[mono] Fix invoking string ctors on wasm in AOT mode. (#52505)

* [mono] Avoid passing a dummy string to string ctors from runtime invokes, pass NULL instead.

The call will go to a wrapper method which will ignore the 'this' argument anyway.

* [mono] Fix invoking string ctors on wasm in AOT mode.

Author: vargaz

src/mono/mono/metadata/marshal-ilgen.c

@@ -1507,43 +1507,21 @@ emit_invoke_call (MonoMethodBuilder *mb, MonoMethod *method,
 				  int loc_res,
 				  gboolean virtual_, gboolean need_direct_wrapper)
 {
-	static MonoString *string_dummy = NULL;
 	int i;
 	int *tmp_nullable_locals;
 	gboolean void_ret = FALSE;
 	gboolean string_ctor = method && method->string_ctor;
 
-	/* to make it work with our special string constructors */
-	if (!string_dummy) {
-		ERROR_DECL (error);
-
-		// FIXME Allow for static construction of MonoString.
-
-		SETUP_ICALL_FUNCTION;
-		SETUP_ICALL_FRAME;
-
-		MONO_GC_REGISTER_ROOT_SINGLE (string_dummy, MONO_ROOT_SOURCE_MARSHAL, NULL, "Marshal Dummy String");
-
-		MonoStringHandle string_dummy_handle = mono_string_new_utf8_len ("dummy", 5, error);
-		string_dummy = MONO_HANDLE_RAW (string_dummy_handle);
-		mono_error_assert_ok (error);
-
-		CLEAR_ICALL_FRAME;
-	}
-
 	if (virtual_) {
 		g_assert (sig->hasthis);
 		g_assert (method->flags & METHOD_ATTRIBUTE_VIRTUAL);
 	}
 
 	if (sig->hasthis) {
 		if (string_ctor) {
-			if (mono_gc_is_moving ()) {
-				mono_mb_emit_ptr (mb, &string_dummy);
-				mono_mb_emit_byte (mb, CEE_LDIND_REF);
-			} else {
-				mono_mb_emit_ptr (mb, string_dummy);
-			}
+			/* This will call the code emitted by mono_marshal_get_native_wrapper () which ignores it */
+			mono_mb_emit_icon (mb, 0);
+			mono_mb_emit_byte (mb, CEE_CONV_I);
 		} else {
 			mono_mb_emit_ldarg (mb, 0);
 		}

src/mono/mono/mini/aot-compiler.c

@@ -4731,6 +4731,20 @@ add_wrappers (MonoAotCompile *acfg)
 		/* FIXME: locking - this is "safe" as full-AOT threads don't mutate the icall data */
 		for (int i = 0; i < MONO_JIT_ICALL_count; ++i)
 			add_jit_icall_wrapper (acfg, mono_find_jit_icall_info ((MonoJitICallId)i));
+
+		if (acfg->aot_opts.llvm_only) {
+			/* String ctors are called directly on llvmonly */
+			for (int i = 0; i < rows; ++i) {
+				ERROR_DECL (error);
+
+				token = MONO_TOKEN_METHOD_DEF | (i + 1);
+				method = mono_get_method_checked (acfg->image, token, NULL, NULL, error);
+				if (method && method->string_ctor) {
+					MonoMethod *w = get_runtime_invoke (acfg, method, FALSE);
+					add_method (acfg, w);
+				}
+			}
+		}
 	}
 
 	/* 

src/mono/mono/mini/mini-runtime.c

@@ -2921,10 +2921,7 @@ create_runtime_invoke_info (MonoMethod *method, gpointer compiled_method, gboole
 	info = g_new0 (RuntimeInvokeInfo, 1);
 	info->compiled_method = compiled_method;
 	info->use_interp = use_interp;
-	if (mono_llvm_only && method->string_ctor)
-		info->sig = mono_marshal_get_string_ctor_signature (method);
-	else
-		info->sig = mono_method_signature_internal (method);
+	info->sig = mono_method_signature_internal (method);
 
 	invoke = mono_marshal_get_runtime_invoke (method, FALSE);
 	(void)invoke;
@@ -3016,7 +3013,13 @@ create_runtime_invoke_info (MonoMethod *method, gpointer compiled_method, gboole
 	}
 
 	if (!info->dyn_call_info) {
-		if (mono_llvm_only) {
+		/*
+		 * Can't use the normal llvmonly code for string ctors since the gsharedvt out wrapper passes
+		 * an extra arg, which the string ctor methods don't have, which causes signature mismatches
+		 * on wasm. Instead, call string ctors normally using a direct runtime invoke wrapper
+		 * which is AOTed for each ctor.
+		 */
+		if (mono_llvm_only && !method->string_ctor) {
 #ifndef MONO_ARCH_GSHAREDVT_SUPPORTED
 			g_assert_not_reached ();
 #endif
@@ -3331,7 +3334,7 @@ mono_jit_runtime_invoke (MonoMethod *method, void *obj, void **params, MonoObjec
 	if (info->use_interp) {
 		result = mini_get_interp_callbacks ()->runtime_invoke (method, obj, params, exc, error);
 		return_val_if_nok (error, NULL);
-	} else if (mono_llvm_only) {
+	} else if (mono_llvm_only && !method->string_ctor) {
 		result = mono_llvmonly_runtime_invoke (method, info, obj, params, exc, error);
 		if (!is_ok (error))
 			return NULL;