src/cmd/compile/internal/amd64/galign.go  |  1 +
 src/cmd/compile/internal/amd64/ggen.go    | 21 +++++++++++++++++++++
 src/cmd/compile/internal/arm/galign.go    |  1 +
 src/cmd/compile/internal/arm/ggen.go      | 21 +++++++++++++++++++++
 src/cmd/compile/internal/arm64/galign.go  |  1 +
 src/cmd/compile/internal/arm64/ggen.go    | 17 +++++++++++++++++
 src/cmd/compile/internal/gc/go.go         |  6 ++++++
 src/cmd/compile/internal/gc/gsubr.go      |  9 +++++++++
 src/cmd/compile/internal/gc/pgen.go       | 23 +++++++++++++++++++++++
 src/cmd/compile/internal/mips/galign.go   |  1 +
 src/cmd/compile/internal/mips/ggen.go     | 17 +++++++++++++++++
 src/cmd/compile/internal/mips64/galign.go |  1 +
 src/cmd/compile/internal/mips64/ggen.go   | 17 +++++++++++++++++
 src/cmd/compile/internal/ppc64/galign.go  |  1 +
 src/cmd/compile/internal/ppc64/ggen.go    | 17 +++++++++++++++++
 src/cmd/compile/internal/s390x/galign.go  |  1 +
 src/cmd/compile/internal/s390x/ggen.go    | 13 +++++++++++++
 src/cmd/compile/internal/x86/galign.go    |  1 +
 src/cmd/compile/internal/x86/ggen.go      | 17 +++++++++++++++++
 test/fixedbugs/issue20029.go              | 32 ++++++++++++++++++++++++++++++++

diff --git a/src/cmd/compile/internal/amd64/galign.go b/src/cmd/compile/internal/amd64/galign.go
index bb3830bca548ce2c15645d9971bbed5fe8862193..6fd7f316172d85318cb165b373f71ee91ba980bc 100644
--- a/src/cmd/compile/internal/amd64/galign.go
+++ b/src/cmd/compile/internal/amd64/galign.go
@@ -27,4 +27,5 @@
 	gc.Thearch.SSAMarkMoves = ssaMarkMoves
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 }
diff --git a/src/cmd/compile/internal/amd64/ggen.go b/src/cmd/compile/internal/amd64/ggen.go
index c137b52d8042fa49f5a6f850b4f427809c42784a..d12153383e1d5984a9daece9ff77b45f71cb5550 100644
--- a/src/cmd/compile/internal/amd64/ggen.go
+++ b/src/cmd/compile/internal/amd64/ggen.go
@@ -166,6 +166,27 @@
 	return p
 }
 
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	op := x86.AMOVQ
+	if gc.Widthptr == 4 {
+		op = x86.AMOVL
+	}
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	for i := int64(0); i < size; i += int64(gc.Widthptr) {
+		p := gc.AddAsmAfter(op, pp)
+		pp = p
+		p.From.Type = obj.TYPE_CONST
+		p.From.Offset = 0
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_AUTO
+		p.To.Reg = x86.REG_SP
+		p.To.Offset = n.Xoffset + i
+		p.To.Sym = sym
+	}
+}
+
 func ginsnop() {
 	// This is actually not the x86 NOP anymore,
 	// but at the point where it gets used, AX is dead
diff --git a/src/cmd/compile/internal/arm/galign.go b/src/cmd/compile/internal/arm/galign.go
index 308b016026df5b15e419d73768c5ed2c4bf32edf..53f57cd978c1d3a3a3e0cbb31bb1f3c76f6ee6ef 100644
--- a/src/cmd/compile/internal/arm/galign.go
+++ b/src/cmd/compile/internal/arm/galign.go
@@ -21,4 +21,5 @@
 	gc.Thearch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 }
diff --git a/src/cmd/compile/internal/arm/ggen.go b/src/cmd/compile/internal/arm/ggen.go
index 6dce0a4e80678e0b9b6ea2de5847bea25c9ed045..1b97a4d0c95a50d619f3af584dabb3c260ed3af6 100644
--- a/src/cmd/compile/internal/arm/ggen.go
+++ b/src/cmd/compile/internal/arm/ggen.go
@@ -92,6 +92,27 @@ 	}
 
 	return p
 }
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	p := gc.Prog(arm.AMOVW)
+	p.From.Type = obj.TYPE_CONST
+	p.From.Offset = 0
+	p.To.Type = obj.TYPE_REG
+	p.To.Reg = arm.REGTMP
+	for i := int64(0); i < size; i += 4 {
+		p := gc.AddAsmAfter(arm.AMOVW, pp)
+		pp = p
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = arm.REGTMP
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_AUTO
+		p.To.Reg = arm.REGSP
+		p.To.Offset = n.Xoffset + i
+		p.To.Sym = sym
+	}
+}
 
 func ginsnop() {
 	p := gc.Prog(arm.AAND)
diff --git a/src/cmd/compile/internal/arm64/galign.go b/src/cmd/compile/internal/arm64/galign.go
index 20a67e398d6924f4ed9ad7e13504116297aec477..15680dd16885296c8e7c1960031d25f3c29ecb45 100644
--- a/src/cmd/compile/internal/arm64/galign.go
+++ b/src/cmd/compile/internal/arm64/galign.go
@@ -21,4 +21,5 @@
 	gc.Thearch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 }
diff --git a/src/cmd/compile/internal/arm64/ggen.go b/src/cmd/compile/internal/arm64/ggen.go
index 16813b642ab4bcb5922ec529f03e39d10c0c29d4..83966a493342bee79df0b2dfea6b52381b1b6829 100644
--- a/src/cmd/compile/internal/arm64/ggen.go
+++ b/src/cmd/compile/internal/arm64/ggen.go
@@ -103,6 +103,23 @@
 	return p
 }
 
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	for i := int64(0); i < size; i += 8 {
+		p := gc.AddAsmAfter(arm64.AMOVD, pp)
+		pp = p
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = arm64.REGZERO
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_AUTO
+		p.To.Reg = arm64.REGSP
+		p.To.Offset = n.Xoffset + i
+		p.To.Sym = sym
+	}
+}
+
 func ginsnop() {
 	p := gc.Prog(arm64.AHINT)
 	p.From.Type = obj.TYPE_CONST
diff --git a/src/cmd/compile/internal/gc/go.go b/src/cmd/compile/internal/gc/go.go
index ff33e9c1c4b1bc4069c98b3c4902837cf3d239cc..a529ca40f7eb6ef5ee81103e7970a11e90d6b41a 100644
--- a/src/cmd/compile/internal/gc/go.go
+++ b/src/cmd/compile/internal/gc/go.go
@@ -361,6 +361,12 @@
 	// SSAGenBlock emits end-of-block Progs. SSAGenValue should be called
 	// for all values in the block before SSAGenBlock.
 	SSAGenBlock func(s *SSAGenState, b, next *ssa.Block)
+
+	// ZeroAuto emits code to zero the given auto stack variable.
+	// Code is added immediately after pp.
+	// ZeroAuto must not use any non-temporary registers.
+	// ZeroAuto will only be called for variables which contain a pointer.
+	ZeroAuto func(n *Node, pp *obj.Prog)
 }
 
 var pcloc int32
diff --git a/src/cmd/compile/internal/gc/gsubr.go b/src/cmd/compile/internal/gc/gsubr.go
index 1e8636347a6ef81e64cf1ddcc3f3d2d9c5d70bde..2a8bedff962786d1cd4149de9e02ee2145e481f1 100644
--- a/src/cmd/compile/internal/gc/gsubr.go
+++ b/src/cmd/compile/internal/gc/gsubr.go
@@ -72,6 +72,15 @@ 	p.Link = q
 	return q
 }
 
+func AddAsmAfter(as obj.As, p *obj.Prog) *obj.Prog {
+	q := Ctxt.NewProg()
+	Clearp(q)
+	q.As = as
+	q.Link = p.Link
+	p.Link = q
+	return q
+}
+
 func ggloblnod(nam *Node) {
 	s := Linksym(nam.Sym)
 	s.Gotype = Linksym(ngotype(nam))
diff --git a/src/cmd/compile/internal/gc/pgen.go b/src/cmd/compile/internal/gc/pgen.go
index 643ba79d631036923b4228094c8fad1624f101dd..dde28f670befa9b7cffd1008c293a46d3e809bc2 100644
--- a/src/cmd/compile/internal/gc/pgen.go
+++ b/src/cmd/compile/internal/gc/pgen.go
@@ -120,7 +120,30 @@ 	gvardefx(n, obj.AVARLIVE)
 }
 
 func removevardef(firstp *obj.Prog) {
+	// At VARKILLs, 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.
 	for p := firstp; p != nil; p = p.Link {
+		if p.As != obj.AVARKILL {
+			continue
+		}
+		n := p.To.Node.(*Node)
+		if !n.Name.Needzero {
+			continue
+		}
+		if n.Class != PAUTO {
+			Fatalf("zero of variable which isn't PAUTO %v", n)
+		}
+		if n.Type.Size()%int64(Widthptr) != 0 {
+			Fatalf("zero of variable not a multiple of ptr size %v", n)
+		}
+		Thearch.ZeroAuto(n, p)
+	}
+
+	for p := firstp; p != nil; p = p.Link {
+
 		for p.Link != nil && (p.Link.As == obj.AVARDEF || p.Link.As == obj.AVARKILL || p.Link.As == obj.AVARLIVE) {
 			p.Link = p.Link.Link
 		}
diff --git a/src/cmd/compile/internal/mips/galign.go b/src/cmd/compile/internal/mips/galign.go
index 39f5d2bf641516d92cb81a26cd77cb6af70680f1..e6d117a806d6f594999b14fe4612842aad14d629 100644
--- a/src/cmd/compile/internal/mips/galign.go
+++ b/src/cmd/compile/internal/mips/galign.go
@@ -23,4 +23,5 @@ 	gc.Thearch.Proginfo = proginfo
 	gc.Thearch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 }
diff --git a/src/cmd/compile/internal/mips/ggen.go b/src/cmd/compile/internal/mips/ggen.go
index ec540f8102aa4f6c3c91e47df477dabac796d8ab..a95db57ffe55be5a5e49d14e019f29391feb2313 100644
--- a/src/cmd/compile/internal/mips/ggen.go
+++ b/src/cmd/compile/internal/mips/ggen.go
@@ -92,6 +92,23 @@
 	return p
 }
 
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	for i := int64(0); i < size; i += 4 {
+		p := gc.AddAsmAfter(mips.AMOVW, pp)
+		pp = p
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = mips.REGZERO
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_AUTO
+		p.To.Reg = mips.REGSP
+		p.To.Offset = n.Xoffset + i
+		p.To.Sym = sym
+	}
+}
+
 func ginsnop() {
 	p := gc.Prog(mips.ANOR)
 	p.From.Type = obj.TYPE_REG
diff --git a/src/cmd/compile/internal/mips64/galign.go b/src/cmd/compile/internal/mips64/galign.go
index 4a36a4ce5b44fac87b335a5b750af3f0241fc309..e8ea073057761ee1da7ff82c22a893f8d4a0a258 100644
--- a/src/cmd/compile/internal/mips64/galign.go
+++ b/src/cmd/compile/internal/mips64/galign.go
@@ -25,4 +25,5 @@
 	gc.Thearch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 }
diff --git a/src/cmd/compile/internal/mips64/ggen.go b/src/cmd/compile/internal/mips64/ggen.go
index 2af4a8b1ce620432605338981c9c92956ab89059..eb48e2b26166d1d80b2ee5e79e1d3e2d6e1182bc 100644
--- a/src/cmd/compile/internal/mips64/ggen.go
+++ b/src/cmd/compile/internal/mips64/ggen.go
@@ -95,6 +95,23 @@
 	return p
 }
 
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	for i := int64(0); i < size; i += 8 {
+		p := gc.AddAsmAfter(mips.AMOVV, pp)
+		pp = p
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = mips.REGZERO
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_AUTO
+		p.To.Reg = mips.REGSP
+		p.To.Offset = n.Xoffset + i
+		p.To.Sym = sym
+	}
+}
+
 func ginsnop() {
 	p := gc.Prog(mips.ANOR)
 	p.From.Type = obj.TYPE_REG
diff --git a/src/cmd/compile/internal/ppc64/galign.go b/src/cmd/compile/internal/ppc64/galign.go
index 186aa2946a892e4c689fe49a4bfe654d90f93368..37802477310d0d0c4afd16f4b27af7e11e36c592 100644
--- a/src/cmd/compile/internal/ppc64/galign.go
+++ b/src/cmd/compile/internal/ppc64/galign.go
@@ -24,6 +24,7 @@
 	gc.Thearch.SSAMarkMoves = ssaMarkMoves
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 
 	initvariants()
 	initproginfo()
diff --git a/src/cmd/compile/internal/ppc64/ggen.go b/src/cmd/compile/internal/ppc64/ggen.go
index b3ce96856721949f14e25151ba3631654c5000ff..4abd18dc3da0dad52a7f2913cf9b82f816e71d66 100644
--- a/src/cmd/compile/internal/ppc64/ggen.go
+++ b/src/cmd/compile/internal/ppc64/ggen.go
@@ -90,6 +90,23 @@
 	return p
 }
 
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	for i := int64(0); i < size; i += 8 {
+		p := gc.AddAsmAfter(ppc64.AMOVD, pp)
+		pp = p
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = ppc64.REGZERO
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_AUTO
+		p.To.Reg = ppc64.REGSP
+		p.To.Offset = n.Xoffset + i
+		p.To.Sym = sym
+	}
+}
+
 func ginsnop() {
 	p := gc.Prog(ppc64.AOR)
 	p.From.Type = obj.TYPE_REG
diff --git a/src/cmd/compile/internal/s390x/galign.go b/src/cmd/compile/internal/s390x/galign.go
index 91b9ed07775984b0f08929db935c0f367d85bd93..9424d204fa01ff85dc2d21f374a0e4a7aeb07680 100644
--- a/src/cmd/compile/internal/s390x/galign.go
+++ b/src/cmd/compile/internal/s390x/galign.go
@@ -20,4 +20,5 @@
 	gc.Thearch.SSAMarkMoves = ssaMarkMoves
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 }
diff --git a/src/cmd/compile/internal/s390x/ggen.go b/src/cmd/compile/internal/s390x/ggen.go
index 15c65546d6914a406ff2130ce1165f62c4a72192..5ecfaa517747433afc78264c9d7d2b3e94d07cb2 100644
--- a/src/cmd/compile/internal/s390x/ggen.go
+++ b/src/cmd/compile/internal/s390x/ggen.go
@@ -143,6 +143,19 @@
 	return p
 }
 
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	p := gc.AddAsmAfter(s390x.ACLEAR, pp)
+	pp = p
+	p.From.Type = obj.TYPE_CONST
+	p.From.Offset = n.Type.Size()
+	p.To.Type = obj.TYPE_MEM
+	p.To.Name = obj.NAME_AUTO
+	p.To.Reg = s390x.REGSP
+	p.To.Offset = n.Xoffset
+	p.To.Sym = gc.Linksym(n.Sym)
+}
+
 func ginsnop() {
 	p := gc.Prog(s390x.AOR)
 	p.From.Type = obj.TYPE_REG
diff --git a/src/cmd/compile/internal/x86/galign.go b/src/cmd/compile/internal/x86/galign.go
index edac6a002abe2a72cb315785da6c503b740430db..bb29d2a02f103ab5ae1474c1215db9edda282974 100644
--- a/src/cmd/compile/internal/x86/galign.go
+++ b/src/cmd/compile/internal/x86/galign.go
@@ -31,4 +31,5 @@
 	gc.Thearch.SSAMarkMoves = ssaMarkMoves
 	gc.Thearch.SSAGenValue = ssaGenValue
 	gc.Thearch.SSAGenBlock = ssaGenBlock
+	gc.Thearch.ZeroAuto = zeroAuto
 }
diff --git a/src/cmd/compile/internal/x86/ggen.go b/src/cmd/compile/internal/x86/ggen.go
index 25769b4de031ea87bc0f3fe220f952fc999d64aa..33ffc5f534f03597170517d951bde1266450b632 100644
--- a/src/cmd/compile/internal/x86/ggen.go
+++ b/src/cmd/compile/internal/x86/ggen.go
@@ -84,6 +84,23 @@
 	return p
 }
 
+func zeroAuto(n *gc.Node, pp *obj.Prog) {
+	// Note: this code must not clobber any registers.
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	for i := int64(0); i < size; i += 4 {
+		p := gc.AddAsmAfter(x86.AMOVL, pp)
+		pp = p
+		p.From.Type = obj.TYPE_CONST
+		p.From.Offset = 0
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_AUTO
+		p.To.Reg = x86.REG_SP
+		p.To.Offset = n.Xoffset + i
+		p.To.Sym = sym
+	}
+}
+
 func ginsnop() {
 	p := gc.Prog(x86.AXCHGL)
 	p.From.Type = obj.TYPE_REG
diff --git a/test/fixedbugs/issue20029.go b/test/fixedbugs/issue20029.go
new file mode 100644
index 0000000000000000000000000000000000000000..db3f8aa5ddd00353874dd6dafe576ad15f1d4b70
--- /dev/null
+++ b/test/fixedbugs/issue20029.go
@@ -0,0 +1,32 @@
+// run
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 20029: make sure we zero at VARKILLs of
+// ambiguously live variables.
+// The ambiguously live variable here is the hiter
+// for the inner range loop.
+
+package main
+
+import "runtime"
+
+func f(m map[int]int) {
+outer:
+	for i := 0; i < 10; i++ {
+		for k := range m {
+			if k == 5 {
+				continue outer
+			}
+		}
+		runtime.GC()
+		break
+	}
+	runtime.GC()
+}
+func main() {
+	m := map[int]int{1: 2, 2: 3, 3: 4}
+	f(m)
+}