[wasm] Patch trapping position into stack trace

src/js/messages.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // -------------------------------------------------------------------
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
@@ skipping 808 matching lines @@
 
 
 function GetStackFrames(raw_stack) {
   var frames = new InternalArray();
   var sloppy_frames = raw_stack[0];
   for (var i = 1; i < raw_stack.length; i += 4) {
     var recv = raw_stack[i];
     var fun = raw_stack[i + 1];
     var code = raw_stack[i + 2];
     var pc = raw_stack[i + 3];
-    var pos = %FunctionGetPositionForOffset(code, pc);
+    // For traps in wasm, the bytecode offset is passed as (-1 - offset).
+    // Otherwise, lookup the position from the pc.
+    var pos = IS_NUMBER(fun) && pc < 0 ? (-1 - pc) :

[Yang, 2016/05/11 08:22:18]

fun can only be a number for wasm, right? Why do we still need to encode the
offset as a negative number?

[Clemens Hammacher, 2016/05/12 07:49:39]

Because there are still two cases for wasm:
1) For "inner" stack frames, the position is determined by the
StackFrameIterator; it a positive number describing the program code offset.
That number needs to be translated via FunctionGetPositionForOffset to a byte
offset.
2) For top frames, the byte offset is patched in directly, there is no
translation needed.

I know that this is not nice; it will be cleaned up together with the whole
stack-frame-patching hack.

+      %FunctionGetPositionForOffset(code, pc);
     sloppy_frames--;
     frames.push(new CallSite(recv, fun, pos, (sloppy_frames < 0)));
   }
   return frames;
 }
 
 
 // Flag to prevent recursive call of Error.prepareStackTrace.
 var formatting_custom_stack_trace = false;
 
@@ skipping 202 matching lines @@
 utils.Export(function(to) {
   to.ErrorToString = ErrorToString;
   to.MakeError = MakeError;
   to.MakeRangeError = MakeRangeError;
   to.MakeSyntaxError = MakeSyntaxError;
   to.MakeTypeError = MakeTypeError;
   to.MakeURIError = MakeURIError;
 });
 
 });