WASM: Add intrinsic implementation for ByReference<T> ctor and fix the Value getter (#5987)

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

@@ -1562,6 +1562,23 @@ namespace Internal.IL
                         return true;
                     }
                     break;
+                case ".ctor":
+                    if (metadataType.IsByReferenceOfT)
+                    {
+                        StackEntry byRefValueParamHolder = _stack.Pop();
+
+                        // Allocate a slot on the shadow stack for the ByReference type
+                        int spillIndex = _spilledExpressions.Count;
+                        SpilledExpressionEntry spillEntry = new SpilledExpressionEntry(StackValueKind.ByRef, "byref" + _currentOffset, metadataType, spillIndex, this);
+                        _spilledExpressions.Add(spillEntry);
+                        LLVMValueRef addrOfValueType = LoadVarAddress(spillIndex, LocalVarKind.Temp, out TypeDesc unused);
+                        var typedAddress = CastIfNecessary(_builder, addrOfValueType, LLVM.PointerType(LLVM.Int32Type(), 0));
+                        LLVM.BuildStore(_builder, byRefValueParamHolder.ValueForStackKind(StackValueKind.ByRef, _builder, false), typedAddress);
+
+                        _stack.Push(spillEntry);
+                        return true;
+                    }
+                    break;
             }
 
             return false;

tests/src/Simple/HelloWasm/Program.cs

@@ -294,6 +294,22 @@ internal static class Program
             PrintLine("float comparison: Ok.");
         }
 
+        // Create a ByReference<char> through the ReadOnlySpan ctor and call the ByReference.Value via the indexer.
+        var span = "123".AsSpan();
+        if (span[0] != '1'
+            || span[1] != '2'
+            || span[2] != '3')
+        {
+            PrintLine("ByReference intrinsics exercise via ReadOnlySpan failed");
+            PrintLine(span[0].ToString());
+            PrintLine(span[1].ToString());
+            PrintLine(span[2].ToString());
+        }
+        else
+        {
+            PrintLine("ByReference intrinsics exercise via ReadOnlySpan OK.");
+        }
+
         // This test should remain last to get other results before stopping the debugger
         PrintLine("Debugger.Break() test: Ok if debugger is open and breaks.");
         System.Diagnostics.Debugger.Break();