mixed context types behavior

Author: adambratschikaye

design/mvp/CanonicalABI.md

@@ -3752,12 +3752,17 @@ validation specifies:
 * `$f` is given type `(func (result $t))`
 
 Calling `$f` invokes the following function, which reads the [thread-local
-storage] of the [current thread]:
+storage] of the [current thread] (taking only the low 32-bits if `$t` is `i32`):
 ```python
 def canon_context_get(t, i, thread):
+  MASK_32BIT = 1 << 32 - 1
+
   assert(t == 'i32' or t == 'i64')
   assert(i < Thread.CONTEXT_LENGTH)
-  return [thread.context[i]]
+  result = thread.context[i]
+  if t == 'i32':
+    result &= MASK_32BIT
+  return [result]
 ```
 
 

design/mvp/Concurrency.md

@@ -414,17 +414,24 @@ current thread's thread-local storage can be read and written from core wasm
 code by calling the [`context.get`] and [`context.set`] built-ins.
 
 The thread-local storage array's length is currently fixed to contain exactly 2
-`i32`s or `i64`s with the goal of allowing this array to be stored inline in
-whatever existing runtime data structure is already efficiently reachable from
-ambient compiled wasm code. Because module instantiation is declarative in the
-Component Model, the imported `context.{get,set}` built-ins can be inlined by
-the core wasm compiler as-if they were instructions, allowing the generated
-machine code to be a single load or store. This makes thread-local storage a
-natural place to store:
+`i64`s with the goal of allowing this array to be stored inline in whatever
+existing runtime data structure is already efficiently reachable from ambient
+compiled wasm code. Because module instantiation is declarative in the Component
+Model, the imported `context.{get,set}` built-ins can be inlined by the core
+wasm compiler as-if they were instructions, allowing the generated machine code
+to be a single load or store. This makes thread-local storage a natural place to
+store:
 1. a pointer to the linear-memory "shadow stack" pointer
 2. a pointer to a struct used by the runtime to implement the language's
    thread-local features
 
+Both of `context.{get,set}` take an immediate argument of `i32` or `i64` to
+indicate the return or argument type. `context.set i32` will zero the high
+bits of the stored value and `context.get i32` will only read the low bits of
+the stored value.  Generally it is expected that 32-bit components always use
+the `i32` immediate and 64-bit components always use the `i64` immediate, but
+mixing these calls is still valid.
+
 When threads are created explicitly by `thread.new-indirect`, the lifetime of
 the thread-local storage array ends when the function passed to
 `thread.new-indirect` returns and thus any linear-memory allocations associated

design/mvp/Explainer.md

@@ -1564,6 +1564,13 @@ The `context.get` built-in returns the `i`th element of the [current thread]'s
 than 2 and `T` to be `i32` or `i64`, but these restrictions may be relaxed in
 the future.
 
+Mixing `i32` and `i64` results in truncating or unsigned extending the
+stored values:
+* If `context.get i32 i` is called after `context.set i64 i v`,
+  only the low 32-bits are read (returning `i32.wrap_i64 v`).
+* If `context.get i64 i` is called after `context.set i32 i v`,
+  only the upper 32-bits will be zeroed (returning `i64.extend_i32_u v`).
+
 For details, see [Thread-Local Storage] in the concurrency explainer and
 [`canon_context_get`] in the Canonical ABI explainer.
 
@@ -1579,6 +1586,13 @@ The `context.set` built-in sets the `i`th element of the [current thread]'s
 `i` to be less than 2 and `T` to be `i32` or `i64`, but these restrictions may
 be relaxed in the future.
 
+Mixing `i32` and `i64` results in truncating or unsigned extending the
+stored values:
+* If `context.get i32 i` is called after `context.set i64 i v`,
+  only the low 32-bits are read (returning `i32.wrap_i64 v`).
+* If `context.get i64 i` is called after `context.set i32 i v`,
+  only the upper 32-bits will be zeroed (returning `i64.extend_i32_u v`).
+
 For details, see [Thread-Local Storage] in the concurrency explainer and
 [`canon_context_set`] in the Canonical ABI explainer.
 

design/mvp/canonical-abi/definitions.py

@@ -2195,9 +2195,14 @@ def canon_resource_rep(rt, thread, i):
 ### 🔀 `canon context.get`
 
 def canon_context_get(t, i, thread):
+  MASK_32BIT = 1 << 32 - 1
+
   assert(t == 'i32' or t == 'i64')
   assert(i < Thread.CONTEXT_LENGTH)
-  return [thread.context[i]]
+  result = thread.context[i]
+  if t == 'i32':
+    result &= MASK_32BIT
+  return [result]
 
 ### 🔀 `canon context.set`