wasm add rethrow support

src/ILCompiler.WebAssembly/src/CodeGen/ILToWebAssemblyImporter.cs

@@ -4404,7 +4404,10 @@ private void ImportInitBlk()
 
         private void ImportRethrow()
         {
-            EmitTrapCall();
+            // rethrows can only occur from a catch handler in which case there should be an exception slot
+            Debug.Assert(_spilledExpressions.Count > 0 && _spilledExpressions[0].Name == "ExceptionSlot");
+
+            ThrowOrRethrow(_spilledExpressions[0]);
         }
 
         private void ImportSizeOf(int token)
@@ -4451,6 +4454,11 @@ private void ImportThrow()
         {
             var exceptionObject = _stack.Pop();
 
+            ThrowOrRethrow(exceptionObject);
+        }
+
+        void ThrowOrRethrow(StackEntry exceptionObject)
+        {
             if (RhpThrowEx.Handle.Equals(IntPtr.Zero))
             {
                 RhpThrowEx = Module.AddFunction("RhpThrowEx", LLVMTypeRef.CreateFunction(LLVMTypeRef.Void, new LLVMTypeRef[] { LLVMTypeRef.CreatePointer(LLVMTypeRef.Int8, 0) }, false));
@@ -5390,7 +5398,6 @@ private ObjectNode.ObjectData EncodeEHInfo()
 //            }
 
             builder.EmitCompressedUInt((uint)totalClauses);
-
             // Iterate backwards to emit the innermost first, but within a try region go forwards to get the first matching catch type
             int i = _exceptionRegions.Length - 1;
             while (i >= 0)

tests/src/Simple/HelloWasm/Program.cs

@@ -1637,6 +1637,8 @@ private static void TestTryCatch()
         TestFilterNested();
 
         TestCatchAndThrow();
+
+        TestRethrow();
     }
 
     private static void TestTryCatchNoException()
@@ -1849,6 +1851,32 @@ private static void TestFilterNested()
         EndTest(exceptionFlowSequence == @"In middle catchRunning outer filterIn outer catchRunning inner filterIn inner catch");
     }
 
+    private static void TestRethrow()
+    {
+        StartTest("Test rethrow");
+        int caught = 0;
+        try
+        {
+            try
+            {
+                throw new Exception("first");
+            }
+            catch
+            {
+                caught++;
+                throw;
+            }
+        }
+        catch(Exception e)
+        {
+            if (e.Message == "first")
+            {
+                caught++;
+            }
+        }
+        EndTest(caught == 2);
+    }
+
     private static void TestCatchAndThrow()
     {
         StartTest("Test catch and throw different exception");