possible the ordering has changed and this is
+			// now the common case. I'm not sure.
+			if f.Nname.Name.Param.Stackcopy != nil {
+				f.Nname.Name.Param.Stackcopy.Xoffset = o
 				f.Nname.Xoffset = 0
 			} else {
 				f.Nname.Xoffset = o

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

@@ -1408,8 +1408,8 @@ func (p *exporter) stmt(n *Node) {
 	switch op := n.Op; op {
 	case ODCL:
 		p.op(ODCL)
-		switch n.Left.Class &^ PHEAP {
+		switch n.Left.Class {
-		case PPARAM, PPARAMOUT, PAUTO:
+		case PPARAM, PPARAMOUT, PAUTO, PAUTOHEAP:
 			// TODO(gri) when is this not PAUTO?
 			// Also, originally this didn't look like
 			// the default case. Investigate.

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

@@ -519,7 +519,7 @@ func cgen_wb(n, res *Node, wb bool) {
 		ODOTPTR,
 		OINDEX,
 		OIND,
-		ONAME: // PHEAP or PPARAMREF var
+		ONAME: // PPARAMREF var
 		var n1 Node
 		Igen(n, &n1, res)
 
@@ -1579,7 +1579,7 @@ func Agen(n *Node, res *Node) {
 		}
 
 		// should only get here for heap vars or paramref
-		if n.Class&PHEAP == 0 && n.Class != PPARAMREF {
+		if n.Class != PPARAMREF {
 			Dump("bad agen", n)
 			Fatalf("agen: bad ONAME class %#x", n.Class)
 		}
@@ -1646,7 +1646,7 @@ func Igen(n *Node, a *Node, res *Node) {
 
 	switch n.Op {
 	case ONAME:
-		if (n.Class&PHEAP != 0) || n.Class == PPARAMREF {
+		if n.Class == PPARAMREF {
 			break
 		}
 		*a = *n

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

@@ -405,7 +405,7 @@ func Complexgen(n *Node, res *Node) {
 		ODOTPTR,
 		OINDEX,
 		OIND,
-		ONAME, // PHEAP or PPARAMREF var
+		ONAME, // PPARAMREF var
 		OCALLFUNC,
 		OCALLMETH,
 		OCALLINTER:

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

@@ -1068,7 +1068,6 @@ func escassign(e *EscState, dst, src *Node, step *EscStep) {
 		OIND,    // dst = *x
 		ODOTPTR, // dst = (*x).f
 		ONAME,
-		OPARAM,
 		ODDDARG,
 		OPTRLIT,
 		OARRAYLIT,
@@ -1835,20 +1834,20 @@ func escwalkBody(e *EscState, level Level, dst *Node, src *Node, step *EscStep,
 		}
 		if leaks {
 			src.Esc = EscHeap
-			addrescapes(src.Left)
 			if Debug['m'] != 0 && osrcesc != src.Esc {
 				p := src
 				if p.Left.Op == OCLOSURE {
 					p = p.Left // merely to satisfy error messages in tests
 				}
 				if Debug['m'] > 2 {
-					Warnl(src.Lineno, "%v escapes to heap, level=%v, dst.eld=%v, src.eld=%v",
+					Warnl(src.Lineno, "%v escapes to heap, level=%v, dst=%v dst.eld=%v, src.eld=%v",
-						Nconv(p, FmtShort), level, dstE.Escloopdepth, modSrcLoopdepth)
+						Nconv(p, FmtShort), level, dst, dstE.Escloopdepth, modSrcLoopdepth)
 				} else {
 					Warnl(src.Lineno, "%v escapes to heap", Nconv(p, FmtShort))
 					step.describe(src)
 				}
 			}
+			addrescapes(src.Left)
 			escwalkBody(e, level.dec(), dst, src.Left, e.stepWalk(dst, src.Left, why, step), modSrcLoopdepth)
 			extraloopdepth = modSrcLoopdepth // passes to recursive case, seems likely a no-op
 		} else {

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

@@ -121,7 +121,7 @@ func reexportdep(n *Node) {
 	//print("reexportdep %+hN\n", n);
 	switch n.Op {
 	case ONAME:
-		switch n.Class &^ PHEAP {
+		switch n.Class {
 		// methods will be printed along with their type
 		// nodes for T.Method expressions
 		case PFUNC:

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

@@ -218,6 +218,7 @@ var classnames = []string{
 	"Pxxx",
 	"PEXTERN",
 	"PAUTO",
+	"PAUTOHEAP",
 	"PPARAM",
 	"PPARAMOUT",
 	"PPARAMREF",
@@ -251,14 +252,10 @@ func jconv(n *Node, flag FmtFlag) string {
 	}
 
 	if n.Class != 0 {
-		s := ""
+		if int(n.Class) < len(classnames) {
-		if n.Class&PHEAP != 0 {
+			fmt.Fprintf(&buf, " class(%s)", classnames[n.Class])
-			s = ",heap"
-		}
-		if int(n.Class&^PHEAP) < len(classnames) {
-			fmt.Fprintf(&buf, " class(%s%s)", classnames[n.Class&^PHEAP], s)
 		} else {
-			fmt.Fprintf(&buf, " class(%d?%s)", n.Class&^PHEAP, s)
+			fmt.Fprintf(&buf, " class(%d?)", n.Class)
 		}
 	}
 
@@ -798,8 +795,8 @@ func stmtfmt(n *Node) string {
 	switch n.Op {
 	case ODCL:
 		if fmtmode == FExp {
-			switch n.Left.Class &^ PHEAP {
+			switch n.Left.Class {
-			case PPARAM, PPARAMOUT, PAUTO:
+			case PPARAM, PPARAMOUT, PAUTO, PAUTOHEAP:
 				f += fmt.Sprintf("var %v %v", n.Left, n.Left.Type)
 				goto ret
 			}

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

@@ -43,53 +43,40 @@ func addrescapes(n *Node) {
 			break
 		}
 
-		switch n.Class {
+		// A PPARAMREF is a closure reference.
-		case PPARAMREF:
+		// Mark the thing it refers to as escaping.
+		if n.Class == PPARAMREF {
 			addrescapes(n.Name.Defn)
-
+			break
-		// if func param, need separate temporary
-		// to hold heap pointer.
-		// the function type has already been checked
-		// (we're in the function body)
-		// so the param already has a valid xoffset.
-
-		// expression to refer to stack copy
-		case PPARAM, PPARAMOUT:
-			n.Name.Param.Stackparam = Nod(OPARAM, n, nil)
-
-			n.Name.Param.Stackparam.Type = n.Type
-			n.Name.Param.Stackparam.Addable = true
-			if n.Xoffset == BADWIDTH {
-				Fatalf("addrescapes before param assignment")
-			}
-			n.Name.Param.Stackparam.Xoffset = n.Xoffset
-			fallthrough
-
-		case PAUTO:
-			n.Class |= PHEAP
-
-			n.Addable = false
-			n.Ullman = 2
-			n.Xoffset = 0
-
-			// create stack variable to hold pointer to heap
-			oldfn := Curfn
-
-			Curfn = n.Name.Curfn
-			if Curfn.Func.Closure != nil && Curfn.Op == OCLOSURE {
-				Curfn = Curfn.Func.Closure
-			}
-			n.Name.Heapaddr = temp(Ptrto(n.Type))
-			buf := fmt.Sprintf("&%v", n.Sym)
-			n.Name.Heapaddr.Sym = Lookup(buf)
-			n.Name.Heapaddr.Orig.Sym = n.Name.Heapaddr.Sym
-			n.Esc = EscHeap
-			if Debug['m'] != 0 {
-				fmt.Printf("%v: moved to heap: %v\n", n.Line(), n)
-			}
-			Curfn = oldfn
 		}
 
+		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.Lineno
+		moveToHeap(n)
+		Curfn = oldfn
+		lineno = ln
+
 	case OIND, ODOTPTR:
 		break
 
@@ -105,6 +92,110 @@ func addrescapes(n *Node) {
 	}
 }
 
+// isParamStackCopy reports whether this is the on-stack copy of a
+// function parameter that moved to the heap.
+func (n *Node) isParamStackCopy() bool {
+	return n.Op == ONAME && (n.Class == PPARAM || n.Class == PPARAMOUT) && n.Name.Heapaddr != nil
+}
+
+// isParamHeapCopy reports whether this is the on-heap copy of
+// a function parameter that moved to the heap.
+func (n *Node) isParamHeapCopy() bool {
+	return n.Op == ONAME && n.Class == PAUTOHEAP && n.Name.Param.Stackcopy != nil
+}
+
+// paramClass reports the parameter class (PPARAM or PPARAMOUT)
+// of the node, which may be an unmoved on-stack parameter
+// or the on-heap or on-stack copy of a parameter that moved to the heap.
+// If the node is not a parameter, paramClass returns Pxxx.
+func (n *Node) paramClass() Class {
+	if n.Op != ONAME {
+		return Pxxx
+	}
+	if n.Class == PPARAM || n.Class == PPARAMOUT {
+		return n.Class
+	}
+	if n.isParamHeapCopy() {
+		return n.Name.Param.Stackcopy.Class
+	}
+	return Pxxx
+}
+
+// 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(Ptrto(n.Type))
+	heapaddr.Sym = Lookup("&" + n.Sym.Name)
+	heapaddr.Orig.Sym = heapaddr.Sym
+
+	// 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 := Nod(ONAME, nil, nil)
+		stackcopy.Sym = n.Sym
+		stackcopy.Type = n.Type
+		stackcopy.Xoffset = n.Xoffset
+		stackcopy.Class = n.Class
+		stackcopy.Name.Heapaddr = heapaddr
+		if n.Class == PPARAM {
+			stackcopy.SetNotLiveAtEnd(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.Class = PAUTOHEAP
+	n.Ullman = 2
+	n.Xoffset = 0
+	n.Name.Heapaddr = heapaddr
+	n.Esc = EscHeap
+	if Debug['m'] != 0 {
+		fmt.Printf("%v: moved to heap: %v\n", n.Line(), n)
+	}
+}
+
 func clearlabels() {
 	for _, l := range labellist {
 		l.Sym.Label = nil
@@ -243,16 +334,9 @@ func cgen_dcl(n *Node) {
 		Fatalf("cgen_dcl")
 	}
 
-	if n.Class&PHEAP == 0 {
+	if n.Class == PAUTOHEAP {
-		return
+		Fatalf("cgen_dcl %v", n)
 	}
-	if compiling_runtime {
-		Yyerror("%v escapes to heap, not allowed in runtime.", n)
-	}
-	if prealloc[n] == nil {
-		prealloc[n] = callnew(n.Type)
-	}
-	Cgen_as(n.Name.Heapaddr, prealloc[n])
 }
 
 // generate discard of value
@@ -263,7 +347,7 @@ func cgen_discard(nr *Node) {
 
 	switch nr.Op {
 	case ONAME:
-		if nr.Class&PHEAP == 0 && nr.Class != PEXTERN && nr.Class != PFUNC && nr.Class != PPARAMREF {
+		if nr.Class != PAUTOHEAP && nr.Class != PEXTERN && nr.Class != PFUNC && nr.Class != PPARAMREF {
 			gused(nr)
 		}
 
@@ -908,11 +992,6 @@ func Cgen_as_wb(nl, nr *Node, wb bool) {
 	}
 
 	if nr == nil || iszero(nr) {
-		// heaps should already be clear
-		if nr == nil && (nl.Class&PHEAP != 0) {
-			return
-		}
-
 		tl := nl.Type
 		if tl == nil {
 			return

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

@@ -91,14 +91,13 @@ const (
 	Pxxx      Class = iota
 	PEXTERN         // global variable
 	PAUTO           // local variables
+	PAUTOHEAP       // local variable or parameter moved to heap
 	PPARAM          // input arguments
 	PPARAMOUT       // output results
 	PPARAMREF       // closure variable reference
 	PFUNC           // global function
 
 	PDISCARD // discard during parse of duplicate import
-
-	PHEAP = 1 << 7 // an extra bit to identify an escaped variable
 )
 
 // note this is the runtime representation

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

@@ -53,7 +53,6 @@ func Ismem(n *Node) bool {
 		OCAP,
 		OINDREG,
 		ONAME,
-		OPARAM,
 		OCLOSUREVAR:
 		return true
 
@@ -349,18 +348,6 @@ func Naddr(a *obj.Addr, n *Node) {
 			a.Width = 0
 		}
 
-		// n->left is PHEAP ONAME for stack parameter.
-	// compute address of actual parameter on stack.
-	case OPARAM:
-		a.Etype = uint8(Simtype[n.Left.Type.Etype])
-
-		a.Width = n.Left.Type.Width
-		a.Offset = n.Xoffset
-		a.Sym = Linksym(n.Left.Sym)
-		a.Type = obj.TYPE_MEM
-		a.Name = obj.NAME_PARAM
-		a.Node = n.Left.Orig
-
 	case OCLOSUREVAR:
 		if !Curfn.Func.Needctxt {
 			Fatalf("closurevar without needctxt")

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

@@ -27,9 +27,7 @@
 
 package gc
 
-import (
+import "fmt"
-	"fmt"
-)
 
 // Get the function's package. For ordinary functions it's on the ->sym, but for imported methods
 // the ->sym can be re-used in the local package, so peel it off the receiver's type.
@@ -180,6 +178,7 @@ func ishairy(n *Node, budget *int32) bool {
 			*budget -= fn.InlCost
 			break
 		}
+
 		if n.Left.Op == ONAME && n.Left.Left != nil && n.Left.Left.Op == OTYPE && n.Left.Right != nil && n.Left.Right.Op == ONAME { // methods called as functions
 			if d := n.Left.Sym.Def; d != nil && d.Func.Inl.Len() != 0 {
 				*budget -= d.Func.InlCost
@@ -568,14 +567,13 @@ func mkinlcall1(n *Node, fn *Node, isddd bool) *Node {
 		if ln.Class == PPARAMOUT { // return values handled below.
 			continue
 		}
+		if ln.isParamStackCopy() { // ignore the on-stack copy of a parameter that moved to the heap
+			continue
+		}
 		if ln.Op == ONAME {
-			ln.Name.Inlvar = inlvar(ln)
+			ln.Name.Inlvar = typecheck(inlvar(ln), Erv)
-
+			if ln.Class == PPARAM || ln.Name.Param.Stackcopy != nil && ln.Name.Param.Stackcopy.Class == PPARAM {
-			// Typecheck because inlvar is not necessarily a function parameter.
+				ninit.Append(Nod(ODCL, ln.Name.Inlvar, nil))
-			ln.Name.Inlvar = typecheck(ln.Name.Inlvar, Erv)
-
-			if ln.Class&^PHEAP != PAUTO {
-				ninit.Append(Nod(ODCL, ln.Name.Inlvar, nil)) // otherwise gen won't emit the allocations for heapallocs
 			}
 		}
 	}

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

@@ -76,7 +76,6 @@ var opnames = []string{
 	OINDEX:           "INDEX",
 	OINDEXMAP:        "INDEXMAP",
 	OKEY:             "KEY",
-	OPARAM:           "PARAM",
 	OLEN:             "LEN",
 	OMAKE:            "MAKE",
 	OMAKECHAN:        "MAKECHAN",

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

@@ -203,6 +203,14 @@ func getvariables(fn *Node) []*Node {
 	var result []*Node
 	for _, ln := range fn.Func.Dcl {
 		if ln.Op == ONAME {
+			switch ln.Class {
+			case PAUTO, PPARAM, PPARAMOUT, PFUNC, PAUTOHEAP:
+				// ok
+			default:
+				Dump("BAD NODE", ln)
+				Fatalf("getvariables")
+			}
+
 			// In order for GODEBUG=gcdead=1 to work, each bitmap needs
 			// to contain information about all variables covered by the bitmap.
 			// For local variables, the bitmap only covers the stkptrsize
@@ -567,7 +575,7 @@ func progeffects(prog *obj.Prog, vars []*Node, uevar bvec, varkill bvec, avarini
 		// read the out arguments - they won't be set until the new
 		// function runs.
 		for i, node := range vars {
-			switch node.Class &^ PHEAP {
+			switch node.Class {
 			case PPARAM:
 				if !node.NotLiveAtEnd() {
 					bvset(uevar, int32(i))
@@ -595,7 +603,7 @@ func progeffects(prog *obj.Prog, vars []*Node, uevar bvec, varkill bvec, avarini
 		// A text instruction marks the entry point to a function and
 		// the definition point of all in arguments.
 		for i, node := range vars {
-			switch node.Class &^ PHEAP {
+			switch node.Class {
 			case PPARAM:
 				if node.Addrtaken {
 					bvset(avarinit, int32(i))
@@ -610,23 +618,24 @@ func progeffects(prog *obj.Prog, vars []*Node, uevar bvec, varkill bvec, avarini
 	if prog.Info.Flags&(LeftRead|LeftWrite|LeftAddr) != 0 {
 		from := &prog.From
 		if from.Node != nil && from.Sym != nil && ((from.Node).(*Node)).Name.Curfn == Curfn {
-			switch ((from.Node).(*Node)).Class &^ PHEAP {
+			switch ((from.Node).(*Node)).Class {
 			case PAUTO, PPARAM, PPARAMOUT:
-				pos, ok := from.Node.(*Node).Opt().(int32) // index in vars
+				n := from.Node.(*Node).Orig // orig needed for certain nodarg results
+				pos, ok := n.Opt().(int32) // index in vars
 				if !ok {
 					break
 				}
-				if pos >= int32(len(vars)) || vars[pos] != from.Node {
+				if pos >= int32(len(vars)) || vars[pos] != n {
-					Fatalf("bad bookkeeping in liveness %v %d", Nconv(from.Node.(*Node), 0), pos)
+					Fatalf("bad bookkeeping in liveness %v %d", Nconv(n, 0), pos)
 				}
-				if ((from.Node).(*Node)).Addrtaken {
+				if n.Addrtaken {
 					bvset(avarinit, pos)
 				} else {
 					if prog.Info.Flags&(LeftRead|LeftAddr) != 0 {
 						bvset(uevar, pos)
 					}
 					if prog.Info.Flags&LeftWrite != 0 {
-						if from.Node != nil && !Isfat(((from.Node).(*Node)).Type) {
+						if !Isfat(n.Type) {
 							bvset(varkill, pos)
 						}
 					}
@@ -638,16 +647,17 @@ func progeffects(prog *obj.Prog, vars []*Node, uevar bvec, varkill bvec, avarini
 	if prog.Info.Flags&(RightRead|RightWrite|RightAddr) != 0 {
 		to := &prog.To
 		if to.Node != nil && to.Sym != nil && ((to.Node).(*Node)).Name.Curfn == Curfn {
-			switch ((to.Node).(*Node)).Class &^ PHEAP {
+			switch ((to.Node).(*Node)).Class {
 			case PAUTO, PPARAM, PPARAMOUT:
-				pos, ok := to.Node.(*Node).Opt().(int32) // index in vars
+				n := to.Node.(*Node).Orig // orig needed for certain nodarg results
+				pos, ok := n.Opt().(int32) // index in vars
 				if !ok {
 					return
 				}
-				if pos >= int32(len(vars)) || vars[pos] != to.Node {
+				if pos >= int32(len(vars)) || vars[pos] != n {
-					Fatalf("bad bookkeeping in liveness %v %d", Nconv(to.Node.(*Node), 0), pos)
+					Fatalf("bad bookkeeping in liveness %v %d", Nconv(n, 0), pos)
 				}
-				if ((to.Node).(*Node)).Addrtaken {
+				if n.Addrtaken {
 					if prog.As != obj.AVARKILL {
 						bvset(avarinit, pos)
 					}
@@ -667,7 +677,7 @@ func progeffects(prog *obj.Prog, vars []*Node, uevar bvec, varkill bvec, avarini
 						bvset(uevar, pos)
 					}
 					if prog.Info.Flags&RightWrite != 0 {
-						if to.Node != nil && (!Isfat(((to.Node).(*Node)).Type) || prog.As == obj.AVARDEF) {
+						if !Isfat(n.Type) || prog.As == obj.AVARDEF {
 							bvset(varkill, pos)
 						}
 					}
@@ -814,8 +824,7 @@ func checkparam(fn *Node, p *obj.Prog, n *Node) {
 		return
 	}
 	for _, a := range fn.Func.Dcl {
-		class := a.Class &^ PHEAP
+		if a.Op == ONAME && (a.Class == PPARAM || a.Class == PPARAMOUT) && a == n {
-		if a.Op == ONAME && (class == PPARAM || class == PPARAMOUT) && a == n {
 			return
 		}
 	}

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

@@ -419,7 +419,6 @@ func instrumentnode(np **Node, init *Nodes, wr int, skip int) {
 	case OPRINT, // don't bother instrumenting it
 		OPRINTN,     // don't bother instrumenting it
 		OCHECKNIL,   // always followed by a read.
-		OPARAM,      // it appears only in fn->exit to copy heap params back
 		OCLOSUREVAR, // immutable pointer to captured variable
 		ODOTMETH,    // either part of CALLMETH or CALLPART (lowered to PTRLIT)
 		OINDREG,     // at this stage, only n(SP) nodes from nodarg
@@ -496,7 +495,7 @@ func callinstr(np **Node, init *Nodes, wr int, skip int) bool {
 	// e.g. if we've got a local variable/method receiver
 	// that has got a pointer inside. Whether it points to
 	// the heap or not is impossible to know at compile time
-	if (class&PHEAP != 0) || class == PPARAMREF || class == PEXTERN || b.Op == OINDEX || b.Op == ODOTPTR || b.Op == OIND {
+	if class == PAUTOHEAP || class == PPARAMREF || class == PEXTERN || b.Op == OINDEX || b.Op == ODOTPTR || b.Op == OIND {
 		hascalls := 0
 		foreach(n, hascallspred, &hascalls)
 		if hascalls != 0 {

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

@@ -516,7 +516,7 @@ func isliteral(n *Node) bool {
 }
 
 func (n *Node) isSimpleName() bool {
-	return n.Op == ONAME && n.Addable && n.Class&PHEAP == 0 && n.Class != PPARAMREF
+	return n.Op == ONAME && n.Addable && n.Class != PAUTOHEAP && n.Class != PPARAMREF
 }
 
 func litas(l *Node, r *Node, init *Nodes) {

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

@@ -165,23 +165,15 @@ func buildssa(fn *Node) *ssa.Func {
 				s.ptrargs = append(s.ptrargs, n)
 				n.SetNotLiveAtEnd(true) // SSA takes care of this explicitly
 			}
-		case PAUTO | PHEAP:
-			// TODO this looks wrong for PAUTO|PHEAP, no vardef, but also no definition
-			aux := s.lookupSymbol(n, &ssa.AutoSymbol{Typ: n.Type, Node: n})
-			s.decladdrs[n] = s.entryNewValue1A(ssa.OpAddr, Ptrto(n.Type), aux, s.sp)
-		case PPARAM | PHEAP, PPARAMOUT | PHEAP:
-		// This ends up wrong, have to do it at the PARAM node instead.
 		case PAUTO:
 			// processed at each use, to prevent Addr coming
 			// before the decl.
+		case PAUTOHEAP:
+			// moved to heap - already handled by frontend
 		case PFUNC:
 			// local function - already handled by frontend
 		default:
-			str := ""
+			s.Unimplementedf("local variable with class %s unimplemented", classnames[n.Class])
-			if n.Class&PHEAP != 0 {
-				str = ",heap"
-			}
-			s.Unimplementedf("local variable with class %s%s unimplemented", classnames[n.Class&^PHEAP], str)
 		}
 	}
 
@@ -294,7 +286,7 @@ type state struct {
 	// list of FwdRef values.
 	fwdRefs []*ssa.Value
 
-	// list of PPARAMOUT (return) variables. Does not include PPARAM|PHEAP vars.
+	// list of PPARAMOUT (return) variables.
 	returns []*Node
 
 	// list of PPARAM SSA-able pointer-shaped args. We ensure these are live
@@ -593,24 +585,9 @@ func (s *state) stmt(n *Node) {
 		return
 
 	case ODCL:
-		if n.Left.Class&PHEAP == 0 {
+		if n.Left.Class == PAUTOHEAP {
-			return
+			Fatalf("DCL %v", n)
 		}
-		if compiling_runtime {
-			Fatalf("%v escapes to heap, not allowed in runtime.", n)
-		}
-
-		// TODO: the old pass hides the details of PHEAP
-		// variables behind ONAME nodes. Figure out if it's better
-		// to rewrite the tree and make the heapaddr construct explicit
-		// or to keep this detail hidden behind the scenes.
-		palloc := prealloc[n.Left]
-		if palloc == nil {
-			palloc = callnew(n.Left.Type)
-			prealloc[n.Left] = palloc
-		}
-		r := s.expr(palloc)
-		s.assign(n.Left.Name.Heapaddr, r, false, false, n.Lineno, 0)
 
 	case OLABEL:
 		sym := n.Left.Sym
@@ -1451,9 +1428,6 @@ func (s *state) expr(n *Node) *ssa.Value {
 	case OCFUNC:
 		aux := s.lookupSymbol(n, &ssa.ExternSymbol{Typ: n.Type, Sym: n.Left.Sym})
 		return s.entryNewValue1A(ssa.OpAddr, n.Type, aux, s.sb)
-	case OPARAM:
-		addr := s.addr(n, false)
-		return s.newValue2(ssa.OpLoad, n.Left.Type, addr, s.mem())
 	case ONAME:
 		if n.Class == PFUNC {
 			// "value" of a function is the address of the function's closure
@@ -2749,10 +2723,10 @@ func (s *state) addr(n *Node, bounded bool) *ssa.Value {
 			// that cse works on their addresses
 			aux := s.lookupSymbol(n, &ssa.ArgSymbol{Typ: n.Type, Node: n})
 			return s.newValue1A(ssa.OpAddr, t, aux, s.sp)
-		case PAUTO | PHEAP, PPARAM | PHEAP, PPARAMOUT | PHEAP, PPARAMREF:
+		case PPARAMREF:
 			return s.expr(n.Name.Heapaddr)
 		default:
-			s.Unimplementedf("variable address class %v not implemented", n.Class)
+			s.Unimplementedf("variable address class %v not implemented", classnames[n.Class])
 			return nil
 		}
 	case OINDREG:
@@ -2795,17 +2769,6 @@ func (s *state) addr(n *Node, bounded bool) *ssa.Value {
 	case OCLOSUREVAR:
 		return s.newValue1I(ssa.OpOffPtr, t, n.Xoffset,
 			s.entryNewValue0(ssa.OpGetClosurePtr, Ptrto(Types[TUINT8])))
-	case OPARAM:
-		p := n.Left
-		if p.Op != ONAME || !(p.Class == PPARAM|PHEAP || p.Class == PPARAMOUT|PHEAP) {
-			s.Fatalf("OPARAM not of ONAME,{PPARAM,PPARAMOUT}|PHEAP, instead %s", nodedump(p, 0))
-		}
-
-		// Recover original offset to address passed-in param value.
-		original_p := *p
-		original_p.Xoffset = n.Xoffset
-		aux := &ssa.ArgSymbol{Typ: n.Type, Node: &original_p}
-		return s.entryNewValue1A(ssa.OpAddr, t, aux, s.sp)
 	case OCONVNOP:
 		addr := s.addr(n.Left, bounded)
 		return s.newValue1(ssa.OpCopy, t, addr) // ensure that addr has the right type
@@ -2833,9 +2796,12 @@ func (s *state) canSSA(n *Node) bool {
 	if n.Addrtaken {
 		return false
 	}
-	if n.Class&PHEAP != 0 {
+	if n.isParamHeapCopy() {
 		return false
 	}
+	if n.Class == PAUTOHEAP {
+		Fatalf("canSSA of PAUTOHEAP %v", n)
+	}
 	switch n.Class {
 	case PEXTERN, PPARAMREF:
 		// TODO: maybe treat PPARAMREF with an Arg-like op to read from closure?

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

@@ -1231,7 +1231,7 @@ func ullmancalc(n *Node) {
 	switch n.Op {
 	case OREGISTER, OLITERAL, ONAME:
 		ul = 1
-		if n.Class == PPARAMREF || (n.Class&PHEAP != 0) {
+		if n.Class == PPARAMREF || n.Class == PAUTOHEAP {
 			ul++
 		}
 		goto out
@@ -2257,6 +2257,7 @@ func isbadimport(path string) bool {
 }
 
 func checknil(x *Node, init *Nodes) {
+	x = walkexpr(x, nil) // caller has not done this yet
 	if x.Type.IsInterface() {
 		x = Nod(OITAB, x, nil)
 		x = typecheck(x, Erv)

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

@@ -143,7 +143,7 @@ func (n *Node) SetOpt(x interface{}) {
 	n.E = x
 }
 
-// Name holds Node fields used only by named nodes (ONAME, OPACK, some OLITERAL).
+// Name holds Node fields used only by named nodes (ONAME, OPACK, OLABEL, ODCLFIELD, some OLITERAL).
 type Name struct {
 	Pack      *Node // real package for import . names
 	Pkg       *Pkg  // pkg for OPACK nodes
@@ -151,7 +151,7 @@ type Name struct {
 	Inlvar    *Node // ONAME substitute while inlining
 	Defn      *Node // initializing assignment
 	Curfn     *Node // function for local variables
-	Param     *Param
+	Param     *Param // additional fields for ONAME, ODCLFIELD
 	Decldepth int32 // declaration loop depth, increased for every loop or label
 	Vargen    int32 // unique name for ONAME within a function.  Function outputs are numbered starting at one.
 	Iota      int32 // value if this name is iota
@@ -167,16 +167,16 @@ type Name struct {
 type Param struct {
 	Ntype *Node
 
-	// ONAME func param with PHEAP
+	// ONAME PAUTOHEAP
-	Outerexpr  *Node // expression copied into closure for variable
+	Outerexpr *Node // expression copied into closure for variable
-	Stackparam *Node // OPARAM node referring to stack copy of param
+	Stackcopy *Node // the PPARAM/PPARAMOUT on-stack slot (moved func params only)
 
 	// ONAME PPARAM
 	Field *Field // TFIELD in arg struct
 
 	// ONAME closure param with PPARAMREF
 	Outer   *Node // outer PPARAMREF in nested closure
-	Closure *Node // ONAME/PHEAP <-> ONAME/PPARAMREF
+	Closure *Node // ONAME/PAUTOHEAP <-> ONAME/PPARAMREF
 }
 
 // Func holds Node fields used only with function-like nodes.
@@ -292,7 +292,7 @@ const (
 	OINDEX     // Left[Right] (index of array or slice)
 	OINDEXMAP  // Left[Right] (index of map)
 	OKEY       // Left:Right (key:value in struct/array/map literal, or slice index pair)
-	OPARAM     // variant of ONAME for on-stack copy of a parameter or return value that escapes.
+	_          // was OPARAM, but cannot remove without breaking binary blob in builtin.go
 	OLEN       // len(Left)
 	OMAKE      // make(List) (before type checking converts to one of the following)
 	OMAKECHAN  // make(Type, Left) (type is chan)

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

@@ -3099,7 +3099,7 @@ func islvalue(n *Node) bool {
 			return false
 		}
 		fallthrough
-	case OIND, ODOTPTR, OCLOSUREVAR, OPARAM:
+	case OIND, ODOTPTR, OCLOSUREVAR:
 		return true
 
 	case ODOT:

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

@@ -27,9 +27,8 @@ func walk(fn *Node) {
 	lno := lineno
 
 	// Final typecheck for any unused variables.
-	// It's hard to be on the heap when not-used, but best to be consistent about &~PHEAP here and below.
 	for i, ln := range fn.Func.Dcl {
-		if ln.Op == ONAME && ln.Class&^PHEAP == PAUTO {
+		if ln.Op == ONAME && (ln.Class == PAUTO || ln.Class == PAUTOHEAP) {
 			ln = typecheck(ln, Erv|Easgn)
 			fn.Func.Dcl[i] = ln
 		}
@@ -37,13 +36,13 @@ func walk(fn *Node) {
 
 	// Propagate the used flag for typeswitch variables up to the NONAME in it's definition.
 	for _, ln := range fn.Func.Dcl {
-		if ln.Op == ONAME && ln.Class&^PHEAP == PAUTO && ln.Name.Defn != nil && ln.Name.Defn.Op == OTYPESW && ln.Used {
+		if ln.Op == ONAME && (ln.Class == PAUTO || ln.Class == PAUTOHEAP) && ln.Name.Defn != nil && ln.Name.Defn.Op == OTYPESW && ln.Used {
 			ln.Name.Defn.Left.Used = true
 		}
 	}
 
 	for _, ln := range fn.Func.Dcl {
-		if ln.Op != ONAME || ln.Class&^PHEAP != PAUTO || ln.Sym.Name[0] == '&' || ln.Used {
+		if ln.Op != ONAME || (ln.Class != PAUTO && ln.Class != PAUTOHEAP) || ln.Sym.Name[0] == '&' || ln.Used {
 			continue
 		}
 		if defn := ln.Name.Defn; defn != nil && defn.Op == OTYPESW {
@@ -97,13 +96,13 @@ func samelist(a, b []*Node) bool {
 func paramoutheap(fn *Node) bool {
 	for _, ln := range fn.Func.Dcl {
 		switch ln.Class {
-		case PPARAMOUT,
+		case PPARAMOUT:
-			PPARAMOUT | PHEAP:
+			if ln.isParamStackCopy() || ln.Addrtaken {
-			return ln.Addrtaken
+				return true
+			}
 
+		case PAUTO:
 			// stop early - parameters are over
-		case PAUTO,
-			PAUTO | PHEAP:
 			return false
 		}
 	}
@@ -212,7 +211,6 @@ func walkstmt(n *Node) *Node {
 		n = addinit(n, init.Slice())
 
 	case OBREAK,
-		ODCL,
 		OCONTINUE,
 		OFALL,
 		OGOTO,
@@ -224,6 +222,21 @@ func walkstmt(n *Node) *Node {
 		OVARLIVE:
 		break
 
+	case ODCL:
+		v := n.Left
+		if v.Class == PAUTOHEAP {
+			if compiling_runtime {
+				Yyerror("%v escapes to heap, not allowed in runtime.", v)
+			}
+			if prealloc[v] == nil {
+				prealloc[v] = callnew(v.Type)
+			}
+			nn := Nod(OAS, v.Name.Heapaddr, prealloc[v])
+			nn.Colas = true
+			nn = typecheck(nn, Etop)
+			return walkstmt(nn)
+		}
+
 	case OBLOCK:
 		walkstmtlist(n.List.Slice())
 
@@ -295,11 +308,14 @@ func walkstmt(n *Node) *Node {
 
 			var cl Class
 			for _, ln := range Curfn.Func.Dcl {
-				cl = ln.Class &^ PHEAP
+				cl = ln.Class
-				if cl == PAUTO {
+				if cl == PAUTO || cl == PAUTOHEAP {
 					break
 				}
 				if cl == PPARAMOUT {
+					if ln.isParamStackCopy() {
+						ln = walkexpr(typecheck(Nod(OIND, ln.Name.Heapaddr, nil), Erv), nil)
+					}
 					rl = append(rl, ln)
 				}
 			}
@@ -487,6 +503,12 @@ func walkexpr(n *Node, init *Nodes) *Node {
 		Fatalf("missed typecheck: %v\n", Nconv(n, FmtSign))
 	}
 
+	if n.Op == ONAME && n.Class == PAUTOHEAP {
+		nn := Nod(OIND, n.Name.Heapaddr, nil)
+		nn = typecheck(nn, Erv)
+		return walkexpr(nn, init)
+	}
+
 opswitch:
 	switch n.Op {
 	default:
@@ -497,7 +519,6 @@ opswitch:
 		ONONAME,
 		OINDREG,
 		OEMPTY,
-		OPARAM,
 		OGETG:
 
 	case ONOT,
@@ -626,7 +647,7 @@ opswitch:
 		n.Addable = true
 
 	case ONAME:
-		if n.Class&PHEAP == 0 && n.Class != PPARAMREF {
+		if n.Class != PPARAMREF {
 			n.Addable = true
 		}
 
@@ -1640,7 +1661,7 @@ func ascompatee(op Op, nl, nr []*Node, init *Nodes) []*Node {
 			break
 		}
 		// Do not generate 'x = x' during return. See issue 4014.
-		if op == ORETURN && nl[i] == nr[i] {
+		if op == ORETURN && samesafeexpr(nl[i], nr[i]) {
 			continue
 		}
 		nn = append(nn, ascompatee1(op, nl[i], nr[i], init))
@@ -2553,11 +2574,6 @@ func vmatch1(l *Node, r *Node) bool {
 func paramstoheap(params *Type, out bool) []*Node {
 	var nn []*Node
 	for _, t := range params.Fields().Slice() {
-		v := t.Nname
-		if v != nil && v.Sym != nil && strings.HasPrefix(v.Sym.Name, "~r") { // unnamed result
-			v = nil
-		}
-
 		// For precise stacks, the garbage collector assumes results
 		// are always live, so zero them always.
 		if out {
@@ -2567,24 +2583,19 @@ func paramstoheap(params *Type, out bool) []*Node {
 			nn = append(nn, Nod(OAS, nodarg(t, -1), nil))
 		}
 
-		if v == nil || v.Class&PHEAP == 0 {
+		v := t.Nname
+		if v != nil && v.Sym != nil && strings.HasPrefix(v.Sym.Name, "~r") { // unnamed result
+			v = nil
+		}
+		if v == nil {
 			continue
 		}
 
-		// generate allocation & copying code
+		if stackcopy := v.Name.Param.Stackcopy; stackcopy != nil {
-		if compiling_runtime {
+			nn = append(nn, walkstmt(Nod(ODCL, v, nil)))
-			Yyerror("%v escapes to heap, not allowed in runtime.", v)
+			if stackcopy.Class == PPARAM {
-		}
+				nn = append(nn, walkstmt(typecheck(Nod(OAS, v, stackcopy), Etop)))
-		if prealloc[v] == nil {
+			}
-			prealloc[v] = callnew(v.Type)
-		}
-		nn = append(nn, Nod(OAS, v.Name.Heapaddr, prealloc[v]))
-		if v.Class&^PHEAP != PPARAMOUT {
-			as := Nod(OAS, v, v.Name.Param.Stackparam)
-			v.Name.Param.Stackparam.Typecheck = 1
-			as = typecheck(as, Etop)
-			as = applywritebarrier(as)
-			nn = append(nn, as)
 		}
 	}
 
@@ -2597,10 +2608,12 @@ func returnsfromheap(params *Type) []*Node {
 	var nn []*Node
 	for _, t := range params.Fields().Slice() {
 		v := t.Nname
-		if v == nil || v.Class != PHEAP|PPARAMOUT {
+		if v == nil {
 			continue
 		}
-		nn = append(nn, Nod(OAS, v.Name.Param.Stackparam, v))
+		if stackcopy := v.Name.Param.Stackcopy; stackcopy != nil && stackcopy.Class == PPARAMOUT {
+			nn = append(nn, walkstmt(typecheck(Nod(OAS, stackcopy, v), Etop)))
+		}
 	}
 
 	return nn

src/cmd/compile/internal/x86/gsubr.go

@@ -643,7 +643,7 @@ func ginscmp(op gc.Op, t *gc.Type, n1, n2 *gc.Node, likely int) *obj.Prog {
 		base = n1.Left
 	}
 
-	if base.Op == gc.ONAME && base.Class&gc.PHEAP == 0 || n1.Op == gc.OINDREG {
+	if base.Op == gc.ONAME && base.Class != gc.PAUTOHEAP || n1.Op == gc.OINDREG {
 		r1 = *n1
 	} else {
 		gc.Regalloc(&r1, t, n1)

test/fixedbugs/issue15747.go

@@ -0,0 +1,41 @@
+// errorcheck -0 -live
+
+// Copyright 2016 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.
+
+// Issue 15747: liveness analysis was marking heap-escaped params live too much,
+// and worse was using the wrong bitmap bits to do so.
+
+package p
+
+var global *[]byte
+
+type Q struct{}
+
+type T struct{ M string }
+
+var b bool
+
+func f1(q *Q, xx []byte) interface{} { // ERROR "live at entry to f1: q xx" "live at call to newobject: q xx" "live at call to writebarrierptr: q &xx"
+	// xx was copied from the stack to the heap on the previous line:
+	// xx was live for the first two prints but then it switched to &xx
+	// being live. We should not see plain xx again.
+	if b {
+		global = &xx // ERROR "live at call to writebarrierptr: q &xx$"
+	}
+	xx, _, err := f2(xx, 5) // ERROR "live at call to newobject: q( d)? &xx( odata.ptr)?" "live at call to writebarrierptr: q (e|err.data err.type)$"
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func f2(d []byte, n int) (odata, res []byte, e interface{}) { // ERROR "live at entry to f2: d"
+	if n > len(d) {
+		return d, nil, &T{M: "hello"} // ERROR "live at call to newobject: d"
+	}
+	res = d[:n]
+	odata = d[n:]
+	return
+}

test/fixedbugs/issue15747b.go

@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2016 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.
+
+// Issue 15747: If a ODCL is dropped, for example when inlining,
+// then it's easy to end up not initializing the '&x' pseudo-variable
+// to point to an actual allocation. The liveness analysis will detect
+// this and abort the computation, so this test just checks that the
+// compilation succeeds.
+
+package p
+
+type R [100]byte
+
+func (x R) New() *R {
+	return &x
+}