cmd/compile: implement slice-to-array conversions

The conversion T(x) is implemented as *(*T)(x). Accordingly, runtime
panic messages for (*T)(x) are made more general.

Fixes #46505.

Change-Id: I76317c0878b6a5908299506d392eed50d7ef6523
Reviewed-on: https://go-review.googlesource.com/c/go/+/430415
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/escape/expr.go

@@ -126,6 +126,10 @@ func (e *escape) exprSkipInit(k hole, n ir.Node) {
 	case ir.OITAB, ir.OIDATA, ir.OSPTR:
 		n := n.(*ir.UnaryExpr)
 		e.expr(k, n.X)
+	case ir.OSLICE2ARR:
+		// Converting a slice to array is effectively a deref.
+		n := n.(*ir.ConvExpr)
+		e.expr(k.deref(n, "slice-to-array"), n.X)
 	case ir.OSLICE2ARRPTR:
 		// the slice pointer flows directly to the result
 		n := n.(*ir.ConvExpr)

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

@@ -289,7 +289,7 @@ func (n *ConvExpr) SetOp(op Op) {
 	switch op {
 	default:
 		panic(n.no("SetOp " + op.String()))
-	case OCONV, OCONVIFACE, OCONVIDATA, OCONVNOP, OBYTES2STR, OBYTES2STRTMP, ORUNES2STR, OSTR2BYTES, OSTR2BYTESTMP, OSTR2RUNES, ORUNESTR, OSLICE2ARRPTR:
+	case OCONV, OCONVIFACE, OCONVIDATA, OCONVNOP, OBYTES2STR, OBYTES2STRTMP, ORUNES2STR, OSTR2BYTES, OSTR2BYTESTMP, OSTR2RUNES, ORUNESTR, OSLICE2ARR, OSLICE2ARRPTR:
 		n.op = op
 	}
 }

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

@@ -208,6 +208,7 @@ var OpPrec = []int{
 	OPRINT:            8,
 	ORUNESTR:          8,
 	OSIZEOF:           8,
+	OSLICE2ARR:        8,
 	OSLICE2ARRPTR:     8,
 	OSTR2BYTES:        8,
 	OSTR2RUNES:        8,
@@ -753,6 +754,7 @@ func exprFmt(n Node, s fmt.State, prec int) {
 		OSTR2BYTES,
 		OSTR2RUNES,
 		ORUNESTR,
+		OSLICE2ARR,
 		OSLICE2ARRPTR:
 		n := n.(*ConvExpr)
 		if n.Type() == nil || n.Type().Sym() == nil {

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

@@ -134,6 +134,7 @@ const (
 	OSTR2BYTES    // Type(X) (Type is []byte, X is a string)
 	OSTR2BYTESTMP // Type(X) (Type is []byte, X is a string, ephemeral)
 	OSTR2RUNES    // Type(X) (Type is []rune, X is a string)
+	OSLICE2ARR    // Type(X) (Type is [N]T, X is a []T)
 	OSLICE2ARRPTR // Type(X) (Type is *[N]T, X is a []T)
 	// X = Y or (if Def=true) X := Y
 	// If Def, then Init includes a DCL node for X.

src/cmd/compile/internal/typecheck/iexport.go

@@ -1974,7 +1974,7 @@ func (w *exportWriter) expr(n ir.Node) {
 		w.expr(n.Y)
 		w.typ(n.Type())
 
-	case ir.OCONV, ir.OCONVIFACE, ir.OCONVIDATA, ir.OCONVNOP, ir.OBYTES2STR, ir.ORUNES2STR, ir.OSTR2BYTES, ir.OSTR2RUNES, ir.ORUNESTR, ir.OSLICE2ARRPTR:
+	case ir.OCONV, ir.OCONVIFACE, ir.OCONVIDATA, ir.OCONVNOP, ir.OBYTES2STR, ir.ORUNES2STR, ir.OSTR2BYTES, ir.OSTR2RUNES, ir.ORUNESTR, ir.OSLICE2ARR, ir.OSLICE2ARRPTR:
 		n := n.(*ir.ConvExpr)
 		w.op(n.Op())
 		w.pos(n.Pos())

src/cmd/compile/internal/typecheck/iimport.go

@@ -1484,7 +1484,7 @@ func (r *importReader) node() ir.Node {
 		n.SetType(r.typ())
 		return n
 
-	case ir.OCONV, ir.OCONVIFACE, ir.OCONVIDATA, ir.OCONVNOP, ir.OBYTES2STR, ir.ORUNES2STR, ir.OSTR2BYTES, ir.OSTR2RUNES, ir.ORUNESTR, ir.OSLICE2ARRPTR:
+	case ir.OCONV, ir.OCONVIFACE, ir.OCONVIDATA, ir.OCONVNOP, ir.OBYTES2STR, ir.ORUNES2STR, ir.OSTR2BYTES, ir.OSTR2RUNES, ir.ORUNESTR, ir.OSLICE2ARR, ir.OSLICE2ARRPTR:
 		n := ir.NewConvExpr(r.pos(), op, r.typ(), r.expr())
 		n.SetImplicit(r.bool())
 		return n

src/cmd/compile/internal/typecheck/subr.go

@@ -579,11 +579,16 @@ func Convertop(srcConstant bool, src, dst *types.Type) (ir.Op, string) {
 		return ir.OCONVNOP, ""
 	}
 
-	// 11. src is a slice and dst is a pointer-to-array.
+	// 11. src is a slice and dst is an array or pointer-to-array.
 	// They must have same element type.
-	if src.IsSlice() && dst.IsPtr() && dst.Elem().IsArray() &&
-		types.Identical(src.Elem(), dst.Elem().Elem()) {
-		return ir.OSLICE2ARRPTR, ""
+	if src.IsSlice() {
+		if dst.IsArray() && types.Identical(src.Elem(), dst.Elem()) {
+			return ir.OSLICE2ARR, ""
+		}
+		if dst.IsPtr() && dst.Elem().IsArray() &&
+			types.Identical(src.Elem(), dst.Elem().Elem()) {
+			return ir.OSLICE2ARRPTR, ""
+		}
 	}
 
 	return ir.OXXX, ""

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

@@ -503,3 +503,21 @@ func walkCheckPtrArithmetic(n *ir.ConvExpr, init *ir.Nodes) ir.Node {
 
 	return cheap
 }
+
+// walkSliceToArray walks an OSLICE2ARR expression.
+func walkSliceToArray(n *ir.ConvExpr, init *ir.Nodes) ir.Node {
+	// Replace T(x) with *(*T)(x).
+	conv := typecheck.Expr(ir.NewConvExpr(base.Pos, ir.OCONV, types.NewPtr(n.Type()), n.X)).(*ir.ConvExpr)
+	deref := typecheck.Expr(ir.NewStarExpr(base.Pos, conv)).(*ir.StarExpr)
+
+	// The OSLICE2ARRPTR conversion handles checking the slice length,
+	// so the dereference can't fail.
+	//
+	// However, this is more than just an optimization: if T is a
+	// zero-length array, then x (and thus (*T)(x)) can be nil, but T(x)
+	// should *not* panic. So suppressing the nil check here is
+	// necessary for correctness in that case.
+	deref.SetBounded(true)
+
+	return walkExpr(deref, init)
+}

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

@@ -227,6 +227,10 @@ func walkExpr1(n ir.Node, init *ir.Nodes) ir.Node {
 		n := n.(*ir.ConvExpr)
 		return walkConv(n, init)
 
+	case ir.OSLICE2ARR:
+		n := n.(*ir.ConvExpr)
+		return walkSliceToArray(n, init)
+
 	case ir.OSLICE2ARRPTR:
 		n := n.(*ir.ConvExpr)
 		n.X = walkExpr(n.X, init)