go/ir: add SliceToArray op

Closes gh-1335

go/ir/doc.go

@@ -93,6 +93,7 @@
 //   *Sigma                ✔               ✔
 //   *Slice                ✔               ✔
 //   *SliceToArrayPointer  ✔               ✔
+//   *SliceToArray         ✔               ✔
 //   *Store                ✔               ✔
 //   *StringLookup         ✔               ✔
 //   *Type                                                 ✔ (type)

go/ir/emit.go

@@ -279,6 +279,25 @@ func emitConv(f *Function, val Value, t_dst types.Type, source ast.Node) Value {
 		return f.emit(c, source)
 	}
 
+	// Conversion from slice to array. This is almost the same as converting from slice to array pointer, then
+	// dereferencing the pointer. Except that a nil slice can be converted to [0]T, whereas converting a nil slice to
+	// (*[0]T) results in a nil pointer, dereferencing which would panic. To hide the extra branching we use a dedicated
+	// instruction, SliceToArray.
+	if tset_src.All(func(termSrc *types.Term) bool {
+		return tset_dst.All(func(termDst *types.Term) bool {
+			if slice, ok := termSrc.Type().Underlying().(*types.Slice); ok {
+				if arr, ok := termDst.Type().Underlying().(*types.Array); ok && types.Identical(slice.Elem(), arr.Elem()) {
+					return true
+				}
+			}
+			return false
+		})
+	}) {
+		c := &SliceToArray{X: val}
+		c.setType(t_dst)
+		return f.emit(c, source)
+	}
+
 	// A representation-changing conversion?
 	// At least one of {ut_src,ut_dst} must be *Basic.
 	// (The other may be []byte or []rune.)

go/ir/lift.go

@@ -1488,6 +1488,10 @@ func splitOnNewInformation(u *BasicBlock, renaming *StackMap) {
 			// A slice to array pointer conversion tells us the minimum length of the slice
 			rename(instr.X, instr, CopyInfoUnspecified, i)
 			i++
+		case *SliceToArray:
+			// A slice to array conversion tells us the minimum length of the slice
+			rename(instr.X, instr, CopyInfoUnspecified, i)
+			i++
 		case *Slice:
 			// Slicing tells us about some of the bounds
 			off := 0

go/ir/print.go

@@ -174,6 +174,7 @@ func (v *ChangeType) String() string          { return printConv("ChangeType", v
 func (v *Convert) String() string             { return printConv("Convert", v, v.X) }
 func (v *ChangeInterface) String() string     { return printConv("ChangeInterface", v, v.X) }
 func (v *SliceToArrayPointer) String() string { return printConv("SliceToArrayPointer", v, v.X) }
+func (v *SliceToArray) String() string        { return printConv("SliceToArray", v, v.X) }
 func (v *MakeInterface) String() string       { return printConv("MakeInterface", v, v.X) }
 
 func (v *MakeClosure) String() string {

go/ir/sanity.go

@@ -142,6 +142,7 @@ func (s *sanity) checkInstr(idx int, instr Instruction) {
 	case *ChangeInterface:
 	case *ChangeType:
 	case *SliceToArrayPointer:
+	case *SliceToArray:
 	case *Convert:
 		if _, ok := instr.X.Type().Underlying().(*types.Basic); !ok {
 			if _, ok := instr.Type().Underlying().(*types.Basic); !ok {

go/ir/ssa.go

@@ -927,13 +927,27 @@ type ChangeInterface struct {
 // from an explicit conversion in the source.
 //
 // Example printed form:
-// 	t1 = SliceToArrayPointer <*[4]byte> t1
 //
+//	t2 = SliceToArrayPointer <*[4]byte> t1
 type SliceToArrayPointer struct {
 	register
 	X Value
 }
 
+// The SliceToArray instruction yields the conversion of slice X to
+// array.
+//
+// Pos() returns the ast.CallExpr.Lparen, if the instruction arose
+// from an explicit conversion in the source.
+//
+// Example printed form:
+//
+//	t2 = SliceToArray <[4]byte> t1
+type SliceToArray struct {
+	register
+	X Value
+}
+
 // MakeInterface constructs an instance of an interface type from a
 // value of a concrete type.
 //
@@ -1911,6 +1925,10 @@ func (v *SliceToArrayPointer) Operands(rands []*Value) []*Value {
 	return append(rands, &v.X)
 }
 
+func (v *SliceToArray) Operands(rands []*Value) []*Value {
+	return append(rands, &v.X)
+}
+
 func (s *DebugRef) Operands(rands []*Value) []*Value {
 	return append(rands, &s.X)
 }

unused/unused.go

@@ -1789,6 +1789,8 @@ func (g *graph) instructions(fn *ir.Function) {
 				// nothing to do
 			case *ir.SliceToArrayPointer:
 				// nothing to do
+			case *ir.SliceToArray:
+				// nothing to do
 			case *ir.CompositeValue:
 				if t, ok := typeutil.CoreType(instr.Type()).(*types.Struct); ok {
 					for i := 0; i < len(instr.Values); i++ {