cmd/compile/internal/ir: add NewZero

This constructs a zero value of any type, which helps address some
corner case scenarios.

It should also eventually handle the predeclared "zero" value, at
least as currently implemented in go.dev/cl/520336.

For #61372.

Change-Id: I3a86a94fd8fa388c9c6bf281da8aa532b3da00fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/527696
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ir/const.go

@@ -34,6 +34,36 @@ func NewUintptr(pos src.XPos, v int64) Node {
 	return NewBasicLit(pos, types.Types[types.TUINTPTR], constant.MakeInt64(v))
 }
 
+// NewZero returns a zero value of the given type.
+func NewZero(pos src.XPos, typ *types.Type) Node {
+	switch {
+	case typ.HasNil():
+		return NewNilExpr(pos, typ)
+	case typ.IsInteger():
+		return NewBasicLit(pos, typ, intZero)
+	case typ.IsFloat():
+		return NewBasicLit(pos, typ, floatZero)
+	case typ.IsComplex():
+		return NewBasicLit(pos, typ, complexZero)
+	case typ.IsBoolean():
+		return NewBasicLit(pos, typ, constant.MakeBool(false))
+	case typ.IsString():
+		return NewBasicLit(pos, typ, constant.MakeString(""))
+	case typ.IsArray() || typ.IsStruct():
+		// TODO(mdempsky): Return a typechecked expression instead.
+		return NewCompLitExpr(pos, OCOMPLIT, typ, nil)
+	}
+
+	base.FatalfAt(pos, "unexpected type: %v", typ)
+	panic("unreachable")
+}
+
+var (
+	intZero     = constant.MakeInt64(0)
+	floatZero   = constant.ToFloat(intZero)
+	complexZero = constant.ToComplex(intZero)
+)
+
 // NewOne returns an OLITERAL representing 1 with the given type.
 func NewOne(pos src.XPos, typ *types.Type) Node {
 	var val constant.Value

src/cmd/compile/internal/noder/codes.go

@@ -58,7 +58,7 @@ const (
 	exprSizeof
 	exprAlignof
 	exprOffsetof
-	exprNil
+	exprZero
 	exprFuncInst
 	exprRecv
 	exprReshape

src/cmd/compile/internal/noder/helpers.go

@@ -59,10 +59,6 @@ func FixValue(typ *types.Type, val constant.Value) constant.Value {
 	return val
 }
 
-func Nil(pos src.XPos, typ *types.Type) ir.Node {
-	return ir.NewNilExpr(pos, typ)
-}
-
 // Expressions
 
 func Addr(pos src.XPos, x ir.Node) *ir.AddrExpr {

src/cmd/compile/internal/noder/reader.go

@@ -1340,16 +1340,9 @@ func (r *reader) syntheticArgs(pos src.XPos) (recvs, params ir.Nodes) {
 			// For anonymous and blank parameters, we don't have an *ir.Name
 			// to use as the argument. However, since we know the shaped
 			// function won't use the value either, we can just pass the
-			// zero value. (Also unfortunately, we don't have an easy
-			// zero-value IR node; so we use a default-initialized temporary
-			// variable.)
+			// zero value.
 			if arg == nil {
-				tmp := typecheck.TempAt(pos, r.curfn, param.Type)
-				r.curfn.Body.Append(
-					typecheck.Stmt(ir.NewDecl(pos, ir.ODCL, tmp)),
-					typecheck.Stmt(ir.NewAssignStmt(pos, tmp, nil)),
-				)
-				arg = tmp
+				arg = ir.NewZero(pos, param.Type)
 			}
 
 			out.Append(arg)
@@ -2177,10 +2170,10 @@ func (r *reader) expr() (res ir.Node) {
 		val := FixValue(typ, r.Value())
 		return ir.NewBasicLit(pos, typ, val)
 
-	case exprNil:
+	case exprZero:
 		pos := r.pos()
 		typ := r.typ()
-		return Nil(pos, typ)
+		return ir.NewZero(pos, typ)
 
 	case exprCompLit:
 		return r.compLit()

src/cmd/compile/internal/noder/writer.go

@@ -1752,7 +1752,7 @@ func (w *writer) expr(expr syntax.Expr) {
 		}
 
 		if _, isNil := obj.(*types2.Nil); isNil {
-			w.Code(exprNil)
+			w.Code(exprZero)
 			w.pos(expr)
 			w.typ(tv.Type)
 			return