cmd/compile,runtime: remove ambiguously live logic

The previous CL introduced stack objects. This CL removes the old
ambiguously live liveness analysis. After this CL we're relying
on stack objects exclusively.

Update a bunch of liveness tests to reflect the new world.

Fixes #22350

Change-Id: I739b26e015882231011ce6bc1a7f426049e59f31
Reviewed-on: https://go-review.googlesource.com/c/134156
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

16 files changed, 375 insertions, 334 deletions

diff --git a/src/cmd/compile/internal/gc/op_string.go b/src/cmd/compile/internal/gc/op_string.go
index 857234e45d..90b95d769f 100644
--- a/src/cmd/compile/internal/gc/op_string.go
+++ b/src/cmd/compile/internal/gc/op_string.go
@@ -4,9 +4,9 @@ package gc
 
 import "strconv"
 
-const _Op_name = "XXXNAMENONAMETYPEPACKLITERALADDSUBORXORADDSTRADDRANDANDAPPENDARRAYBYTESTRARRAYBYTESTRTMPARRAYRUNESTRSTRARRAYBYTESTRARRAYBYTETMPSTRARRAYRUNEASAS2AS2FUNCAS2RECVAS2MAPRAS2DOTTYPEASOPCALLCALLFUNCCALLMETHCALLINTERCALLPARTCAPCLOSECLOSURECMPIFACECMPSTRCOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVNOPCOPYDCLDCLFUNCDCLFIELDDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTINDINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMULDIVMODLSHRSHANDANDNOTNEWNOTCOMPLUSMINUSORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRRECOVERRECVRUNESTRSELRECVSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFBLOCKBREAKCASEXCASECONTINUEDEFEREMPTYFALLFORFORUNTILGOTOIFLABELPROCRANGERETURNSELECTSWITCHTYPESWTCHANTMAPTSTRUCTTINTERTFUNCTARRAYDDDDDDARGINLCALLEFACEITABIDATASPTRCLOSUREVARCFUNCCHECKNILVARKILLVARLIVEINDREGSPRETJMPGETGEND"
+const _Op_name = "XXXNAMENONAMETYPEPACKLITERALADDSUBORXORADDSTRADDRANDANDAPPENDARRAYBYTESTRARRAYBYTESTRTMPARRAYRUNESTRSTRARRAYBYTESTRARRAYBYTETMPSTRARRAYRUNEASAS2AS2FUNCAS2RECVAS2MAPRAS2DOTTYPEASOPCALLCALLFUNCCALLMETHCALLINTERCALLPARTCAPCLOSECLOSURECMPIFACECMPSTRCOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVNOPCOPYDCLDCLFUNCDCLFIELDDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTINDINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMULDIVMODLSHRSHANDANDNOTNEWNOTCOMPLUSMINUSORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRRECOVERRECVRUNESTRSELRECVSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFBLOCKBREAKCASEXCASECONTINUEDEFEREMPTYFALLFORFORUNTILGOTOIFLABELPROCRANGERETURNSELECTSWITCHTYPESWTCHANTMAPTSTRUCTTINTERTFUNCTARRAYDDDDDDARGINLCALLEFACEITABIDATASPTRCLOSUREVARCFUNCCHECKNILVARDEFVARKILLVARLIVEINDREGSPRETJMPGETGEND"
 
-var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 36, 39, 45, 49, 55, 61, 73, 88, 100, 112, 127, 139, 141, 144, 151, 158, 165, 175, 179, 183, 191, 199, 208, 216, 219, 224, 231, 239, 245, 252, 258, 267, 275, 283, 289, 293, 302, 309, 313, 316, 323, 331, 339, 346, 352, 355, 361, 368, 376, 380, 387, 395, 397, 399, 401, 403, 405, 407, 410, 415, 423, 426, 435, 438, 442, 450, 457, 466, 469, 472, 475, 478, 481, 484, 490, 493, 496, 499, 503, 508, 512, 517, 522, 528, 533, 537, 542, 550, 558, 564, 573, 580, 584, 591, 598, 606, 610, 614, 618, 625, 632, 640, 646, 651, 656, 660, 665, 673, 678, 683, 687, 690, 698, 702, 704, 709, 713, 718, 724, 730, 736, 742, 747, 751, 758, 764, 769, 775, 778, 784, 791, 796, 800, 805, 809, 819, 824, 832, 839, 846, 854, 860, 864, 867}
+var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 36, 39, 45, 49, 55, 61, 73, 88, 100, 112, 127, 139, 141, 144, 151, 158, 165, 175, 179, 183, 191, 199, 208, 216, 219, 224, 231, 239, 245, 252, 258, 267, 275, 283, 289, 293, 302, 309, 313, 316, 323, 331, 339, 346, 352, 355, 361, 368, 376, 380, 387, 395, 397, 399, 401, 403, 405, 407, 410, 415, 423, 426, 435, 438, 442, 450, 457, 466, 469, 472, 475, 478, 481, 484, 490, 493, 496, 499, 503, 508, 512, 517, 522, 528, 533, 537, 542, 550, 558, 564, 573, 580, 584, 591, 598, 606, 610, 614, 618, 625, 632, 640, 646, 651, 656, 660, 665, 673, 678, 683, 687, 690, 698, 702, 704, 709, 713, 718, 724, 730, 736, 742, 747, 751, 758, 764, 769, 775, 778, 784, 791, 796, 800, 805, 809, 819, 824, 832, 838, 845, 852, 860, 866, 870, 873}
 
 func (i Op) String() string {
 	if i >= Op(len(_Op_index)-1) {
diff --git a/src/cmd/compile/internal/gc/plive.go b/src/cmd/compile/internal/gc/plive.go
index e070a5cd1a..563b425db7 100644
--- a/src/cmd/compile/internal/gc/plive.go
+++ b/src/cmd/compile/internal/gc/plive.go
@@ -78,6 +78,10 @@ import (
 // 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
@@ -85,23 +89,15 @@ type BlockEffects struct {
 	//
 	//	uevar: upward exposed variables (used before set in block)
 	//	varkill: killed variables (set in block)
-	//	avarinit: addrtaken variables set or used (proof of initialization)
-	uevar    varRegVec
-	varkill  varRegVec
-	avarinit bvec
+	uevar   varRegVec
+	varkill varRegVec
 
 	// Computed during Liveness.solve using control flow information:
 	//
 	//	livein: variables live at block entry
 	//	liveout: variables live at block exit
-	//	avarinitany: addrtaken variables possibly initialized at block exit
-	//		(initialized in block or at exit from any predecessor block)
-	//	avarinitall: addrtaken variables certainly initialized at block exit
-	//		(initialized in block or at exit from all predecessor blocks)
-	livein      varRegVec
-	liveout     varRegVec
-	avarinitany bvec
-	avarinitall bvec
+	livein  varRegVec
+	liveout varRegVec
 }
 
 // A collection of global state used by liveness analysis.
@@ -186,10 +182,9 @@ func (idx LivenessIndex) Valid() bool {
 }
 
 type progeffectscache struct {
-	textavarinit []int32
-	retuevar     []int32
-	tailuevar    []int32
-	initialized  bool
+	retuevar    []int32
+	tailuevar   []int32
+	initialized bool
 }
 
 // varRegVec contains liveness bitmaps for variables and registers.
@@ -264,24 +259,13 @@ func (lv *Liveness) initcache() {
 			// 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))
 
-			if node.Addrtaken() {
-				lv.cache.textavarinit = append(lv.cache.textavarinit, int32(i))
-			}
-
 		case PPARAMOUT:
-			// If the result had its address taken, it is being tracked
-			// by the avarinit code, which does not use uevar.
-			// If we added it to uevar too, we'd not see any kill
-			// and decide that the variable was live entry, which it is not.
-			// So only use uevar in the non-addrtaken case.
-			// The p.to.type == obj.TYPE_NONE limits the bvset to
-			// non-tail-call return instructions; see note below for details.
-			if !node.Addrtaken() {
-				lv.cache.retuevar = append(lv.cache.retuevar, int32(i))
-			}
+			// 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))
 		}
 	}
 }
@@ -291,21 +275,13 @@ func (lv *Liveness) initcache() {
 //
 // The possible flags are:
 //	uevar - used by the instruction
-//	varkill - killed by the instruction
-//		for variables without address taken, means variable was set
-//		for variables with address taken, means variable was marked dead
-//	avarinit - initialized or referred to by the instruction,
-//		only for variables with address taken but not escaping to heap
-//
-// The avarinit output serves as a signal that the data has been
-// initialized, because any use of a variable must come after its
-// initialization.
+//	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
-	avarinit
 )
 
 // valueEffects returns the index of a variable in lv.vars and the
@@ -329,27 +305,15 @@ func (lv *Liveness) valueEffects(v *ssa.Value) (int32, liveEffect) {
 	}
 
 	var effect liveEffect
-	if n.Addrtaken() {
-		if v.Op != ssa.OpVarKill {
-			effect |= avarinit
-		}
-		if v.Op == ssa.OpVarDef || v.Op == ssa.OpVarKill {
-			effect |= varkill
-		}
-	} else {
-		// Read is a read, obviously.
-		// Addr by itself is also implicitly a read.
-		//
-		// Addr|Write means that the address is being taken
-		// but only so that the instruction can write to the value.
-		// It is not a read.
-
-		if e&ssa.SymRead != 0 || e&(ssa.SymAddr|ssa.SymWrite) == ssa.SymAddr {
-			effect |= uevar
-		}
-		if e&ssa.SymWrite != 0 && (!isfat(n.Type) || v.Op == ssa.OpVarDef) {
-			effect |= varkill
-		}
+	// Read is a read, obviously.
+	//
+	// Addr is a read also, as any subseqent holder of the pointer must be able
+	// to see all the values (including initialization) written so far.
+	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 {
@@ -545,9 +509,6 @@ func newliveness(fn *Node, f *ssa.Func, vars []*Node, idx map[*Node]int32, stkpt
 		be.varkill = varRegVec{vars: bulk.next()}
 		be.livein = varRegVec{vars: bulk.next()}
 		be.liveout = varRegVec{vars: bulk.next()}
-		be.avarinit = bulk.next()
-		be.avarinitany = bulk.next()
-		be.avarinitall = bulk.next()
 	}
 	lv.livenessMap.reset(lv.f.NumValues())
 
@@ -869,19 +830,6 @@ func (lv *Liveness) prologue() {
 			}
 			be.uevar.regs |= regUevar
 		}
-
-		// Walk the block instructions forward to update avarinit bits.
-		// avarinit describes the effect at the end of the block, not the beginning.
-		for _, val := range b.Values {
-			pos, e := lv.valueEffects(val)
-			// No need for regEffects because registers never appear in avarinit.
-			if e&varkill != 0 {
-				be.avarinit.Unset(pos)
-			}
-			if e&avarinit != 0 {
-				be.avarinit.Set(pos)
-			}
-		}
 	}
 }
 
@@ -892,51 +840,10 @@ func (lv *Liveness) solve() {
 	nvars := int32(len(lv.vars))
 	newlivein := varRegVec{vars: bvalloc(nvars)}
 	newliveout := varRegVec{vars: bvalloc(nvars)}
-	any := bvalloc(nvars)
-	all := bvalloc(nvars)
 
-	// Push avarinitall, avarinitany forward.
-	// avarinitall says the addressed var is initialized along all paths reaching the block exit.
-	// avarinitany says the addressed var is initialized along some path reaching the block exit.
-	for _, b := range lv.f.Blocks {
-		be := lv.blockEffects(b)
-		if b == lv.f.Entry {
-			be.avarinitall.Copy(be.avarinit)
-		} else {
-			be.avarinitall.Clear()
-			be.avarinitall.Not()
-		}
-		be.avarinitany.Copy(be.avarinit)
-	}
-
-	// Walk blocks in the general direction of propagation (RPO
-	// for avarinit{any,all}, and PO for live{in,out}). This
-	// improves convergence.
+	// Walk blocks in postorder ordering. This improves convergence.
 	po := lv.f.Postorder()
 
-	for change := true; change; {
-		change = false
-		for i := len(po) - 1; i >= 0; i-- {
-			b := po[i]
-			be := lv.blockEffects(b)
-			lv.avarinitanyall(b, any, all)
-
-			any.AndNot(any, be.varkill.vars)
-			all.AndNot(all, be.varkill.vars)
-			any.Or(any, be.avarinit)
-			all.Or(all, be.avarinit)
-			if !any.Eq(be.avarinitany) {
-				change = true
-				be.avarinitany.Copy(any)
-			}
-
-			if !all.Eq(be.avarinitall) {
-				change = true
-				be.avarinitall.Copy(all)
-			}
-		}
-	}
-
 	// 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.
@@ -957,7 +864,7 @@ func (lv *Liveness) solve() {
 					newliveout.vars.Set(pos)
 				}
 			case ssa.BlockExit:
-				// nothing to do
+				// panic exit - nothing to do
 			default:
 				// A variable is live on output from this block
 				// if it is live on input to some successor.
@@ -975,7 +882,7 @@ func (lv *Liveness) solve() {
 			}
 
 			// A variable is live on input to this block
-			// if it is live on output from this block and
+			// 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])
@@ -990,8 +897,6 @@ func (lv *Liveness) solve() {
 func (lv *Liveness) epilogue() {
 	nvars := int32(len(lv.vars))
 	liveout := varRegVec{vars: bvalloc(nvars)}
-	any := bvalloc(nvars)
-	all := bvalloc(nvars)
 	livedefer := bvalloc(nvars) // always-live variables
 
 	// If there is a defer (that could recover), then all output
@@ -1017,6 +922,9 @@ func (lv *Liveness) epilogue() {
 				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.Name.SetNeedzero(true)
 				livedefer.Set(int32(i))
 			}
@@ -1033,9 +941,6 @@ func (lv *Liveness) epilogue() {
 	{
 		// Reserve an entry for function entry.
 		live := bvalloc(nvars)
-		for _, pos := range lv.cache.textavarinit {
-			live.Set(pos)
-		}
 		lv.livevars = append(lv.livevars, varRegVec{vars: live})
 	}
 
@@ -1043,53 +948,14 @@ func (lv *Liveness) epilogue() {
 		be := lv.blockEffects(b)
 		firstBitmapIndex := len(lv.livevars)
 
-		// Compute avarinitany and avarinitall for entry to block.
-		// This duplicates information known during Liveness.solve
-		// but avoids storing two more vectors for each block.
-		lv.avarinitanyall(b, any, all)
-
 		// Walk forward through the basic block instructions and
 		// allocate liveness maps for those instructions that need them.
-		// Seed the maps with information about the addrtaken variables.
 		for _, v := range b.Values {
-			pos, e := lv.valueEffects(v)
-			// No need for regEffects because registers never appear in avarinit.
-			if e&varkill != 0 {
-				any.Unset(pos)
-				all.Unset(pos)
-			}
-			if e&avarinit != 0 {
-				any.Set(pos)
-				all.Set(pos)
-			}
-
 			if !lv.issafepoint(v) {
 				continue
 			}
 
-			// Annotate ambiguously live variables so that they can
-			// be zeroed at function entry and at VARKILL points.
-			// liveout is dead here and used as a temporary.
-			liveout.vars.AndNot(any, all)
-			if !liveout.vars.IsEmpty() {
-				for pos := int32(0); pos < liveout.vars.n; pos++ {
-					if !liveout.vars.Get(pos) {
-						continue
-					}
-					all.Set(pos) // silence future warnings in this block
-					n := lv.vars[pos]
-					if !n.Name.Needzero() {
-						n.Name.SetNeedzero(true)
-						if debuglive >= 1 {
-							Warnl(v.Pos, "%v: %L is ambiguously live", lv.fn.Func.Nname, n)
-						}
-					}
-				}
-			}
-
-			// Live stuff first.
 			live := bvalloc(nvars)
-			live.Copy(any)
 			lv.livevars = append(lv.livevars, varRegVec{vars: live})
 		}
 
@@ -1128,6 +994,17 @@ func (lv *Liveness) epilogue() {
 				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.vars.Get(int32(i)) {
+					continue
+				}
+				if n.Class() == PPARAM {
+					continue // ok
+				}
+				Fatalf("bad live variable at entry of %v: %L", lv.fn.Func.Nname, n)
+			}
+
 			// Record live variables.
 			live := &lv.livevars[index]
 			live.Or(*live, liveout)
@@ -1330,28 +1207,6 @@ func clobberPtr(b *ssa.Block, v *Node, offset int64) {
 	b.NewValue0IA(src.NoXPos, ssa.OpClobber, types.TypeVoid, offset, v)
 }
 
-func (lv *Liveness) avarinitanyall(b *ssa.Block, any, all bvec) {
-	if len(b.Preds) == 0 {
-		any.Clear()
-		all.Clear()
-		for _, pos := range lv.cache.textavarinit {
-			any.Set(pos)
-			all.Set(pos)
-		}
-		return
-	}
-
-	be := lv.blockEffects(b.Preds[0].Block())
-	any.Copy(be.avarinitany)
-	all.Copy(be.avarinitall)
-
-	for _, pred := range b.Preds[1:] {
-		be := lv.blockEffects(pred.Block())
-		any.Or(any, be.avarinitany)
-		all.And(all, be.avarinitall)
-	}
-}
-
 // Compact coalesces identical bitmaps from lv.livevars into the sets
 // lv.stackMapSet and lv.regMaps.
 //
@@ -1559,7 +1414,6 @@ func (lv *Liveness) printDebug() {
 			printed = false
 			printed = lv.printeffect(printed, "uevar", pos, effect&uevar != 0, regUevar)
 			printed = lv.printeffect(printed, "varkill", pos, effect&varkill != 0, regKill)
-			printed = lv.printeffect(printed, "avarinit", pos, effect&avarinit != 0, 0)
 			if printed {
 				fmt.Printf("\n")
 			}
@@ -1596,9 +1450,6 @@ func (lv *Liveness) printDebug() {
 		printed = false
 		printed = lv.printbvec(printed, "varkill", be.varkill)
 		printed = lv.printbvec(printed, "liveout", be.liveout)
-		printed = lv.printbvec(printed, "avarinit", varRegVec{vars: be.avarinit})
-		printed = lv.printbvec(printed, "avarinitany", varRegVec{vars: be.avarinitany})
-		printed = lv.printbvec(printed, "avarinitall", varRegVec{vars: be.avarinitall})
 		if printed {
 			fmt.Printf("\n")
 		}
diff --git a/src/cmd/compile/internal/gc/sinit.go b/src/cmd/compile/internal/gc/sinit.go
index f76b02828f..9d1114fa43 100644
--- a/src/cmd/compile/internal/gc/sinit.go
+++ b/src/cmd/compile/internal/gc/sinit.go
@@ -839,6 +839,10 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
 			a = nod(OAS, x, nil)
 			a = typecheck(a, Etop)
 			init.Append(a) // zero new temp
+		} else {
+			// Declare that we're about to initialize all of x.
+			// (Which happens at the *vauto = vstat below.)
+			init.Append(nod(OVARDEF, x, nil))
 		}
 
 		a = nod(OADDR, x, nil)
@@ -849,6 +853,8 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
 			a = typecheck(a, Etop)
 			init.Append(a) // zero new temp
 			a = a.Left
+		} else {
+			init.Append(nod(OVARDEF, a, nil))
 		}
 
 		a = nod(OADDR, a, nil)
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index 663042754e..138ce08fec 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -755,8 +755,8 @@ func (s *state) stmtList(l Nodes) {
 
 // stmt converts the statement n to SSA and adds it to s.
 func (s *state) stmt(n *Node) {
-	if !(n.Op == OVARKILL || n.Op == OVARLIVE) {
-		// OVARKILL and OVARLIVE are invisible to the programmer, so we don't use their line numbers to avoid confusion in debugging.
+	if !(n.Op == OVARKILL || n.Op == OVARLIVE || n.Op == OVARDEF) {
+		// OVARKILL, OVARLIVE, and OVARDEF are invisible to the programmer, so we don't use their line numbers to avoid confusion in debugging.
 		s.pushLine(n.Pos)
 		defer s.popLine()
 	}
@@ -1170,6 +1170,10 @@ func (s *state) stmt(n *Node) {
 		}
 		s.startBlock(bEnd)
 
+	case OVARDEF:
+		if !s.canSSA(n.Left) {
+			s.vars[&memVar] = s.newValue1Apos(ssa.OpVarDef, types.TypeMem, n.Left, s.mem(), false)
+		}
 	case OVARKILL:
 		// Insert a varkill op to record that a variable is no longer live.
 		// We only care about liveness info at call sites, so putting the
@@ -4977,6 +4981,12 @@ func emitStackObjects(e *ssafn, pp *Progs) {
 	p.To.Type = obj.TYPE_MEM
 	p.To.Name = obj.NAME_EXTERN
 	p.To.Sym = x
+
+	if debuglive != 0 {
+		for _, v := range vars {
+			Warnl(v.Pos, "stack object %v %s", v, v.Type.String())
+		}
+	}
 }
 
 // genssa appends entries to pp for each instruction in f.
@@ -5056,24 +5066,8 @@ func genssa(f *ssa.Func, pp *Progs) {
 			case ssa.OpGetG:
 				// nothing to do when there's a g register,
 				// and checkLower complains if there's not
-			case ssa.OpVarDef, ssa.OpVarLive, ssa.OpKeepAlive:
+			case ssa.OpVarDef, ssa.OpVarLive, ssa.OpKeepAlive, ssa.OpVarKill:
 				// nothing to do; already used by liveness
-			case ssa.OpVarKill:
-				// Zero variable if it is ambiguously live.
-				// After the VARKILL anything this variable references
-				// might be collected. If it were to become live again later,
-				// the GC will see references to already-collected objects.
-				// See issue 20029.
-				n := v.Aux.(*Node)
-				if n.Name.Needzero() {
-					if n.Class() != PAUTO {
-						v.Fatalf("zero of variable which isn't PAUTO %v", n)
-					}
-					if n.Type.Size()%int64(Widthptr) != 0 {
-						v.Fatalf("zero of variable not a multiple of ptr size %v", n)
-					}
-					thearch.ZeroAuto(s.pp, n)
-				}
 			case ssa.OpPhi:
 				CheckLoweredPhi(v)
 			case ssa.OpConvert:
diff --git a/src/cmd/compile/internal/gc/syntax.go b/src/cmd/compile/internal/gc/syntax.go
index df23b83f29..eb2ab6b916 100644
--- a/src/cmd/compile/internal/gc/syntax.go
+++ b/src/cmd/compile/internal/gc/syntax.go
@@ -739,6 +739,7 @@ const (
 	OCLOSUREVAR // variable reference at beginning of closure function
 	OCFUNC      // reference to c function pointer (not go func value)
 	OCHECKNIL   // emit code to ensure pointer/interface not nil
+	OVARDEF     // variable is about to be fully initialized
 	OVARKILL    // variable is dead
 	OVARLIVE    // variable is alive
 	OINDREGSP   // offset plus indirect of REGSP, such as 8(SP).
diff --git a/src/cmd/compile/internal/ssa/gen/genericOps.go b/src/cmd/compile/internal/ssa/gen/genericOps.go
index 7292012d26..ee9c6fa0f6 100644
--- a/src/cmd/compile/internal/ssa/gen/genericOps.go
+++ b/src/cmd/compile/internal/ssa/gen/genericOps.go
@@ -470,8 +470,9 @@ var genericOps = []opData{
 
 	{name: "VarDef", argLength: 1, aux: "Sym", typ: "Mem", symEffect: "None", zeroWidth: true}, // aux is a *gc.Node of a variable that is about to be initialized.  arg0=mem, returns mem
 	{name: "VarKill", argLength: 1, aux: "Sym", symEffect: "None"},                             // aux is a *gc.Node of a variable that is known to be dead.  arg0=mem, returns mem
-	{name: "VarLive", argLength: 1, aux: "Sym", symEffect: "Read", zeroWidth: true},            // aux is a *gc.Node of a variable that must be kept live.  arg0=mem, returns mem
-	{name: "KeepAlive", argLength: 2, typ: "Mem", zeroWidth: true},                             // arg[0] is a value that must be kept alive until this mark.  arg[1]=mem, returns mem
+	// TODO: what's the difference betweeen VarLive and KeepAlive?
+	{name: "VarLive", argLength: 1, aux: "Sym", symEffect: "Read", zeroWidth: true}, // aux is a *gc.Node of a variable that must be kept live.  arg0=mem, returns mem
+	{name: "KeepAlive", argLength: 2, typ: "Mem", zeroWidth: true},                  // arg[0] is a value that must be kept alive until this mark.  arg[1]=mem, returns mem
 
 	// Ops for breaking 64-bit operations on 32-bit architectures
 	{name: "Int64Make", argLength: 2, typ: "UInt64"}, // arg0=hi, arg1=lo
diff --git a/src/runtime/chan.go b/src/runtime/chan.go
index a4ee51ca39..5cf0b86f58 100644
--- a/src/runtime/chan.go
+++ b/src/runtime/chan.go
@@ -232,6 +232,11 @@ func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 	gp.param = nil
 	c.sendq.enqueue(mysg)
 	goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)
+	// Ensure the value being sent is kept alive until the
+	// receiver copies it out. The sudog has a pointer to the
+	// stack object, but sudogs aren't considered as roots of the
+	// stack tracer.
+	KeepAlive(ep)
 
 	// someone woke us up.
 	if mysg != gp.waiting {
diff --git a/src/runtime/gcinfo_test.go b/src/runtime/gcinfo_test.go
index 7dd1a5607c..0741f6361c 100644
--- a/src/runtime/gcinfo_test.go
+++ b/src/runtime/gcinfo_test.go
@@ -35,14 +35,46 @@ func TestGCInfo(t *testing.T) {
 	verifyGCInfo(t, "data eface", &dataEface, infoEface)
 	verifyGCInfo(t, "data iface", &dataIface, infoIface)
 
-	verifyGCInfo(t, "stack Ptr", new(Ptr), infoPtr)
-	verifyGCInfo(t, "stack ScalarPtr", new(ScalarPtr), infoScalarPtr)
-	verifyGCInfo(t, "stack PtrScalar", new(PtrScalar), infoPtrScalar)
-	verifyGCInfo(t, "stack BigStruct", new(BigStruct), infoBigStruct())
-	verifyGCInfo(t, "stack string", new(string), infoString)
-	verifyGCInfo(t, "stack slice", new([]string), infoSlice)
-	verifyGCInfo(t, "stack eface", new(interface{}), infoEface)
-	verifyGCInfo(t, "stack iface", new(Iface), infoIface)
+	{
+		var x Ptr
+		verifyGCInfo(t, "stack Ptr", &x, infoPtr)
+		runtime.KeepAlive(x)
+	}
+	{
+		var x ScalarPtr
+		verifyGCInfo(t, "stack ScalarPtr", &x, infoScalarPtr)
+		runtime.KeepAlive(x)
+	}
+	{
+		var x PtrScalar
+		verifyGCInfo(t, "stack PtrScalar", &x, infoPtrScalar)
+		runtime.KeepAlive(x)
+	}
+	{
+		var x BigStruct
+		verifyGCInfo(t, "stack BigStruct", &x, infoBigStruct())
+		runtime.KeepAlive(x)
+	}
+	{
+		var x string
+		verifyGCInfo(t, "stack string", &x, infoString)
+		runtime.KeepAlive(x)
+	}
+	{
+		var x []string
+		verifyGCInfo(t, "stack slice", &x, infoSlice)
+		runtime.KeepAlive(x)
+	}
+	{
+		var x interface{}
+		verifyGCInfo(t, "stack eface", &x, infoEface)
+		runtime.KeepAlive(x)
+	}
+	{
+		var x Iface
+		verifyGCInfo(t, "stack iface", &x, infoIface)
+		runtime.KeepAlive(x)
+	}
 
 	for i := 0; i < 10; i++ {
 		verifyGCInfo(t, "heap Ptr", escape(new(Ptr)), trimDead(padDead(infoPtr)))
diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index 5301e692e0..87fa027b4e 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -1994,7 +1994,9 @@ func reflect_gcbits(x interface{}) []byte {
 	return ret
 }
 
-// Returns GC type info for object p for testing.
+// Returns GC type info for the pointer stored in ep for testing.
+// If ep points to the stack, only static live information will be returned
+// (i.e. not for objects which are only dynamically live stack objects).
 func getgcmask(ep interface{}) (mask []byte) {
 	e := *efaceOf(&ep)
 	p := e.data
@@ -2051,11 +2053,6 @@ func getgcmask(ep interface{}) (mask []byte) {
 		_g_ := getg()
 		gentraceback(_g_.m.curg.sched.pc, _g_.m.curg.sched.sp, 0, _g_.m.curg, 0, nil, 1000, getgcmaskcb, noescape(unsafe.Pointer(&frame)), 0)
 		if frame.fn.valid() {
-			// TODO: once stack objects are enabled (and their pointers
-			// are no longer described by the stack pointermap directly),
-			// tests using this will probably need fixing. We might need
-			// to loop through the stackobjects and if we're inside one,
-			// use the pointermap from that object.
 			locals, _, _ := getStackMap(&frame, nil, false)
 			if locals.n == 0 {
 				return
diff --git a/test/fixedbugs/issue20250.go b/test/fixedbugs/issue20250.go
index 6fc861a8dc..47879385d2 100644
--- a/test/fixedbugs/issue20250.go
+++ b/test/fixedbugs/issue20250.go
@@ -15,10 +15,10 @@ type T struct {
 }
 
 func f(a T) { // ERROR "live at entry to f: a"
-	var e interface{}
-	func() { // ERROR "live at entry to f.func1: a &e"
-		e = a.s // ERROR "live at call to convT2Estring: a &e"
-	}() // ERROR "live at call to f.func1: e$"
+	var e interface{} // ERROR "stack object e interface \{\}$"
+	func() {          // ERROR "live at entry to f.func1: a &e"
+		e = a.s // ERROR "live at call to convT2Estring: &e" "stack object a T$"
+	}()
 	// Before the fix, both a and e were live at the previous line.
 	_ = e
 }
diff --git a/test/live.go b/test/live.go
index 13bdc4aae1..679562d9bf 100644
--- a/test/live.go
+++ b/test/live.go
@@ -32,9 +32,9 @@ func printbytepointer(*byte)
 func printint(int)
 
 func f1() {
-	var x *int
-	printpointer(&x) // ERROR "live at call to printpointer: x$"
+	var x *int       // ERROR "stack object x \*int$"
 	printpointer(&x) // ERROR "live at call to printpointer: x$"
+	printpointer(&x)
 }
 
 func f2(b bool) {
@@ -42,9 +42,9 @@ func f2(b bool) {
 		printint(0) // nothing live here
 		return
 	}
-	var x *int
-	printpointer(&x) // ERROR "live at call to printpointer: x$"
+	var x *int       // ERROR "stack object x \*int$"
 	printpointer(&x) // ERROR "live at call to printpointer: x$"
+	printpointer(&x)
 }
 
 func f3(b1, b2 bool) {
@@ -60,15 +60,15 @@ func f3(b1, b2 bool) {
 	}
 
 	if b2 {
-		var x *int
-		printpointer(&x) // ERROR "live at call to printpointer: x$"
+		var x *int       // ERROR "stack object x \*int$"
 		printpointer(&x) // ERROR "live at call to printpointer: x$"
+		printpointer(&x)
 	} else {
-		var y *int
-		printpointer(&y) // ERROR "live at call to printpointer: y$"
+		var y *int       // ERROR "stack object y \*int$"
 		printpointer(&y) // ERROR "live at call to printpointer: y$"
+		printpointer(&y)
 	}
-	printint(0) // ERROR "f3: x \(type \*int\) is ambiguously live$" "f3: y \(type \*int\) is ambiguously live$" "live at call to printint: x y$"
+	printint(0) // nothing is live here
 }
 
 // The old algorithm treated x as live on all code that
@@ -83,7 +83,7 @@ func f4(b1, b2 bool) { // x not live here
 		return
 	}
 	var z **int
-	x := new(int)
+	x := new(int) // ERROR "stack object x \*int$"
 	*x = 42
 	z = &x
 	printint(**z) // ERROR "live at call to printint: x$"
@@ -99,15 +99,15 @@ func f4(b1, b2 bool) { // x not live here
 func f5(b1 bool) {
 	var z **int
 	if b1 {
-		x := new(int)
+		x := new(int) // ERROR "stack object x \*int$"
 		*x = 42
 		z = &x
 	} else {
-		y := new(int)
+		y := new(int) // ERROR "stack object y \*int$"
 		*y = 54
 		z = &y
 	}
-	printint(**z) // ERROR "f5: x \(type \*int\) is ambiguously live$" "f5: y \(type \*int\) is ambiguously live$" "live at call to printint: x y$"
+	printint(**z) // nothing live here
 }
 
 // confusion about the _ result used to cause spurious "live at entry to f6: _".
@@ -119,7 +119,7 @@ func f6() (_, y string) {
 
 // confusion about addressed results used to cause "live at entry to f7: x".
 
-func f7() (x string) {
+func f7() (x string) { // ERROR "stack object x string"
 	_ = &x
 	x = "hello"
 	return
@@ -141,7 +141,7 @@ var i9 interface{}
 func f9() bool {
 	g8()
 	x := i9
-	y := interface{}(str()) // ERROR "live at call to convT2Estring: .autotmp_[0-9]+ x.data$" "live at call to str: x.data$"
+	y := interface{}(str()) // ERROR "live at call to convT2Estring: x.data$" "live at call to str: x.data$" "stack object .autotmp_[0-9]+ string$"
 	i9 = y                  // make y escape so the line above has to call convT2E
 	return x != y
 }
@@ -163,7 +163,7 @@ var b bool
 
 // this used to have a spurious "live at entry to f11a: ~r0"
 func f11a() *int {
-	select { // ERROR "live at call to selectgo: .autotmp_[0-9]+$"
+	select { // ERROR "stack object .autotmp_[0-9]+ \[2\]struct"
 	case <-c:
 		return nil
 	case <-c:
@@ -178,7 +178,7 @@ func f11b() *int {
 		// get to the bottom of the function.
 		// This used to have a spurious "live at call to printint: p".
 		printint(1) // nothing live here!
-		select {    // ERROR "live at call to selectgo: .autotmp_[0-9]+$"
+		select {    // ERROR "stack object .autotmp_[0-9]+ \[2\]struct"
 		case <-c:
 			return nil
 		case <-c:
@@ -198,7 +198,7 @@ func f11c() *int {
 		// Unlike previous, the cases in this select fall through,
 		// so we can get to the println, so p is not dead.
 		printint(1) // ERROR "live at call to printint: p$"
-		select {    // ERROR "live at call to selectgo: .autotmp_[0-9]+ p$"
+		select {    // ERROR "live at call to selectgo: p$" "stack object .autotmp_[0-9]+ \[2\]struct"
 		case <-c:
 		case <-c:
 		}
@@ -233,8 +233,8 @@ func h13(string, string) string
 // more incorrectly placed VARDEF.
 
 func f14() {
-	x := g14()
-	printstringpointer(&x) // ERROR "live at call to printstringpointer: x$"
+	x := g14() // ERROR "stack object x string$"
+	printstringpointer(&x)
 }
 
 func g14() string
@@ -254,10 +254,10 @@ func iface() interface{}
 
 func f16() {
 	if b {
-		delete(mi, iface()) // ERROR "live at call to mapdelete: .autotmp_[0-9]+$"
+		delete(mi, iface()) // ERROR "stack object .autotmp_[0-9]+ interface \{\}$"
 	}
-	delete(mi, iface()) // ERROR "live at call to mapdelete: .autotmp_[0-9]+$"
-	delete(mi, iface()) // ERROR "live at call to mapdelete: .autotmp_[0-9]+$"
+	delete(mi, iface()) // ERROR "stack object .autotmp_[0-9]+ interface \{\}$"
+	delete(mi, iface()) // ERROR "stack object .autotmp_[0-9]+ interface \{\}$"
 }
 
 var m2s map[string]*byte
@@ -300,10 +300,10 @@ func f18() {
 	// temporary introduced by orderexpr.
 	var z *byte
 	if b {
-		z = m2[g18()] // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
+		z = m2[g18()] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 	}
-	z = m2[g18()] // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
-	z = m2[g18()] // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
+	z = m2[g18()] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
+	z = m2[g18()] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 	printbytepointer(z)
 }
 
@@ -317,30 +317,30 @@ func f19() {
 	var z *byte
 
 	if b {
-		z = <-ch // ERROR "live at call to chanrecv1: .autotmp_[0-9]+$"
+		z = <-ch // ERROR "stack object .autotmp_[0-9]+ \*byte$"
 	}
-	z = <-ch // ERROR "live at call to chanrecv1: .autotmp_[0-9]+$"
-	z = <-ch // ERROR "live at call to chanrecv1: .autotmp_[0-9]+$"
+	z = <-ch // ERROR "stack object .autotmp_[0-9]+ \*byte$"
+	z = <-ch // ERROR "stack object .autotmp_[0-9]+ \*byte$" "live at call to chanrecv1: .autotmp_[0-9]+$"
 	printbytepointer(z)
 }
 
 func f20() {
 	// src temporary for channel send
 	if b {
-		ch <- byteptr() // ERROR "live at call to chansend1: .autotmp_[0-9]+$"
+		ch <- byteptr() // ERROR "stack object .autotmp_[0-9]+ \*byte$"
 	}
-	ch <- byteptr() // ERROR "live at call to chansend1: .autotmp_[0-9]+$"
-	ch <- byteptr() // ERROR "live at call to chansend1: .autotmp_[0-9]+$"
+	ch <- byteptr() // ERROR "stack object .autotmp_[0-9]+ \*byte$"
+	ch <- byteptr() // ERROR "stack object .autotmp_[0-9]+ \*byte$"
 }
 
 func f21() {
 	// key temporary for mapaccess using array literal key.
 	var z *byte
 	if b {
-		z = m2[[2]string{"x", "y"}] // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
+		z = m2[[2]string{"x", "y"}] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 	}
-	z = m2[[2]string{"x", "y"}] // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
-	z = m2[[2]string{"x", "y"}] // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
+	z = m2[[2]string{"x", "y"}] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
+	z = m2[[2]string{"x", "y"}] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 	printbytepointer(z)
 }
 
@@ -349,10 +349,10 @@ func f23() {
 	var z *byte
 	var ok bool
 	if b {
-		z, ok = m2[[2]string{"x", "y"}] // ERROR "live at call to mapaccess2: .autotmp_[0-9]+$"
+		z, ok = m2[[2]string{"x", "y"}] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 	}
-	z, ok = m2[[2]string{"x", "y"}] // ERROR "live at call to mapaccess2: .autotmp_[0-9]+$"
-	z, ok = m2[[2]string{"x", "y"}] // ERROR "live at call to mapaccess2: .autotmp_[0-9]+$"
+	z, ok = m2[[2]string{"x", "y"}] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
+	z, ok = m2[[2]string{"x", "y"}] // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 	printbytepointer(z)
 	print(ok)
 }
@@ -361,10 +361,10 @@ func f24() {
 	// key temporary for map access using array literal key.
 	// value temporary too.
 	if b {
-		m2[[2]string{"x", "y"}] = nil // ERROR "live at call to mapassign: .autotmp_[0-9]+$"
+		m2[[2]string{"x", "y"}] = nil // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 	}
-	m2[[2]string{"x", "y"}] = nil // ERROR "live at call to mapassign: .autotmp_[0-9]+$"
-	m2[[2]string{"x", "y"}] = nil // ERROR "live at call to mapassign: .autotmp_[0-9]+$"
+	m2[[2]string{"x", "y"}] = nil // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
+	m2[[2]string{"x", "y"}] = nil // ERROR "stack object .autotmp_[0-9]+ \[2\]string$"
 }
 
 // defer should not cause spurious ambiguously live variables
@@ -387,10 +387,10 @@ func g25()
 
 func f26(b bool) {
 	if b {
-		print26((*int)(nil), (*int)(nil), (*int)(nil)) // ERROR "live at call to print26: .autotmp_[0-9]+$"
+		print26((*int)(nil), (*int)(nil), (*int)(nil)) // ERROR "stack object .autotmp_[0-9]+ \[3\]interface \{\}$"
 	}
-	print26((*int)(nil), (*int)(nil), (*int)(nil)) // ERROR "live at call to print26: .autotmp_[0-9]+$"
-	print26((*int)(nil), (*int)(nil), (*int)(nil)) // ERROR "live at call to print26: .autotmp_[0-9]+$"
+	print26((*int)(nil), (*int)(nil), (*int)(nil)) // ERROR "stack object .autotmp_[0-9]+ \[3\]interface \{\}$"
+	print26((*int)(nil), (*int)(nil), (*int)(nil)) // ERROR "stack object .autotmp_[0-9]+ \[3\]interface \{\}$"
 	printnl()
 }
 
@@ -402,10 +402,10 @@ func print26(...interface{})
 func f27(b bool) {
 	x := 0
 	if b {
-		call27(func() { x++ }) // ERROR "live at call to call27: .autotmp_[0-9]+$"
+		call27(func() { x++ }) // ERROR "stack object .autotmp_[0-9]+ struct \{"
 	}
-	call27(func() { x++ }) // ERROR "live at call to call27: .autotmp_[0-9]+$"
-	call27(func() { x++ }) // ERROR "live at call to call27: .autotmp_[0-9]+$"
+	call27(func() { x++ }) // ERROR "stack object .autotmp_[0-9]+ struct \{"
+	call27(func() { x++ }) // ERROR "stack object .autotmp_[0-9]+ struct \{"
 	printnl()
 }
 
@@ -414,11 +414,11 @@ func f27(b bool) {
 func f27defer(b bool) {
 	x := 0
 	if b {
-		defer call27(func() { x++ }) // ERROR "live at call to deferproc: .autotmp_[0-9]+$" "live at call to deferreturn: .autotmp_[0-9]+$"
+		defer call27(func() { x++ }) // ERROR "stack object .autotmp_[0-9]+ struct \{"
 	}
-	defer call27(func() { x++ }) // ERROR "f27defer: .autotmp_[0-9]+ \(type struct { F uintptr; x \*int }\) is ambiguously live$" "live at call to deferproc: .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to deferreturn: .autotmp_[0-9]+ .autotmp_[0-9]+$"
-	printnl()                    // ERROR "live at call to printnl: .autotmp_[0-9]+ .autotmp_[0-9]+$"
-} // ERROR "live at call to deferreturn: .autotmp_[0-9]+ .autotmp_[0-9]+$"
+	defer call27(func() { x++ }) // ERROR "stack object .autotmp_[0-9]+ struct \{"
+	printnl()
+}
 
 // and newproc (go) escapes to the heap
 
@@ -440,24 +440,24 @@ var s1, s2, s3, s4, s5, s6, s7, s8, s9, s10 string
 
 func f28(b bool) {
 	if b {
-		printstring(s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10) // ERROR "live at call to concatstrings: .autotmp_[0-9]+$" "live at call to printstring: .autotmp_[0-9]+$"
+		printstring(s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10) // ERROR "stack object .autotmp_[0-9]+ \[10\]string$"
 	}
-	printstring(s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10) // ERROR "live at call to concatstrings: .autotmp_[0-9]+$" "live at call to printstring: .autotmp_[0-9]+$"
-	printstring(s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10) // ERROR "live at call to concatstrings: .autotmp_[0-9]+$" "live at call to printstring: .autotmp_[0-9]+$"
+	printstring(s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10) // ERROR "stack object .autotmp_[0-9]+ \[10\]string$"
+	printstring(s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10) // ERROR "stack object .autotmp_[0-9]+ \[10\]string$"
 }
 
 // map iterator should die on end of range loop
 
 func f29(b bool) {
 	if b {
-		for k := range m { // ERROR "live at call to mapiterinit: .autotmp_[0-9]+$" "live at call to mapiternext: .autotmp_[0-9]+$"
+		for k := range m { // ERROR "live at call to mapiterinit: .autotmp_[0-9]+$" "live at call to mapiternext: .autotmp_[0-9]+$" "stack object .autotmp_[0-9]+ map.iter\[string\]int$"
 			printstring(k) // ERROR "live at call to printstring: .autotmp_[0-9]+$"
 		}
 	}
-	for k := range m { // ERROR "live at call to mapiterinit: .autotmp_[0-9]+$" "live at call to mapiternext: .autotmp_[0-9]+$"
+	for k := range m { // ERROR "live at call to mapiterinit: .autotmp_[0-9]+$" "live at call to mapiternext: .autotmp_[0-9]+$" "stack object .autotmp_[0-9]+ map.iter\[string\]int$"
 		printstring(k) // ERROR "live at call to printstring: .autotmp_[0-9]+$"
 	}
-	for k := range m { // ERROR "live at call to mapiterinit: .autotmp_[0-9]+$" "live at call to mapiternext: .autotmp_[0-9]+$"
+	for k := range m { // ERROR "live at call to mapiterinit: .autotmp_[0-9]+$" "live at call to mapiternext: .autotmp_[0-9]+$" "stack object .autotmp_[0-9]+ map.iter\[string\]int$"
 		printstring(k) // ERROR "live at call to printstring: .autotmp_[0-9]+$"
 	}
 }
@@ -473,20 +473,19 @@ type pstruct struct {
 }
 
 func f30(b bool) {
-	// two live temps during printintpointer(p):
-	// in the copy of p.intp and
+	// live temp during printintpointer(p):
 	// the internal iterator pointer if a pointer to pstruct in pstructarr
 	// can not be easily computed by strength reduction.
 	if b {
-		for _, p := range pstructarr {
-			printintpointer(p.intp) // ERROR "live at call to printintpointer: .autotmp_[0-9]+ .autotmp_[0-9]+$"
+		for _, p := range pstructarr { // ERROR "stack object .autotmp_[0-9]+ \[10\]pstruct$"
+			printintpointer(p.intp) // ERROR "live at call to printintpointer: .autotmp_[0-9]+$"
 		}
 	}
-	for _, p := range pstructarr {
-		printintpointer(p.intp) // ERROR "live at call to printintpointer: .autotmp_[0-9]+ .autotmp_[0-9]+$"
+	for _, p := range pstructarr { // ERROR "stack object .autotmp_[0-9]+ \[10\]pstruct$"
+		printintpointer(p.intp) // ERROR "live at call to printintpointer: .autotmp_[0-9]+$"
 	}
-	for _, p := range pstructarr {
-		printintpointer(p.intp) // ERROR "live at call to printintpointer: .autotmp_[0-9]+ .autotmp_[0-9]+$"
+	for _, p := range pstructarr { // ERROR "stack object .autotmp_[0-9]+ \[10\]pstruct$"
+		printintpointer(p.intp) // ERROR "live at call to printintpointer: .autotmp_[0-9]+$"
 	}
 }
 
@@ -494,13 +493,13 @@ func f30(b bool) {
 
 func f31(b1, b2, b3 bool) {
 	if b1 {
-		g31(str()) // ERROR "live at call to convT2Estring: .autotmp_[0-9]+$" "live at call to g31: .autotmp_[0-9]+$"
+		g31(str()) // ERROR "stack object .autotmp_[0-9]+ string$"
 	}
 	if b2 {
-		h31(str()) // ERROR "live at call to convT2Estring: .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to h31: .autotmp_[0-9]+$" "live at call to newobject: .autotmp_[0-9]+$"
+		h31(str()) // ERROR "live at call to convT2Estring: .autotmp_[0-9]+$" "live at call to newobject: .autotmp_[0-9]+$" "stack object .autotmp_[0-9]+ string$"
 	}
 	if b3 {
-		panic(str()) // ERROR "live at call to convT2Estring: .autotmp_[0-9]+$" "live at call to gopanic: .autotmp_[0-9]+$"
+		panic(str()) // ERROR "stack object .autotmp_[0-9]+ string$"
 	}
 	print(b3)
 }
@@ -520,10 +519,10 @@ var t32 T32
 
 func f32(b bool) {
 	if b {
-		call32(t32.Inc) // ERROR "live at call to call32: .autotmp_[0-9]+$"
+		call32(t32.Inc) // ERROR "stack object .autotmp_[0-9]+ struct \{"
 	}
-	call32(t32.Inc) // ERROR "live at call to call32: .autotmp_[0-9]+$"
-	call32(t32.Inc) // ERROR "live at call to call32: .autotmp_[0-9]+$"
+	call32(t32.Inc) // ERROR "stack object .autotmp_[0-9]+ struct \{"
+	call32(t32.Inc) // ERROR "stack object .autotmp_[0-9]+ struct \{"
 }
 
 //go:noescape
@@ -535,7 +534,7 @@ func call32(func())
 var m33 map[interface{}]int
 
 func f33() {
-	if m33[byteptr()] == 0 { // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
+	if m33[byteptr()] == 0 { // ERROR "stack object .autotmp_[0-9]+ interface \{\}$"
 		printnl()
 		return
 	} else {
@@ -545,7 +544,7 @@ func f33() {
 }
 
 func f34() {
-	if m33[byteptr()] == 0 { // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$"
+	if m33[byteptr()] == 0 { // ERROR "stack object .autotmp_[0-9]+ interface \{\}$"
 		printnl()
 		return
 	}
@@ -553,7 +552,8 @@ func f34() {
 }
 
 func f35() {
-	if m33[byteptr()] == 0 && m33[byteptr()] == 0 { // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$" "f35: .autotmp_[0-9]+ \(type interface \{\}\) is ambiguously live$"
+	if m33[byteptr()] == 0 && // ERROR "stack object .autotmp_[0-9]+ interface \{\}"
+		m33[byteptr()] == 0 { // ERROR "stack object .autotmp_[0-9]+ interface \{\}"
 		printnl()
 		return
 	}
@@ -561,7 +561,8 @@ func f35() {
 }
 
 func f36() {
-	if m33[byteptr()] == 0 || m33[byteptr()] == 0 { // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$" "f36: .autotmp_[0-9]+ \(type interface \{\}\) is ambiguously live$"
+	if m33[byteptr()] == 0 || // ERROR "stack object .autotmp_[0-9]+ interface \{\}"
+		m33[byteptr()] == 0 { // ERROR "stack object .autotmp_[0-9]+ interface \{\}"
 		printnl()
 		return
 	}
@@ -569,7 +570,9 @@ func f36() {
 }
 
 func f37() {
-	if (m33[byteptr()] == 0 || m33[byteptr()] == 0) && m33[byteptr()] == 0 { // ERROR "live at call to mapaccess1: .autotmp_[0-9]+$" "f37: .autotmp_[0-9]+ \(type interface \{\}\) is ambiguously live$"
+	if (m33[byteptr()] == 0 || // ERROR "stack object .autotmp_[0-9]+ interface \{\}"
+		m33[byteptr()] == 0) && // ERROR "stack object .autotmp_[0-9]+ interface \{\}"
+		m33[byteptr()] == 0 { // ERROR "stack object .autotmp_[0-9]+ interface \{\}"
 		printnl()
 		return
 	}
@@ -589,14 +592,14 @@ func f38(b bool) {
 	// we care that the println lines have no live variables
 	// and therefore no output.
 	if b {
-		select { // ERROR "live at call to selectgo:( .autotmp_[0-9]+)+$"
+		select { // ERROR "live at call to selectgo:( .autotmp_[0-9]+)+$" "stack object .autotmp_[0-9]+ \[4\]struct \{"
 		case <-fc38():
 			printnl()
-		case fc38() <- *fi38(1): // ERROR "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$"
+		case fc38() <- *fi38(1): // ERROR "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$" "stack object .autotmp_[0-9]+ string$"
 			printnl()
-		case *fi38(2) = <-fc38(): // ERROR "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$"
+		case *fi38(2) = <-fc38(): // ERROR "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$" "stack object .autotmp_[0-9]+ string$"
 			printnl()
-		case *fi38(3), *fb38() = <-fc38(): // ERROR "live at call to fb38:( .autotmp_[0-9]+)+$" "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$"
+		case *fi38(3), *fb38() = <-fc38(): // ERROR "stack object .autotmp_[0-9]+ string$" "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$"
 			printnl()
 		}
 		printnl()
@@ -655,15 +658,17 @@ func bad40() {
 }
 
 func good40() {
-	ret := T40{}
-	ret.m = make(map[int]int) // ERROR "live at call to fastrand: .autotmp_[0-9]+ ret$"
+	ret := T40{}              // ERROR "stack object ret T40$"
+	ret.m = make(map[int]int) // ERROR "live at call to fastrand: .autotmp_[0-9]+ ret$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
 	t := &ret
-	printnl() // ERROR "live at call to printnl: .autotmp_[0-9]+ ret$"
-	useT40(t) // ERROR "live at call to useT40: .autotmp_[0-9]+ ret$"
+	printnl() // ERROR "live at call to printnl: ret$"
+	// Note: ret is live at the printnl because the compiler moves &ret
+	// from before the printnl to after.
+	useT40(t)
 }
 
 func ddd1(x, y *int) { // ERROR "live at entry to ddd1: x y$"
-	ddd2(x, y) // ERROR "live at call to ddd2: .autotmp_[0-9]+$"
+	ddd2(x, y) // ERROR "stack object .autotmp_[0-9]+ \[2\]\*int$"
 	printnl()
 	// Note: no .?autotmp live at printnl.  See issue 16996.
 }
diff --git a/test/live2.go b/test/live2.go
index cc1b0b7acf..cea312f075 100644
--- a/test/live2.go
+++ b/test/live2.go
@@ -10,7 +10,6 @@
 package main
 
 // issue 8142: lost 'addrtaken' bit on inlined variables.
-// no inlining in this test, so just checking that non-inlined works.
 
 func printnl()
 
@@ -28,15 +27,15 @@ func newT40() *T40 {
 }
 
 func bad40() {
-	t := newT40() // ERROR "live at call to makemap: .autotmp_[0-9]+ ret$"
-	printnl()     // ERROR "live at call to printnl: .autotmp_[0-9]+ ret$"
-	useT40(t)     // ERROR "live at call to useT40: .autotmp_[0-9]+ ret$"
+	t := newT40() // ERROR "live at call to makemap: ret$" "stack object ret T40$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
+	printnl()     // ERROR "live at call to printnl: ret$"
+	useT40(t)
 }
 
 func good40() {
-	ret := T40{}
-	ret.m = make(map[int]int, 42) // ERROR "live at call to makemap: .autotmp_[0-9]+ ret$"
+	ret := T40{}                  // ERROR "stack object ret T40$"
+	ret.m = make(map[int]int, 42) // ERROR "live at call to makemap: ret$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
 	t := &ret
-	printnl() // ERROR "live at call to printnl: .autotmp_[0-9]+ ret$"
-	useT40(t) // ERROR "live at call to useT40: .autotmp_[0-9]+ ret$"
+	printnl() // ERROR "live at call to printnl: ret$"
+	useT40(t)
 }
diff --git a/test/live_syscall.go b/test/live_syscall.go
index 65a161c028..b7b85bcabf 100644
--- a/test/live_syscall.go
+++ b/test/live_syscall.go
@@ -19,22 +19,22 @@ func f(uintptr) // ERROR "f assuming arg#1 is unsafe uintptr"
 
 func g() {
 	var t int
-	f(uintptr(unsafe.Pointer(&t))) // ERROR "live at call to f: .?autotmp" "g &t does not escape"
+	f(uintptr(unsafe.Pointer(&t))) // ERROR "live at call to f: .?autotmp" "g &t does not escape" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
 
 func h() {
 	var v int
-	syscall.Syscall(0, 1, uintptr(unsafe.Pointer(&v)), 2) // ERROR "live at call to Syscall: .?autotmp" "h &v does not escape"
+	syscall.Syscall(0, 1, uintptr(unsafe.Pointer(&v)), 2) // ERROR "live at call to Syscall: .?autotmp" "h &v does not escape" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
 
 func i() {
 	var t int
 	p := unsafe.Pointer(&t) // ERROR "i &t does not escape"
-	f(uintptr(p))           // ERROR "live at call to f: .?autotmp"
+	f(uintptr(p))           // ERROR "live at call to f: .?autotmp" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
 
 func j() {
 	var v int
 	p := unsafe.Pointer(&v)              // ERROR "j &v does not escape"
-	syscall.Syscall(0, 1, uintptr(p), 2) // ERROR "live at call to Syscall: .?autotmp"
+	syscall.Syscall(0, 1, uintptr(p), 2) // ERROR "live at call to Syscall: .?autotmp" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
diff --git a/test/stackobj.go b/test/stackobj.go
new file mode 100644
index 0000000000..b6af4bd8f3
--- /dev/null
+++ b/test/stackobj.go
@@ -0,0 +1,57 @@
+// run
+
+// Copyright 2018 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 main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+type HeapObj [8]int64
+
+type StkObj struct {
+	h *HeapObj
+}
+
+var n int
+var c int = -1
+
+func gc() {
+	// encourage heap object to be collected, and have its finalizer run.
+	runtime.GC()
+	runtime.GC()
+	runtime.GC()
+	n++
+}
+
+func main() {
+	f()
+	gc() // prior to stack objects, heap object is not collected until here
+	if c < 0 {
+		panic("heap object never collected")
+	}
+	if c != 1 {
+		panic(fmt.Sprintf("expected collection at phase 1, got phase %d", c))
+	}
+}
+
+func f() {
+	var s StkObj
+	s.h = new(HeapObj)
+	runtime.SetFinalizer(s.h, func(h *HeapObj) {
+		// Remember at what phase the heap object was collected.
+		c = n
+	})
+	g(&s)
+	gc()
+}
+
+func g(s *StkObj) {
+	gc() // heap object is still live here
+	runtime.KeepAlive(s)
+	gc() // heap object should be collected here
+}
diff --git a/test/stackobj3.go b/test/stackobj3.go
new file mode 100644
index 0000000000..8bb8fb3270
--- /dev/null
+++ b/test/stackobj3.go
@@ -0,0 +1,93 @@
+// run
+
+// Copyright 2018 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.
+
+// This test makes sure that ambiguously live arguments work correctly.
+
+package main
+
+import (
+	"runtime"
+)
+
+type HeapObj [8]int64
+
+type StkObj struct {
+	h *HeapObj
+}
+
+var n int
+var c int = -1
+
+func gc() {
+	// encourage heap object to be collected, and have its finalizer run.
+	runtime.GC()
+	runtime.GC()
+	runtime.GC()
+	n++
+}
+
+var null StkObj
+
+var sink *HeapObj
+
+//go:noinline
+func use(p *StkObj) {
+}
+
+//go:noinline
+func f(s StkObj, b bool) {
+	var p *StkObj
+	if b {
+		p = &s
+	} else {
+		p = &null
+	}
+	// use is required here to prevent the conditional
+	// code above from being executed after the first gc() call.
+	use(p)
+	// If b==false, h should be collected here.
+	gc() // 0
+	sink = p.h
+	gc() // 1
+	sink = nil
+	// If b==true, h should be collected here.
+	gc() // 2
+}
+
+func fTrue() {
+	var s StkObj
+	s.h = new(HeapObj)
+	c = -1
+	n = 0
+	runtime.SetFinalizer(s.h, func(h *HeapObj) {
+		// Remember at what phase the heap object was collected.
+		c = n
+	})
+	f(s, true)
+	if c != 2 {
+		panic("bad liveness")
+	}
+}
+
+func fFalse() {
+	var s StkObj
+	s.h = new(HeapObj)
+	c = -1
+	n = 0
+	runtime.SetFinalizer(s.h, func(h *HeapObj) {
+		// Remember at what phase the heap object was collected.
+		c = n
+	})
+	f(s, false)
+	if c != 0 {
+		panic("bad liveness")
+	}
+}
+
+func main() {
+	fTrue()
+	fFalse()
+}
diff --git a/test/uintptrescapes2.go b/test/uintptrescapes2.go
index c94bc148c8..e7b5d721f5 100644
--- a/test/uintptrescapes2.go
+++ b/test/uintptrescapes2.go
@@ -30,14 +30,14 @@ func F4(...uintptr) {} // ERROR "escaping ...uintptr"
 
 func G() {
 	var t int                        // ERROR "moved to heap"
-	F1(uintptr(unsafe.Pointer(&t)))  // ERROR "live at call to F1: .?autotmp" "&t escapes to heap"
+	F1(uintptr(unsafe.Pointer(&t)))  // ERROR "live at call to F1: .?autotmp" "&t escapes to heap" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 	var t2 int                       // ERROR "moved to heap"
-	F3(uintptr(unsafe.Pointer(&t2))) // ERROR "live at call to F3: .?autotmp" "&t2 escapes to heap"
+	F3(uintptr(unsafe.Pointer(&t2))) // ERROR "live at call to F3: .?autotmp" "&t2 escapes to heap" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
 
 func H() {
 	var v int                                 // ERROR "moved to heap"
-	F2(0, 1, uintptr(unsafe.Pointer(&v)), 2)  // ERROR "live at call to newobject: .?autotmp" "live at call to F2: .?autotmp" "escapes to heap"
+	F2(0, 1, uintptr(unsafe.Pointer(&v)), 2)  // ERROR "live at call to newobject: .?autotmp" "live at call to F2: .?autotmp" "escapes to heap" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 	var v2 int                                // ERROR "moved to heap"
-	F4(0, 1, uintptr(unsafe.Pointer(&v2)), 2) // ERROR "live at call to newobject: .?autotmp" "live at call to F4: .?autotmp" "escapes to heap"
+	F4(0, 1, uintptr(unsafe.Pointer(&v2)), 2) // ERROR "live at call to newobject: .?autotmp" "live at call to F4: .?autotmp" "escapes to heap" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }