Model tableswitch in a more sparse way.

AstSemantics.md

@@ -258,10 +258,17 @@ All control flow structures, except `case`, are statements.
  * `br`: branch to a given label in an enclosing construct (see below)
  * `br_if`: conditionally branch to a given label in an enclosing construct
  * `tableswitch`: a jump table transferring which may jump either to enclosed
-                  `case` blocks or to labels in enclosing constructs (see below
-                  for a more detailed description)
- * `case`: must be an immediate child of `tableswitch`; has a label declared
-           in the `tableswitch`'s table and a body (as above, see below)
+                  `case` and `default` blocks or to labels in enclosing constructs
+                  identified by enclosed `case_br` and `default_br` nodes (see
+                  below for a more detailed description)
+ * `case`: must be an immediate child of `tableswitch`; has an immediate index
+           value and a body (as above, see below)
+ * `case_br`: must be an immediate child of `tableswitch`; has an immediate index
+              value and a label (as above, see below)
+ * `default`: must be an immediate child of `tableswitch`; has a body
+              (as above, see below)
+ * `default_br`: must be an immediate child of `tableswitch`; has a a label
+                 (as above, see below)
  * `return`: return zero or more values from this function
 
 References to labels must occur within an *enclosing construct* that defined
@@ -272,18 +279,31 @@ one can arrange `block`s to put labels wherever one wants to jump to, except
 for one restriction: one can't jump into the middle of a loop from outside
 it. This restriction ensures the well-structured property discussed below.
 
-`tableswitch` instructions have a zero-based array of labels, a label index,
-a "default" label, an index operand, and a list of `case` nodes. A `tableswitch`
-selects which label to branch to by looking up the index value in the label
-array, and transferring control to that label. If the index is out of bounds,
-it transfers control to the "default" label.
-
-`case` nodes can only appear as immediate children of `tableswitch` statements.
-They have a label, which must be declared in the immediately enclosing
-`tableswitch`'s array, and a body which can contain arbitrary code. Control
-falls through the end of a `case` block into the following `case` block, or
-the end of the `tableswitch` in the case of the last `case`.
-
+`tableswitch` instructions have a label index operand, and contain a sequence
+of `case`, `case_br`, `default`, and `default_br` nodes. The following
+restrictions apply to the sequence:
+
+ - There must be either exactly one `default` node or exactly one
+   `default_br` node.
+ - Each `case` and `case_br` must have a unique index immediate value
+   among all `case` and `case_br` nodes.
+ - For each `case` and `case_br` with a non-zero index immediate value,
+   there must also be a `case` or `case_br` present with an index
+   immediate value of one less.
+
+A `tableswitch` first chooses a child node by selecting the `case` or
+`case_br` node with the immediate index value equal to the `index` operand,
+if there is such a `case` or `case_br` node present, or the `default` or
+`default_br` node otherwise.
+
+If the chosen child node is a `case`, or `default`, `tableswitch` transfers
+control to the node's body. If the chosen child node is a `case_br` or
+`default_br`, `tableswitch` transfers control directly to the node's label.
+
+`case`, `case_br`, `default`, and `default_br` nodes can only appear as
+immediate children of `tableswitch` statements. Control falls through the end
+of `case` and `default` bodies, into the next `tableswitch` child node in AST
+order, or exiting the `tableswitch` in the case of the last child node.
 
 ## Calls