Merge pull request #31 from WebAssembly/update-function-pointers

Move and merge 'Function pointers' section from V1.md into AstSematics.md

2 files changed, 27 insertions, 25 deletions

diff --git a/AstSemantics.md b/AstSemantics.md
index f6e28ee..dc54bc0 100644
--- a/AstSemantics.md
+++ b/AstSemantics.md
@@ -102,24 +102,37 @@ all global accesses can be considered "non-atomic".
 
 ## Calls
 
-Calls to functions can be direction (by index), or indirect. Each function
-has a signature in terms of local types, and calls must match the function
-signature exactly. Multiple return value calls will be possible.
-
-The details of indirect calls need clarification.
+Direct calls to a function specify the callee by index into a function table.
 
   * CallDirect - call function directly
-  * CallIndirect - call function indirectly
 
-The details of multiple-return-value calls needs clarification.
-Calling a function that returns multiple values will likely have to be a
-statement that specifies multiple local variables to which to assign the
-corresponding return values.
+Each function has a signature in terms of local types, and calls must match the
+function signature exactly. [Imported functions](V1.md#code-loading-and-imports) 
+also have signatures and are added to the same function table and are thus also
+callable via `CallDirect`.
 
-Calls to external (stdlib) functions can be expressed as direct calls to
-an "external" function in the function table. The linking of that function
-will be specified through the module system. A call to an unlinked function
-should be a runtime error.
+Indirect calls may be made to a value of function-pointer type. A function-
+pointer value may be obtained for a given function as specified by its index
+in the function table.
+
+  * CallIndirect - call function indirectly
+  * AddressOf - obtain a function pointer value for a given function
+
+Function-pointer values are comparable for equality and the `AddressOf` operator
+is monomorphic. Function-pointer values can be explicitly coerced to and from
+integers (which, in particular, is necessary when loading/storing to the heap
+since the heap only provides integer types). For security and safety reasons,
+the integer value of a coerced function-pointer value is an abstract index and
+does not reveal the actual machine code address of the target function.
+
+In v.1 function pointer values are
+local to a single module. The [dynamic linking](FutureFeatures.md#dynamic-linking)
+feature is necessary for two modules to pass function pointers back and forth.
+
+Multiple return value calls will be possible, though possibly not in v.1. The
+details of multiple-return-value calls needs clarification. Calling a function
+that returns multiple values will likely have to be a statement that specifies
+multiple local variables to which to assign the corresponding return values.
 
 ## Literals
 
diff --git a/V1.md b/V1.md
index 265ab86..7e28143 100644
--- a/V1.md
+++ b/V1.md
@@ -110,17 +110,6 @@ precise descriptions of:
      but when aliased by JS, it appears as a regular ArrayBuffer.
    * To keep an ArrayBuffer's length immutable, resizing a module's heap detaches any existant ArrayBuffers.
  
-## Function pointers
- * In v.1, function pointers are local to a single module.
-   * The [dynamic linking](FutureFeatures.md#dynamic-linking) feature will be necessary for two modules
-     to pass function pointers back and forth.
- * Function pointers have a unique type per signature that is coercible to and from an int32.
-   * The heap can only load/store integer types, so a coercion is required when loading/storing
-     function pointers to the heap.
- * Values of function pointer types are comparable for equality and callable.
- * Function pointer values are created via special `AddressOf` ops that take a function's index
-   and return a function pointer value unique to the function index.
- 
 ## Non-browser embedding
  * Host environments can define builtin modules that are implemented natively and thus be imported 
    directly by WebAssembly modules.