[wasm] Patch trapping position into stack trace

src/js/messages.js

Index: src/js/messages.js
diff --git a/src/js/messages.js b/src/js/messages.js
index 2ff2dd052baaad0256d9a71d6a82c0d811548a66..be33ff759b79dd0945f529636d881958f14924bc 100644
--- a/src/js/messages.js
+++ b/src/js/messages.js
@@ -826,7 +826,10 @@ function GetStackFrames(raw_stack) {
     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)));
   }