Adjust element segment encoding to match the bulk-memory proposal (#53)

* Adjust element segment encoding to match the bulk-memory proposal * Update interpreter/binary/encode.ml Co-Authored-By: Andreas Rossberg <[email protected]> * Address comments
WebAssembly · Sep 11, 2019 · 3d87cfa · 3d87cfa
1 parent 1f0d158
commit 3d87cfa
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Showing 4 changed files with 40 additions and 28 deletions.
diff --git a/document/core/binary/modules.rst b/document/core/binary/modules.rst
@@ -321,7 +321,7 @@ It decodes into a vector of :ref:`element segments <syntax-elem>` that represent
    \production{element segment} & \Belem &::=&
      \hex{00}~~e{:}\Bexpr~~y^\ast{:}\Bvec(\Bfuncidx)
        &\Rightarrow& \{ \ETABLE~0, \EOFFSET~e, \EINIT~y^\ast \} \\ &&|&
-     \hex{02}~~x{:}\Btableidx~~e{:}\Bexpr~~y^\ast{:}\Bvec(\Bfuncidx)
+     \hex{02}~~x{:}\Btableidx~~e{:}\Bexpr~~\hex{00}~~y^\ast{:}\Bvec(\Bfuncidx)
        &\Rightarrow& \{ \ETABLE~x, \EOFFSET~e, \EINIT~y^\ast \} \\
    \end{array}
 

diff --git a/interpreter/binary/decode.ml b/interpreter/binary/decode.ml
@@ -550,6 +550,10 @@ let table s =
 let table_section s =
   section `TableSection (vec (at table)) [] s
 
+let elem_kind s =
+  match vs7 s with
+  | 0x00 -> FuncRefType
+  | _ -> error s (pos s - 1) "invalid elem kind"
 
 (* Memory section *)
 
@@ -621,21 +625,23 @@ let code_section s =
 
 (* Element section *)
 
-let segment dat s =
-  let pos = pos s in
-  let x = at vu32 s in
-  let index =
-    match x.Source.it with
-    | 0l -> x
-    | 2l -> at var s
-    | _ -> error s pos "invalid segment kind"
-  in
-  let offset = const s in
-  let init = dat s in
-  {index; offset; init}
+let segment dat kind s =
+  match u8 s with
+  | 0 ->
+    let index = Source.(0l @@ Source.no_region) in
+    let offset = const s in
+    let init = dat s in
+    {index; offset; init}
+  | 2 ->
+    let index = at var s in
+    let offset = const s in
+    let _ = kind s in
+    let init = dat s in
+    {index; offset; init}
+  | _ -> error s (pos s - 1) "invalid segment kind"
 
 let table_segment s =
-  segment (vec (at var)) s
+  segment (vec (at var)) elem_kind s
 
 let elem_section s =
   section `ElemSection (vec (at table_segment)) [] s
@@ -644,7 +650,7 @@ let elem_section s =
 (* Data section *)
 
 let memory_segment s =
-  segment string s
+  segment string (fun s -> ()) s
 
 let data_section s =
   section `DataSection (vec (at memory_segment)) [] s

diff --git a/interpreter/binary/encode.ml b/interpreter/binary/encode.ml
@@ -490,13 +490,11 @@ let encode m =
 
     (* Element section *)
     let segment dat seg =
-      let {index; offset; init} = seg.it in
-      if index.it = 0l then
-        u8 0x00
-      else begin
-        u8 0x02; var index
-      end;
-      const offset; dat init
+      match seg.it with
+      | {index; offset; init} when index.it = 0l ->
+        u8 0x00; const offset; dat init
+      | {index; offset; init} ->
+        u8 0x02; var index; const offset; u8 0x00; dat init
 
     let table_segment seg =
       segment (vec var) seg

diff --git a/test/core/binary.wast b/test/core/binary.wast
@@ -114,26 +114,34 @@
   "\0b"                                ;; end
 )
 
-;; Data segment memory index can have non-minimal length
 (module binary
   "\00asm" "\01\00\00\00"
   "\05\03\01"                          ;; Memory section with 1 entry
   "\00\00"                             ;; no max, minimum 0
-  "\0b\07\01"                          ;; Data section with 1 entry
-  "\80\00"                             ;; Memory index 0, encoded with 2 bytes
+  "\0b\06\01"                          ;; Data section with 1 entry
+  "\00"                                ;; Memory index 0
   "\41\00\0b\00"                       ;; (i32.const 0) with contents ""
 )
 
-;; Element segment table index can have non-minimal length
 (module binary
   "\00asm" "\01\00\00\00"
   "\04\04\01"                          ;; Table section with 1 entry
   "\70\00\00"                          ;; no max, minimum 0, funcref
-  "\09\07\01"                          ;; Element section with 1 entry
-  "\80\00"                             ;; Table index 0, encoded with 2 bytes
+  "\09\06\01"                          ;; Element section with 1 entry
+  "\00"                                ;; Table index 0
   "\41\00\0b\00"                       ;; (i32.const 0) with no elements
 )
 
+;; Element segment table index can have non-minimal length
+(module binary
+  "\00asm" "\01\00\00\00"
+  "\04\04\01"                          ;; Table section with 1 entry
+  "\70\00\00"                          ;; no max, minimum 0, funcref
+  "\09\09\01"                          ;; Element section with 1 entry
+  "\02\80\00"                          ;; Table index 0, encoded with 2 bytes
+  "\41\00\0b\00\00"                       ;; (i32.const 0) with no elements
+)
+
 ;; Unsigned LEB128 must not be overlong
 (assert_malformed
   (module binary