Add explicit argument counts (#672)

* Add explicit arities

* Rename

Author: rossberg

BinaryEncoding.md

@@ -321,27 +321,30 @@ It is legal to have several entries with the same type.
 
 ## Control flow operators ([described here](AstSemantics.md#control-flow-structures))
 
-| Name | Opcode | Immediate | Description |
+| Name | Opcode | Immediates | Description |
 | ---- | ---- | ---- | ---- |
 | `nop` | `0x00` | | no operation |
 | `block` | `0x01` |  | begin a sequence of expressions, the last of which yields a value |
 | `loop` | `0x02` |  | begin a block which can also form control flow loops |
 | `if` | `0x03` | | begin if expression |
 | `else` | `0x04` | | begin else expression of if |
 | `select` | `0x05` | | select one of two values based on condition |
-| `br` | `0x06` | relative_depth = `varuint32` | break that targets an outer nested block |
-| `br_if` | `0x07` | relative_depth = `varuint32` | conditional break that targets an outer nested block |
+| `br` | `0x06` | argument_count : `varuint1`, relative_depth : `varuint32` | break that targets an outer nested block |
+| `br_if` | `0x07` | argument_count : `varuint1`, relative_depth : `varuint32` | conditional break that targets an outer nested block |
 | `br_table` | `0x08` | see below | branch table control flow construct |
-| `return` | `0x09` | | return zero or one value from this function |
+| `return` | `0x09` | argument_count : `varuint1` | return zero or one value from this function |
 | `unreachable` | `0x0a` | | trap immediately |
 | `end` | `0x0f` | | end a block, loop, or if |
 
 Note that there is no explicit `if_else` opcode, as the else clause is encoded with the `else` bytecode.
 
+The counts following the break and return operators specify how many preceding operands are taken as transfer arguments; in the MVP, all these values must be either 0 or 1.
+
 The `br_table` operator has an immediate operand which is encoded as follows:
 
 | Field | Type | Description |
 | ---- | ---- | ---- |
+| arity | `varuint1` | number of arguments |
 | target_count | `varuint32` | number of targets in the target_table |
 | target_table | `uint32*` | target entries that indicate an outer block or loop to which to break |
 | default_target | `uint32` | an outer block or loop to which to break in the default case |
@@ -352,17 +355,20 @@ branches to the block or loop at the given offset within the `target_table`. If
 out of range, `br_table` branches to the default target.
 
 ## Basic operators ([described here](AstSemantics.md#constants))
-| Name | Opcode | Immediate | Description |
+
+| Name | Opcode | Immediates | Description |
 | ---- | ---- | ---- | ---- |
-| `i32.const` | `0x10` | value = `varint32` | a constant value interpreted as `i32` |
-| `i64.const` | `0x11` | value = `varint64` | a constant value interpreted as `i64` |
-| `f64.const` | `0x12` | value = `uint64` | a constant value interpreted as `f64` |
-| `f32.const` | `0x13` | value = `uint32` | a constant value interpreted as `f32` |
-| `get_local` | `0x14` | local_index = `varuint32` | read a local variable or parameter |
-| `set_local` | `0x15` | local_index = `varuint32` | write a local variable or parameter |
-| `call` | `0x16` | function_index = `varuint32` | call a function by its index |
-| `call_indirect` | `0x17` | signature_index = `varuint32` | call a function indirect with an expected signature |
-| `call_import` | `0x18` | import_index = `varuint32` | call an imported function by its index |
+| `i32.const` | `0x10` | value : `varint32` | a constant value interpreted as `i32` |
+| `i64.const` | `0x11` | value : `varint64` | a constant value interpreted as `i64` |
+| `f64.const` | `0x12` | value : `uint64` | a constant value interpreted as `f64` |
+| `f32.const` | `0x13` | value : `uint32` | a constant value interpreted as `f32` |
+| `get_local` | `0x14` | local_index : `varuint32` | read a local variable or parameter |
+| `set_local` | `0x15` | local_index : `varuint32` | write a local variable or parameter |
+| `call` | `0x16` | argument_count : `varuint1`, function_index : `varuint32` | call a function by its index |
+| `call_indirect` | `0x17` | argument_count : `varuint1`, type_index : `varuint32` | call a function indirect with an expected signature |
+| `call_import` | `0x18` | argument_count : `varuint1`, import_index : `varuint32` | call an imported function by its index |
+
+The counts following the different call opcodes specify the number of preceding operands taken as arguments.
 
 ## Memory-related operators ([described here](AstSemantics.md#linear-memory-accesses))