[WASM] Implements catch for the wasm low level exception mechanism.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index bd607e2f3712c03a7f071a884fd5c355684c4912..b5e9988306ae46ef92b4bea95efeef08ca9c0276 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -1144,9 +1144,11 @@ Object* Isolate::UnwindAndFindHandler() {
   Address handler_sp = nullptr;
   Address handler_fp = nullptr;
 
-  // Special handling of termination exceptions, uncatchable by JavaScript code,
-  // we unwind the handlers until the top ENTRY handler is found.
+  // Special handling of termination exceptions, uncatchable by JavaScript and
+  // Wasm code, we unwind the handlers until the top ENTRY handler is found.
   bool catchable_by_js = is_catchable_by_javascript(exception);
+  bool catchable_by_wasm =
+      FLAG_wasm_eh_prototype && is_catchable_by_wasm(exception);
 
   // Compute handler and stack unwinding information by performing a full walk
   // over the stack and dispatching according to the frame type.
@@ -1167,6 +1169,26 @@ Object* Isolate::UnwindAndFindHandler() {
       break;
     }
 
+    if (frame->is_wasm() && catchable_by_wasm) {

[titzer, 2016/09/28 12:53:31]

Maybe inline this condition? Since frame->is_wasm() is probably very often
false, that avoids evaluating the other conditions in the common case.

[John, 2016/09/28 13:37:18]

I am cargo-culting on the catchable_by_js. I also find it more clear to read

catchable_by_wasm

than reading an expression in an if condition. I'll leave as is, but I will
change if you feel strongly about it.

+      int stack_slots = 0;  // Will contain stack slot count of frame.
+      WasmFrame* wasm_frame = static_cast<WasmFrame*>(frame);
+      offset = wasm_frame->LookupExceptionHandlerInTable(&stack_slots);
+      if (offset >= 0) {
+        // Compute the stack pointer from the frame pointer. This ensures that
+        // argument slots on the stack are dropped as returning would.
+        Address return_sp = frame->fp() +
+                            StandardFrameConstants::kFixedFrameSizeAboveFp -
+                            stack_slots * kPointerSize;
+
+        // Gather information from the frame.
+        code = frame->LookupCode();
+
+        handler_sp = return_sp;
+        handler_fp = frame->fp();
+        break;
+      }
+    }
+
     // For optimized frames we perform a lookup in the handler table.
     if (frame->is_optimized() && catchable_by_js) {
       OptimizedFrame* js_frame = static_cast<OptimizedFrame*>(frame);