cmd/compile: casts from slices to array pointers are known to be non-nil

The cast is proceeded by a bounds check. If the bounds check passes
then we know the pointer in the slice is non-nil.

... except casts to pointers of 0-sized arrays. They are strange, as
the bounds check can pass for a nil input.

Change-Id: Ic01cf4a82d59fbe3071d4b271c94efca9cafaec1
Reviewed-on: https://go-review.googlesource.com/c/go/+/479335
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -3277,10 +3277,15 @@ func (s *state) exprCheckPtr(n ir.Node, checkPtrOK bool) *ssa.Value {
 		// slice.ptr
 		n := n.(*ir.ConvExpr)
 		v := s.expr(n.X)
-		arrlen := s.constInt(types.Types[types.TINT], n.Type().Elem().NumElem())
+		nelem := n.Type().Elem().NumElem()
+		arrlen := s.constInt(types.Types[types.TINT], nelem)
 		cap := s.newValue1(ssa.OpSliceLen, types.Types[types.TINT], v)
 		s.boundsCheck(arrlen, cap, ssa.BoundsConvert, false)
-		return s.newValue1(ssa.OpSlicePtrUnchecked, n.Type(), v)
+		op := ssa.OpSlicePtr
+		if nelem == 0 {
+			op = ssa.OpSlicePtrUnchecked
+		}
+		return s.newValue1(op, n.Type(), v)
 
 	case ir.OCALLFUNC:
 		n := n.(*ir.CallExpr)

test/nilptr3.go

@@ -248,3 +248,10 @@ func f10(p **int) int {
 	/* */
 	*p // ERROR "removed nil check"
 }
+
+func f11(x []byte) {
+	p := (*[0]byte)(x)
+	_ = *p // ERROR "generated nil check"
+	q := (*[4]byte)(x)
+	_ = *q // ERROR "removed nil check"
+}