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diff --git a/AstSemantics.md b/AstSemantics.md index cc099c8..0e3c753 100644 --- a/AstSemantics.md +++ b/AstSemantics.md @@ -237,39 +237,43 @@ others, etc. ## Control flow structures -WebAssembly offers basic structured control flow. All control flow structures -are statements. - - * `block`: a fixed-length sequence of statements - * `if`: if statement - * `do_while`: do while statement, basically a loop with a conditional branch - (back to the top of the loop) - * `forever`: infinite loop statement (like `while (1)`), basically an - unconditional branch (back to the top of the loop) - * `continue`: continue to start of nested loop - * `break`: break to end from nested loop or block - * `return`: return zero or more values from this function - * `switch`: switch statement with fallthrough - -Loops (`do_while` and `forever`) may only be entered via fallthrough at the top. -In particular, loops may not be entered directly via a `break`, `continue`, or -`switch` destination. Break and continue statements can only target blocks or -loops in which they are nested. These rules guarantee that all control flow -graphs are well-structured. - -Structured control flow provides simple and size-efficient binary encoding and -compilation. Any control flow—even irreducible—can be transformed into structured -control flow with the -[Relooper](https://github.com/kripken/emscripten/raw/master/docs/paper.pdf) -[algorithm](http://dl.acm.org/citation.cfm?id=2048224&CFID=670868333&CFTOKEN=46181900), -with guaranteed low code size overhead, and typically minimal throughput -overhead (except for pathological cases of irreducible control -flow). Alternative approaches can generate reducible control flow via node -splitting, which can reduce throughput overhead, at the cost of increasing -code size (potentially very significantly in pathological cases). -Also, -[more expressive control flow constructs](FutureFeatures.md#more-expressive-control-flow) -may be added in the future. +WebAssembly offers basic structured control flow with the following constructs. +All control flow structures, except `case`, are statements. + + * `block`: a fixed-length sequence of statements with a label at the end + * `loop`: a fixed-length sequence of statements with a label at the end + and a loop header label at the top + * `if`: if statement with then body + * `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 + +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 +within descendents of the node in the tree. For example, references to a +`block`'s label can only occur within the `block`'s body. In practice, +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`. + ## Calls |