Add thread.spawn_indirect

This change codifies the conclusions we arrived to in [WebAssembly#89]. It adds a
new way to spawn threads, `thread.spawn_indirect`, which retrieves the
thread start function from a table. This prompted me to rename
`thread.spawn` to `thread.spawn_ref`.

[WebAssembly#89]: WebAssembly/shared-everything-threads#89

Author: abrown

design/mvp/Async.md

@@ -78,12 +78,11 @@ these languages' concurrency features are already bound (making the Component
 Model "just another OS" from the language toolchains' perspective).
 
 Moreover, this async ABI does not require components to use preemptive
-multi-threading ([`thread.spawn`]) in order to achieve concurrency. Instead,
-concurrency can be achieved by cooperatively switching between different
-logical tasks running on a single thread. This switching may require the use of
-[fibers] or a [CPS transform], but may also be avoided entirely when a
-component's producer toolchain is engineered to always return to an
-[event loop].
+multi-threading ([`thread.spawn*`]) in order to achieve concurrency. Instead,
+concurrency can be achieved by cooperatively switching between different logical
+tasks running on a single thread. This switching may require the use of [fibers]
+or a [CPS transform], but may also be avoided entirely when a component's
+producer toolchain is engineered to always return to an [event loop].
 
 To avoid partitioning the world along sync/async lines as mentioned in the
 Goals section, the Component Model allows *every* component-level function type
@@ -672,11 +671,11 @@ by declarative instantiation and `start` above.
 
 ## Interaction with multi-threading
 
-For now, the integration between multi-threading (via [`thread.spawn`]) and
-native async is limited. In particular, because all [lift and lower
-definitions] produce non-`shared` functions, any threads spawned by a component
-via `thread.spawn` will not be able to directly call imports (synchronously
-*or* asynchronously) and will thus have to use Core WebAssembly `atomics.*`
+For now, the integration between multi-threading (via [`thread.spawn*`]) and
+native async is limited. In particular, because all [lift and lower definitions]
+produce non-`shared` functions, any threads spawned by a component via
+`thread.spawn*` will not be able to directly call imports (synchronously *or*
+asynchronously) and will thus have to use Core WebAssembly `atomics.*`
 instructions to switch back to a non-`shared` function running on the "main"
 thread (i.e., whichever thread was used to call the component's exports).
 
@@ -693,8 +692,8 @@ composition story described above could naturally be extended to a
 sync+async+shared composition story, continuing to avoid the "what color is
 your function" problem (where `shared` is the [color]).
 
-Even without any use of `thread.new`, native async provides an opportunity to
-achieve some automatic parallelism "for free". In particular, due to the
+Even without any use of [`thread.spawn*`], native async provides an opportunity
+to achieve some automatic parallelism "for free". In particular, due to the
 shared-nothing nature of components, each component instance could be given a
 separate thread on which to interleave all tasks executing in that instance.
 Thus, in a cross-component call from `C1` to `C2`, `C2`'s task can run in a
@@ -750,7 +749,7 @@ comes after:
 [`yield`]: Explainer.md#-yield
 [`waitable-set.wait`]: Explainer.md#-waitable-setwait
 [`waitable-set.poll`]: Explainer.md#-waitable-setpoll
-[`thread.spawn`]: Explainer.md#-threadspawn
+[`thread.spawn*`]: Explainer.md#-threadspawnref
 [ESM-integration]: Explainer.md#ESM-integration
 
 [Canonical ABI Explainer]: CanonicalABI.md

design/mvp/Binary.md

@@ -286,9 +286,10 @@ canon    ::= 0x00 0x00 f:<core:funcidx> opts:<opts> ft:<typeidx> => (canon lift
            | 0x01 0x00 f:<funcidx> opts:<opts>                   => (canon lower f opts (core func))
            | 0x02 rt:<typeidx>                                   => (canon resource.new rt (core func))
            | 0x03 rt:<typeidx>                                   => (canon resource.drop rt (core func))
-           | 0x07 rt:<typdidx>                                   => (canon resource.drop rt async (core func)) 🔀
+           | 0x07 rt:<typeidx>                                   => (canon resource.drop rt async (core func)) 🔀
            | 0x04 rt:<typeidx>                                   => (canon resource.rep rt (core func))
-           | 0x05 ft:<typeidx>                                   => (canon thread.spawn ft (core func)) 🧵
+           | 0x05 ft:<typeidx>                                   => (canon thread.spawn_ref ft (core func)) 🧵
+           | 0x24 ft:<typeidx> t:<core:tabidx>                   => (canon thread.spawn_indirect ft (table t) (core func)) 🧵
            | 0x06                                                => (canon thread.available_parallelism (core func)) 🧵
            | 0x08                                                => (canon backpressure.set (core func)) 🔀
            | 0x09 rs:<resultlist> opts:<opts>                    => (canon task.return rs opts (core func)) 🔀

design/mvp/CanonicalABI.md

@@ -3773,11 +3773,11 @@ async def canon_error_context_drop(task, i):
 ```
 
 
-### 🧵 `canon thread.spawn`
+### 🧵 `canon thread.spawn_ref`
 
 For a canonical definition:
 ```wat
-(canon thread.spawn (type $ft) (core func $st))
+(canon thread.spawn_ref (type $ft) (core func $st))
 ```
 validation specifies:
 * `$ft` must refer to a `shared` function type; initially, only the type `(func
@@ -3802,7 +3802,7 @@ thread which:
 In pseudocode, `$st` looks like:
 
 ```python
-def canon_thread_spawn(f, c):
+def canon_thread_spawn_ref(f, c):
   trap_if(f is None)
   if DETERMINISTIC_PROFILE:
     return [-1]
@@ -3820,6 +3820,48 @@ def canon_thread_spawn(f, c):
 ```
 
 
+### 🧵 `canon thread.spawn_indirect`
+
+For a canonical definition:
+```wat
+(canon thread.spawn_indirect (type $ft) (table $t) (core func $st))
+```
+validation specifies:
+* `$ft` must refer to a `shared` function type; initially, only the type `(func
+  shared (param $c i32))` is allowed (see explanation in `thread.spawn_ref`
+  above)
+* `$t` must refer to a table containing `ft`-typed items
+* `$st` is given type `(func (param $i i32) (param $c i32) (result $e
+  i32))`.
+
+Calling `$st` retrieves a function `$f` of type `$ft` from table `$t`. If that
+succeeds, it spawns a thread which:
+  - invokes `$f` with `$c`
+  - executes `$f` until completion or trap in a `shared` context as described by
+    the [shared-everything threads] proposal.
+
+In pseudocode, `$st` looks like:
+
+```python
+def canon_thread_spawn_indirect(t, i, c):
+  trap_if(t[i] is None)
+  f = t[i]
+  if DETERMINISTIC_PROFILE:
+    return [-1]
+
+  def thread_start():
+    try:
+      f(c)
+    except CoreWebAssemblyException:
+      trap()
+
+  if spawn(thread_start):
+    return [0]
+  else:
+    return [-1]
+```
+
+
 ### 🧵 `canon thread.available_parallelism`
 
 For a canonical definition:

design/mvp/Explainer.md

@@ -1438,7 +1438,8 @@ canon ::= ...
         | (canon error-context.new <canonopt>* (core func <id>?))
         | (canon error-context.debug-message <canonopt>* (core func <id>?))
         | (canon error-context.drop (core func <id>?))
-        | (canon thread.spawn <typeidx> (core func <id>?)) 🧵
+        | (canon thread.spawn_ref <typeidx> (core func <id>?)) 🧵
+        | (canon thread.spawn_indirect <typeidx> <core:tableidx> (core func <id>?)) 🧵
         | (canon thread.available_parallelism (core func <id>?)) 🧵
 ```
 
@@ -1945,19 +1946,34 @@ thread management. These are specified as built-ins and not core WebAssembly
 instructions because browsers expect this functionality to come from existing
 Web/JS APIs.
 
-###### 🧵 `thread.spawn`
+###### 🧵 `thread.spawn_ref`
 
 | Synopsis                   |                                                           |
 | -------------------------- | --------------------------------------------------------- |
 | Approximate WIT signature  | `func<FuncT>(f: FuncT, c: FuncT.params[0]) -> bool`       |
 | Canonical ABI signature    | `[f:(ref null (func shared (param i32))) c:i32] -> [i32]` |
 
-The `thread.spawn` built-in spawns a new thread by invoking the shared function
-`f` while passing `c` to it, returning whether a thread was successfully
-spawned. While it's designed to allow different types in the future, the type
-of `c` is currently hard-coded to always be `i32`.
+The `thread.spawn_ref` built-in spawns a new thread by invoking the shared
+function `f` while passing `c` to it, returning whether a thread was
+successfully spawned. While it's designed to allow different types in the
+future, the type of `c` is currently hard-coded to always be `i32`.
 
-(See also [`canon_thread_spawn`] in the Canonical ABI explainer.)
+(See also [`canon_thread_spawn_ref`] in the Canonical ABI explainer.)
+
+
+###### 🧵 `thread.spawn_indirect`
+
+| Synopsis                   |                                                   |
+| -------------------------- | ------------------------------------------------- |
+| Approximate WIT signature  | `func<FuncT>(i: i32, c: FuncT.params[0]) -> bool` |
+| Canonical ABI signature    | `[i:i32 c:i32] -> [i32]`                          |
+
+The `thread.spawn_indirect` built-in spawns a new thread by retrieving the
+shared function `f` from a table using index `i` (much like the `call_indirect`
+core instruction). Once `f` is retrieved, this built-in operates like
+`thread.spawn_ref` above, including the limitations on `f`'s parameters.
+
+(See also [`canon_thread_spawn_indirect`] in the Canonical ABI explainer.)
 
 ###### 🧵 `thread.available_parallelism`
 
@@ -2806,7 +2822,8 @@ For some use-case-focused, worked examples, see:
 [`canon_error_context_new`]: CanonicalABI.md#-canon-error-contextnew
 [`canon_error_context_debug_message`]: CanonicalABI.md#-canon-error-contextdebug-message
 [`canon_error_context_drop`]: CanonicalABI.md#-canon-error-contextdrop
-[`canon_thread_spawn`]: CanonicalABI.md#-canon-theadspawn
+[`canon_thread_spawn_ref`]: CanonicalABI.md#-canon-threadspawnref
+[`canon_thread_spawn_indirect`]: CanonicalABI.md#-canon-threadspawnindirect
 [`canon_thread_available_parallelism`]: CanonicalABI.md#-canon-threadavailable_parallelism
 [`pack_async_copy_result`]: CanonicalABI.md#-canon-streamfuturereadwrite
 [the `close` built-ins]: CanonicalABI.md#-canon-streamfutureclose-readablewritable