cmd/compile: don't nilcheck newobject and return values from mapaccess{1,2}

They are guaranteed to be non-nil, no point in inserting
nil checks for them.

Fixes #15390

Change-Id: I3b9a0f2319affc2139dcc446d0a56c6785ae5a86
Reviewed-on: https://go-review.googlesource.com/22291Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -978,7 +978,11 @@ func Agenr(n *Node, a *Node, res *Node) {
 
 	case OIND:
 		Cgenr(n.Left, a, res)
+		if !n.Left.NonNil {
 			Cgen_checknil(a)
+		} else if Debug_checknil != 0 && n.Lineno > 1 {
+			Warnl(n.Lineno, "removed nil check")
+		}
 
 	case OINDEX:
 		if Ctxt.Arch.Family == sys.ARM {
@@ -1587,7 +1591,11 @@ func Agen(n *Node, res *Node) {
 
 	case OIND:
 		Cgen(nl, res)
+		if !nl.NonNil {
 			Cgen_checknil(res)
+		} else if Debug_checknil != 0 && n.Lineno > 1 {
+			Warnl(n.Lineno, "removed nil check")
+		}
 
 	case ODOT:
 		Agen(nl, res)
@@ -1597,7 +1605,11 @@ func Agen(n *Node, res *Node) {
 
 	case ODOTPTR:
 		Cgen(nl, res)
+		if !nl.NonNil {
 			Cgen_checknil(res)
+		} else if Debug_checknil != 0 && n.Lineno > 1 {
+			Warnl(n.Lineno, "removed nil check")
+		}
 		if n.Xoffset != 0 {
 			addOffset(res, n.Xoffset)
 		}
@@ -1658,7 +1670,11 @@ func Igen(n *Node, a *Node, res *Node) {
 
 	case ODOTPTR:
 		Cgenr(n.Left, a, res)
+		if !n.Left.NonNil {
 			Cgen_checknil(a)
+		} else if Debug_checknil != 0 && n.Lineno > 1 {
+			Warnl(n.Lineno, "removed nil check")
+		}
 		a.Op = OINDREG
 		a.Xoffset += n.Xoffset
 		a.Type = n.Type

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

@@ -319,6 +319,12 @@ func Jconv(n *Node, flag FmtFlag) string {
 	if n.Assigned {
 		buf.WriteString(" assigned")
 	}
+	if n.Bounded {
+		buf.WriteString(" bounded")
+	}
+	if n.NonNil {
+		buf.WriteString(" nonnil")
+	}
 
 	if c == 0 && n.Used {
 		fmt.Fprintf(&buf, " used(%v)", n.Used)

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

@@ -1938,8 +1938,7 @@ func (s *state) expr(n *Node) *ssa.Value {
 		return s.newValue2(ssa.OpLoad, n.Type, addr, s.mem())
 
 	case OIND:
-		p := s.expr(n.Left)
-		s.nilCheck(p)
+		p := s.exprPtr(n.Left, false, n.Lineno)
 		return s.newValue2(ssa.OpLoad, n.Type, p, s.mem())
 
 	case ODOT:
@@ -1952,8 +1951,7 @@ func (s *state) expr(n *Node) *ssa.Value {
 		return s.newValue2(ssa.OpLoad, n.Type, p, s.mem())
 
 	case ODOTPTR:
-		p := s.expr(n.Left)
-		s.nilCheck(p)
+		p := s.exprPtr(n.Left, false, n.Lineno)
 		p = s.newValue1I(ssa.OpOffPtr, p.Type, n.Xoffset, p)
 		return s.newValue2(ssa.OpLoad, n.Type, p, s.mem())
 
@@ -2778,19 +2776,12 @@ func (s *state) addr(n *Node, bounded bool) *ssa.Value {
 			return s.newValue2(ssa.OpPtrIndex, Ptrto(n.Left.Type.Elem()), a, i)
 		}
 	case OIND:
-		p := s.expr(n.Left)
-		if !bounded {
-			s.nilCheck(p)
-		}
-		return p
+		return s.exprPtr(n.Left, bounded, n.Lineno)
 	case ODOT:
 		p := s.addr(n.Left, bounded)
 		return s.newValue1I(ssa.OpOffPtr, t, n.Xoffset, p)
 	case ODOTPTR:
-		p := s.expr(n.Left)
-		if !bounded {
-			s.nilCheck(p)
-		}
+		p := s.exprPtr(n.Left, bounded, n.Lineno)
 		return s.newValue1I(ssa.OpOffPtr, t, n.Xoffset, p)
 	case OCLOSUREVAR:
 		return s.newValue1I(ssa.OpOffPtr, t, n.Xoffset,
@@ -2892,6 +2883,19 @@ func canSSAType(t *Type) bool {
 	}
 }
 
+// exprPtr evaluates n to a pointer and nil-checks it.
+func (s *state) exprPtr(n *Node, bounded bool, lineno int32) *ssa.Value {
+	p := s.expr(n)
+	if bounded || n.NonNil {
+		if s.f.Config.Debug_checknil() && lineno > 1 {
+			s.f.Config.Warnl(lineno, "removed nil check")
+		}
+		return p
+	}
+	s.nilCheck(p)
+	return p
+}
+
 // nilCheck generates nil pointer checking code.
 // Starts a new block on return, unless nil checks are disabled.
 // Used only for automatically inserted nil checks,

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

@@ -54,6 +54,7 @@ type Node struct {
 	Addable   bool  // addressable
 	Etype     EType // op for OASOP, etype for OTYPE, exclam for export, 6g saved reg, ChanDir for OTCHAN
 	Bounded   bool  // bounds check unnecessary
+	NonNil    bool  // guaranteed to be non-nil
 	Class     Class // PPARAM, PAUTO, PEXTERN, etc
 	Embedded  uint8 // ODCLFIELD embedded type
 	Colas     bool  // OAS resulting from :=

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

@@ -886,6 +886,7 @@ opswitch:
 		if !isblank(a) {
 			var_ := temp(Ptrto(t.Val()))
 			var_.Typecheck = 1
+			var_.NonNil = true // mapaccess always returns a non-nil pointer
 			n.List.SetIndex(0, var_)
 			n = walkexpr(n, init)
 			init.Append(n)
@@ -895,8 +896,6 @@ opswitch:
 		n = typecheck(n, Etop)
 		n = walkexpr(n, init)
 
-		// TODO: ptr is always non-nil, so disable nil check for this OIND op.
-
 	case ODELETE:
 		init.AppendNodes(&n.Ninit)
 		map_ := n.List.First()
@@ -1224,7 +1223,6 @@ opswitch:
 			// standard version takes key by reference.
 			// orderexpr made sure key is addressable.
 			key = Nod(OADDR, n.Right, nil)
-
 			p = "mapaccess1"
 		}
 
@@ -1235,6 +1233,7 @@ opswitch:
 			z := zeroaddr(w)
 			n = mkcall1(mapfn(p, t), Ptrto(t.Val()), init, typename(t), n.Left, key, z)
 		}
+		n.NonNil = true // mapaccess always returns a non-nil pointer
 		n = Nod(OIND, n, nil)
 		n.Type = t.Val()
 		n.Typecheck = 1
@@ -2015,7 +2014,9 @@ func callnew(t *Type) *Node {
 	dowidth(t)
 	fn := syslook("newobject")
 	fn = substArgTypes(fn, t)
-	return mkcall1(fn, Ptrto(t), nil, typename(t))
+	v := mkcall1(fn, Ptrto(t), nil, typename(t))
+	v.NonNil = true
+	return v
 }
 
 func iscallret(n *Node) bool {

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

@@ -4,8 +4,6 @@
 
 package ssa
 
-// TODO: return value from newobject/newarray is non-nil.
-
 // nilcheckelim eliminates unnecessary nil checks.
 func nilcheckelim(f *Func) {
 	// A nil check is redundant if the same nil check was successful in a

test/nilptr3.go

@@ -193,3 +193,24 @@ func f4(x *[10]int) {
 	x = y
 	_ = &x[9] // ERROR "removed repeated nil check"
 }
+
+func m1(m map[int][80]byte) byte {
+	v := m[3] // ERROR "removed nil check"
+	return v[5]
+}
+func m2(m map[int][800]byte) byte {
+	v := m[3] // ERROR "removed nil check"
+	return v[5]
+}
+func m3(m map[int][80]byte) (byte, bool) {
+	v, ok := m[3] // ERROR "removed nil check"
+	return v[5], ok
+}
+func m4(m map[int][800]byte) (byte, bool) {
+	v, ok := m[3] // ERROR "removed nil check"
+	return v[5], ok
+}
+func p1() byte {
+	p := new([100]byte)
+	return p[5] // ERROR "removed nil check"
+}

test/nilptr3_ssa.go

@@ -207,3 +207,24 @@ func f6(p, q *T) {
 	x := *p // ERROR "removed nil check"
 	*q = x  // ERROR "removed nil check"
 }
+
+func m1(m map[int][80]byte) byte {
+	v := m[3] // ERROR "removed nil check"
+	return v[5]
+}
+func m2(m map[int][800]byte) byte {
+	v := m[3] // ERROR "removed nil check"
+	return v[5]
+}
+func m3(m map[int][80]byte) (byte, bool) {
+	v, ok := m[3] // ERROR "removed nil check"
+	return v[5], ok
+}
+func m4(m map[int][800]byte) (byte, bool) {
+	v, ok := m[3] // ERROR "removed nil check"
+	return v[5], ok
+}
+func p1() byte {
+	p := new([100]byte)
+	return p[5] // ERROR "removed nil check"
+}