cmd/compile: zero ambiguously live variables at VARKILLs

At VARKILLs, zero a 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.

We don't know a variable is ambiguously live until very
late in compilation (after lowering, register allocation, ...),
so it is hard to generate the code in an arch-independent way.
We also have to be careful not to clobber any registers.
Fortunately, this almost never happens so performance is ~irrelevant.

There are only 2 instances where this triggers in the stdlib.

Fixes #20029

Change-Id: Ia9585a91d7b823fad4a9d141d954464cc7af31f4
Reviewed-on: https://go-review.googlesource.com/41076
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -22,6 +22,7 @@ func Init(arch *gc.Arch) {
 	arch.MAXWIDTH = 1 << 50
 
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = ssaMarkMoves

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

@@ -121,6 +121,26 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, state *uint32) *obj.Pr
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// 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 := pp.Prog(op)
+		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(pp *gc.Progs) {
 	// This is actually not the x86 NOP anymore,
 	// but at the point where it gets used, AX is dead

src/cmd/compile/internal/arm/galign.go

@@ -16,6 +16,7 @@ func Init(arch *gc.Arch) {
 	arch.MAXWIDTH = (1 << 32) - 1
 
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}

src/cmd/compile/internal/arm/ggen.go

@@ -47,6 +47,27 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, r0 *uint32) *obj.Prog
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// Note: this code must not clobber any registers.
+	sym := gc.Linksym(n.Sym)
+	size := n.Type.Size()
+	p := pp.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 := pp.Prog(arm.AMOVW)
+		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(pp *gc.Progs) {
 	p := pp.Prog(arm.AAND)
 	p.From.Type = obj.TYPE_REG

src/cmd/compile/internal/arm64/galign.go

@@ -17,6 +17,7 @@ func Init(arch *gc.Arch) {
 
 	arch.PadFrame = padframe
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}

src/cmd/compile/internal/arm64/ggen.go

@@ -58,6 +58,22 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, _ *uint32) *obj.Prog {
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// 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 := pp.Prog(arm64.AMOVD)
+		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(pp *gc.Progs) {
 	p := pp.Prog(arm64.AHINT)
 	p.From.Type = obj.TYPE_CONST

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

@@ -243,6 +243,11 @@ type Arch struct {
 	// 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.
+	// ZeroAuto must not use any non-temporary registers.
+	// ZeroAuto will only be called for variables which contain a pointer.
+	ZeroAuto func(*Progs, *Node)
 }
 
 var thearch Arch

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

@@ -711,7 +711,7 @@ func livenessepilogue(lv *Liveness) {
 			}
 
 			// Annotate ambiguously live variables so that they can
-			// be zeroed at function entry.
+			// be zeroed at function entry and at VARKILL points.
 			// liveout is dead here and used as a temporary.
 			liveout.AndNot(any, all)
 			if !liveout.IsEmpty() {

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

@@ -4375,8 +4375,24 @@ 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.OpVarKill, ssa.OpVarLive, ssa.OpKeepAlive:
+			case ssa.OpVarDef, ssa.OpVarLive, ssa.OpKeepAlive:
 				// 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)
 

src/cmd/compile/internal/mips/galign.go

@@ -19,6 +19,7 @@ func Init(arch *gc.Arch) {
 	arch.REGSP = mips.REGSP
 	arch.MAXWIDTH = (1 << 31) - 1
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop
 	arch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}
 	arch.SSAGenValue = ssaGenValue

src/cmd/compile/internal/mips/ggen.go

@@ -43,6 +43,22 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, _ *uint32) *obj.Prog {
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// 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 := pp.Prog(mips.AMOVW)
+		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(pp *gc.Progs) {
 	p := pp.Prog(mips.ANOR)
 	p.From.Type = obj.TYPE_REG

src/cmd/compile/internal/mips64/galign.go

@@ -20,6 +20,7 @@ func Init(arch *gc.Arch) {
 	arch.MAXWIDTH = 1 << 50
 
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = func(s *gc.SSAGenState, b *ssa.Block) {}

src/cmd/compile/internal/mips64/ggen.go

@@ -47,6 +47,22 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, _ *uint32) *obj.Prog {
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// 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 := pp.Prog(mips.AMOVV)
+		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(pp *gc.Progs) {
 	p := pp.Prog(mips.ANOR)
 	p.From.Type = obj.TYPE_REG

src/cmd/compile/internal/ppc64/galign.go

@@ -19,6 +19,7 @@ func Init(arch *gc.Arch) {
 	arch.MAXWIDTH = 1 << 50
 
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop2
 
 	arch.SSAMarkMoves = ssaMarkMoves

src/cmd/compile/internal/ppc64/ggen.go

@@ -42,6 +42,22 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, _ *uint32) *obj.Prog {
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// 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 := pp.Prog(ppc64.AMOVD)
+		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(pp *gc.Progs) {
 	p := pp.Prog(ppc64.AOR)
 	p.From.Type = obj.TYPE_REG

src/cmd/compile/internal/s390x/galign.go

@@ -15,6 +15,7 @@ func Init(arch *gc.Arch) {
 	arch.MAXWIDTH = 1 << 50
 
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = ssaMarkMoves

src/cmd/compile/internal/s390x/ggen.go

@@ -93,6 +93,18 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, _ *uint32) *obj.Prog {
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// Note: this code must not clobber any registers.
+	p := pp.Prog(s390x.ACLEAR)
+	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(pp *gc.Progs) {
 	p := pp.Prog(s390x.AOR)
 	p.From.Type = obj.TYPE_REG

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

@@ -30,6 +30,7 @@ func Init(arch *gc.Arch) {
 	arch.MAXWIDTH = (1 << 32) - 1
 
 	arch.ZeroRange = zerorange
+	arch.ZeroAuto = zeroAuto
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = ssaMarkMoves

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

@@ -37,6 +37,22 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, ax *uint32) *obj.Prog
 	return p
 }
 
+func zeroAuto(pp *gc.Progs, n *gc.Node) {
+	// 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 := pp.Prog(x86.AMOVL)
+		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(pp *gc.Progs) {
 	p := pp.Prog(x86.AXCHGL)
 	p.From.Type = obj.TYPE_REG

test/fixedbugs/issue20029.go

+// 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)
+}