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Streamline tableswitch notation #443

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Streamline tableswitch notation #443

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35 changes: 17 additions & 18 deletions AstSemantics.md
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Expand Up @@ -247,12 +247,18 @@ All control flow structures, except `case`, are statements.
* `if_else`: if statement with then and else bodies
* `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)
* `return`: return zero or more values from this function
* `tableswitch`: a jump table with an index operand selecting one of the
targets described by the following enclosed pseudo operators:
* `case` or `case_br`: carries the index constant for which it is selected
* `default` or `default_br`: selected when the index is larger than any
occurring constant

The regular variants of these have an immediate body that is executed and
then fall through to the next target (or the end of the `tableswitch`),
whereas the `_br` variants just contain a reference to a label that control
is transferred to directly.

* `return`: return zero or one value from this function

References to labels must occur within an *enclosing construct* that defined
the label. This means that references to an AST node's label can only happen
Expand All @@ -262,18 +268,11 @@ 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`.

A `tableswitch` represents a jump table. For that reason, its case constants
must form a zero-based and dense range, i.e., be distinct and cover all values
from 0 to the maximum occurring constant.
There must be exactly one default target as well. The ordering among these
targets is unconstrained, however.

## Calls

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