interpreter: fix function start offset of metadata.code.branch_hint

The source information of functions consider them starting from the
first byte of their size, while code metadata sections consider the
start to be the first byte after the size, so we have to account for
that.

Co-authored-by: Andreas Rossberg <rossberg@mpi-sws.org>

Author: yuri91

interpreter/custom/handler_branch_hint.ml

@@ -65,6 +65,7 @@ let get_nth_inst locs idx =
   | Some (_, i) -> Some i
   | None -> None
 
+
 (* Decoding *)
 
 (* TODO: make Decode module reusable instead of duplicating code *)
@@ -144,21 +145,27 @@ let decode_hint locs foff s =
 
 let decode_func_hints locs foff = decode_vec (decode_hint locs foff)
 
-let decode_func m s =
+let get_func_start_off f bs =
+  let s = stream bs in
+  skip f.at.left.column s;
+  let _ = decode_u32 s in
+  pos s
+
+let decode_func m bs s =
   let fidx = decode_u32 s in
   let f = get_func m fidx in
-  let foff = Int32.of_int f.at.left.column in
+  let foff = Int32.of_int (get_func_start_off f bs) in
   let locs = flatten_instr_locs f.it.body in
   let hs = decode_func_hints locs foff s in
   (fidx, List.rev hs)
 
-let decode_funcs m s =
-  let fs = decode_vec (decode_func m) s in
+let decode_funcs m bs s =
+  let fs = decode_vec (decode_func m bs) s in
   IdxMap.add_seq (List.to_seq fs) IdxMap.empty
 
-let decode m _ custom =
+let decode m bs custom =
   let s = stream custom.it.content in
-  try { func_hints = decode_funcs m s } @@ custom.at
+  try { func_hints = decode_funcs m bs s } @@ custom.at
   with EOS -> decode_error (pos s) "unexpected end of name section"
 
 (* Encoding *)
@@ -200,22 +207,22 @@ let encode_hint locs foff buf h =
 
 let encode_func_hints buf locs foff = encode_vec buf (encode_hint locs foff)
 
-let encode_func m buf t =
+let encode_func m bs buf t =
   let fidx, hs = t in
   encode_u32 buf fidx;
   let f = get_func m fidx in
-  let foff = f.at.left.column in
+  let foff = get_func_start_off f bs in
   let locs = flatten_instr_locs f.it.body in
   encode_func_hints buf locs foff hs
 
-let encode_funcs buf m fhs =
-  encode_vec buf (encode_func m) (List.of_seq (IdxMap.to_seq fhs))
+let encode_funcs buf m bs fhs =
+  encode_vec buf (encode_func m bs) (List.of_seq (IdxMap.to_seq fhs))
 
 let encode m bs sec =
   let { func_hints } = sec.it in
   let m2 = Decode.decode "" bs in
   let buf = Buffer.create 200 in
-  encode_funcs buf m2 func_hints;
+  encode_funcs buf m2 bs func_hints;
   let content = Buffer.contents buf in
   {
     name = Utf8.decode "metadata.code.branch_hint";