Add support for the event section (#70)

Author: aheejin

Linking.md

@@ -20,6 +20,7 @@ be performed:
 
 - Merging of function sections (re-numbering functions)
 - Merging of globals sections (re-numbering globals)
+- Merging of event sections (re-numbering events)
 - Merging of data segments (re-positioning data)
 - Resolving undefined external references
 
@@ -84,6 +85,9 @@ A relocation type can be one of the following:
   The offsets start at the actual function code excluding its size field.
 - `9 / R_WEBASSEMBLY_SECTION_OFFSET_I32` - an byte offset from start of the
   specified section encoded as a [uint32].
+- `10 / R_WEBASSEMBLY_EVENT_INDEX_LEB` - an event index encoded as a 5-byte
+  [varuint32]. Used for the immediate argument of a `throw` and `if_except`
+  instruction.
 
 [varuint32]: https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#varuintn
 [varint32]: https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#varintn
@@ -188,15 +192,16 @@ where a `syminfo` is encoded as:
 |              |                |   `1 / SYMTAB_DATA`                         |
 |              |                |   `2 / SYMTAB_GLOBAL`                       |
 |              |                |   `3 / SYMTAB_SECTION`                      |
+|              |                |   `4 / SYMTAB_EVENT`                        |
 | flags        | `varuint32`    | a bitfield containing flags for this symbol |
 
-For functions and globals, we reference an existing Wasm object, which is either
-an import or a defined function/global (recall that the operand of a Wasm
-`call` instruction uses an index space consisting of the function imports
+For functions, globals and events, we reference an existing Wasm object, which
+is either an import or a defined function/global/event (recall that the operand of a
+Wasm `call` instruction uses an index space consisting of the function imports
 followed by the defined functions, and similarly `get_global` for global imports
-and definitions).  If a function or global symbol references an import, then the
-name is taken from the import; otherwise the `syminfo` specifies the symbol's
-name.
+and definitions and `throw` for event imports and definitions). If a function,
+global, or event symbol references an import, then the name is taken from the
+import; otherwise the `syminfo` specifies the symbol's name.
 
 | Field        | Type           | Description                                 |
 | ------------ | -------------- | ------------------------------------------- |
@@ -236,8 +241,8 @@ The current set of valid flags for symbols are:
   Hidden symbols are not to be exported when performing the final link, but
   may be linked to other modules.
 - `0x10 / WASM_SYM_UNDEFINED` - Indicating that this symbol is not defined.
-  For function/global symbols, must match whether the symbol is an import or
-  is defined; for data symbols, determines whether a segment is specified.
+  For function/global/event symbols, must match whether the symbol is an import
+  or is defined; for data symbols, determines whether a segment is specified.
 
 For `WASM_COMDAT_INFO` the following fields are present in the
 subsection:
@@ -262,10 +267,11 @@ and where a `comdat_sym` is encoded as:
 | Field    | Type           | Description                                 |
 | -------- | -------------- | ------------------------------------------- |
 | kind     | `uint8`        | Type of symbol, one of:                     |
-|          |                |   * `0 / WASM_COMDATA_DATA`, a data segment |
-|          |                |   * `1 / WASM_COMDATA_FUNCTION`             |
-|          |                |   * `2 / WASM_COMDATA_GLOBAL`               |
-| index    | `varuint32`    | Index of the data segment/function/global in the Wasm module (depending on kind).  The function/global must not be an import. |
+|          |                |   * `0 / WASM_COMDAT_DATA`, a data segment  |
+|          |                |   * `1 / WASM_COMDAT_FUNCTION`              |
+|          |                |   * `2 / WASM_COMDAT_GLOBAL`                |
+|          |                |   * `3 / WASM_COMDAT_EVENT`                 |
+| index    | `varuint32`    | Index of the data segment/function/global/event in the Wasm module (depending on kind). The function/global/event must not be an import. |
 
 
 Merging Global Sections
@@ -285,6 +291,26 @@ object file.  For example, the `__stack_pointer` symbol may be provided at
 link-time.
 
 
+Merging Event Sections
+-----------------------
+
+Events are meant to represent various control-flow changing constructs of wasm.
+Currently we have a
+[proposal](https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md)
+for one kind of events: exceptions, but the event section can be used to support
+other kinds of events in future as well. The event section is a list of declared
+events associated with the module.
+
+Merging of the event sections requires the re-numbering of events. This follows
+the normal rules for defining symbols: if two object files provide the same
+event symbol strongly, there is a link error; if two object files provide the
+symbol weakly, one is chosen.
+
+When creating non-relocatable output, the Wasm output shall have an import for
+each undefined strong symbol, and an export for each defined symbol with
+non-local linkage and non-hidden visibility.
+
+
 Merging Function Sections
 -------------------------
 
@@ -369,9 +395,9 @@ symbols via a `call_indirect` instruction.
 `R_WEBASSEMBLY_GLOBAL_INDEX_LEB` relocations may fail to be processed, in which
 case linking fails.  This occurs if there is a weakly-undefined global symbol,
 in which case there is no legal value that can be written as the target of any
-`get_global` or `set_global` instruction.  The frontend must not weak globals
-which may not be defined; a definition or import must exist for all global
-symbols in the linked output.
+`get_global` or `set_global` instruction. (This means the frontend must not
+generate weak globals which may not be defined; a definition or import must
+exist for all global symbols in the linked output.)
 
 `R_WEBASSEMBLY_MEMORY_ADDR_LEB`, `R_WEBASSEMBLY_MEMORY_ADDR_SLEB` and
 `R_WEBASSEMBLY_MEMORY_ADDR_I32` relocations cannot fail.  The relocation's value
@@ -385,4 +411,11 @@ adjusted to reflect new offsets in the code section.
 `R_WEBASSEMBLY_SECTION_OFFSET_I32` relocation cannot fail. The values shall be
 adjusted to reflect new offsets in the combined sections.
 
+`R_WEBASSEMBLY_EVENT_INDEX_LEB` relocations may fail to be processed, in which
+case linking fails. This occurs if there is a weakly-undefined event symbol, in
+which case there is no legal value that can be written as the target of any
+`throw` and `if_except` instruction. (This means the frontend must not generate
+weak events which may not be defined; a definition or import must exist for all
+event symbols in the linked output.)
+
 [names_sec]: https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#name-section