Add wasm exception handling support (#10577)

- Adds a new interal setting `EXCEPTION_HANDLING` for the new wasm
  exception handling, and a new option `-fwasm-exceptions`. Setting
  `-fwasm-exceptions` sets `EXCEPTION_HANDLING` setting.
- `NoExceptLibrary` in system_libs.py now support three modes:
  none, emscripten, and wasm. Libraries are built separately for each of
  these modes according to options.
- Adds libcxxabi support. We share a lot of existing code with SjLj EH
  because our LSDA table structure is similar to that of SjLj EH, but
  there are some differenct parts. One important deviation of the wasm
  EH from other schemes is wasm EH does not have two-phase unwinding.
  Itanium-style two-phase unwinding typically consists of two phases:
  search and cleanup. Wasm EH only does one phase unwinding that does
  both search and cleanup together.
- This patch replaces `throw()` with `_THROW` in libcxxabi, which uses
  `noexcept` keyword instead, because wasm EH's clang frontend does not
  yet support exception specifications, such as `throw()`.
- Adds wasm libunwind implementation. Unlike other targets, we don't do
  the actual stack unwinding in libunwind. Instead, it contains
  target-specific method hooks that are used by libcxxabi. For example,
  `_Unwind_RaiseException` in libunwind is called by `__cxa_throw` in
  libcxxabi, whose call is generated when you use `throw` keyword in
  C++.
- Adds `@with_both_exception_handling` decorator in test_core.py, which
  runs the same test on both emscripten and the new wasm EH. Currently
  many tests still fail on the wasm EH, so only passing tests are using
  this decorator and failing ones are marked as TODOs. The reason for
  most of the failures is the lack of support for exception
  specification (such as `throw()`), as noted above.

Detailed info on toolchain convention of wasm EH handling is in
https://github.com/WebAssembly/tool-conventions/blob/master/EHScheme.md.

Author: aheejin

embuilder.py

@@ -37,8 +37,10 @@
     'libcompiler_rt',
     'libc',
     'libc++abi',
+    'libc++abi-except',
     'libc++abi-noexcept',
     'libc++',
+    'libc++-except',
     'libc++-noexcept',
     'libal',
     'libdlmalloc',
@@ -51,6 +53,7 @@
     'libc-wasm',
     'libstandalonewasm',
     'crt1',
+    'libunwind-except'
 ]
 
 USER_TASKS = [

emcc.py

@@ -1432,7 +1432,7 @@ def is_supported_link_flag(f):
 
     # if exception catching is disabled, we can prevent that code from being
     # generated in the frontend
-    if shared.Settings.DISABLE_EXCEPTION_CATCHING == 1 and shared.Settings.WASM_BACKEND:
+    if shared.Settings.DISABLE_EXCEPTION_CATCHING == 1 and shared.Settings.WASM_BACKEND and not shared.Settings.EXCEPTION_HANDLING:
       newargs.append('-fignore-exceptions')
 
     if shared.Settings.DEAD_FUNCTIONS:
@@ -2729,6 +2729,8 @@ def parse_args(newargs):
   options = EmccOptions()
   settings_changes = []
   should_exit = False
+  eh_enabled = False
+  wasm_eh_enabled = False
 
   def check_bad_eq(arg):
     if '=' in arg:
@@ -2980,13 +2982,15 @@ def check_bad_eq(arg):
       settings_changes.append('PTHREADS_PROFILING=1')
       newargs[i] = ''
     elif newargs[i] == '-fno-exceptions':
-      settings_changes.append('DISABLE_EXCEPTION_CATCHING=1')
-      settings_changes.append('DISABLE_EXCEPTION_THROWING=1')
+      shared.Settings.DISABLE_EXCEPTION_CATCHING = 1
+      shared.Settings.DISABLE_EXCEPTION_THROWING = 1
+      shared.Settings.EXCEPTION_HANDLING = 0
     elif newargs[i] == '-fexceptions':
-      settings_changes.append('DISABLE_EXCEPTION_CATCHING=0')
-      settings_changes.append('DISABLE_EXCEPTION_THROWING=0')
+      eh_enabled = True
+    elif newargs[i] == '-fwasm-exceptions':
+      wasm_eh_enabled = True
     elif newargs[i] == '-fignore-exceptions':
-      settings_changes.append('DISABLE_EXCEPTION_CATCHING=1')
+      shared.Settings.DISABLE_EXCEPTION_CATCHING = 1
     elif newargs[i] == '--default-obj-ext':
       newargs[i] = ''
       options.default_object_extension = newargs[i + 1]
@@ -3028,6 +3032,18 @@ def check_bad_eq(arg):
     elif newargs[i] == '-fno-rtti':
       shared.Settings.USE_RTTI = 0
 
+    # TODO Currently -fexceptions only means Emscripten EH. Switch to wasm
+    # exception handling by default when -fexceptions is given when wasm
+    # exception handling becomes stable.
+    if wasm_eh_enabled:
+      shared.Settings.EXCEPTION_HANDLING = 1
+      shared.Settings.DISABLE_EXCEPTION_THROWING = 1
+      shared.Settings.DISABLE_EXCEPTION_CATCHING = 1
+    elif eh_enabled:
+      shared.Settings.EXCEPTION_HANDLING = 0
+      shared.Settings.DISABLE_EXCEPTION_THROWING = 0
+      shared.Settings.DISABLE_EXCEPTION_CATCHING = 0
+
   if should_exit:
     sys.exit(0)
 

src/settings.js

@@ -589,6 +589,10 @@ var ENVIRONMENT = '';
 //   * LZ4 files are read-only.
 var LZ4 = 0;
 
+// Emscripten exception handling options.
+// These options only pertain to Emscripten exception handling and do not
+// control the experimental native wasm exception handling option.
+
 // Disables generating code to actually catch exceptions. This disabling is on
 // by default as the overhead of exceptions is quite high in size and speed
 // currently (in the future, wasm should improve that). When exceptions are

src/settings_internal.js

@@ -180,3 +180,6 @@ var LTO = 0;
 // sections.
 // This has no effect if DWARF is not being emitted.
 var SEPARATE_DWARF = 0;
+
+// New WebAssembly exception handling (experimental)
+var EXCEPTION_HANDLING = 0;

system/lib/libcxxabi/include/cxxabi.h

@@ -32,13 +32,13 @@ class type_info; // forward declaration
 #endif
 }
 
-// XXX EMSCRIPTEN Wasm exception handling has not yet implemented support for
+// XXX EMSCRIPTEN: Wasm exception handling has not yet implemented support for
 // exception specification. This temporarily changes 'throw()` with 'noexcept'
 // to make wasm EH working in the interim.
 #ifdef __USING_WASM_EXCEPTIONS__
-#define NOTHROW noexcept
+#define _NOTHROW noexcept
 #else
-#define NOTHROW throw()
+#define _NOTHROW throw()
 #endif
 
 // runtime routines use C calling conventions, but are in __cxxabiv1 namespace
@@ -47,20 +47,24 @@ extern "C"  {
 
 // 2.4.2 Allocating the Exception Object
 extern _LIBCXXABI_FUNC_VIS void *
-__cxa_allocate_exception(size_t thrown_size) NOTHROW;
+__cxa_allocate_exception(size_t thrown_size) _NOTHROW;
 extern _LIBCXXABI_FUNC_VIS void
-__cxa_free_exception(void *thrown_exception) NOTHROW;
+__cxa_free_exception(void *thrown_exception) _NOTHROW;
 
 // 2.4.3 Throwing the Exception Object
 extern _LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void
 __cxa_throw(void *thrown_exception, std::type_info *tinfo,
+#ifdef __USING_WASM_EXCEPTIONS__
+            void *(*dest)(void *));
+#else
             void (*dest)(void *));
+#endif
 
 // 2.5.3 Exception Handlers
 extern _LIBCXXABI_FUNC_VIS void *
-__cxa_get_exception_ptr(void *exceptionObject) NOTHROW;
+__cxa_get_exception_ptr(void *exceptionObject) _NOTHROW;
 extern _LIBCXXABI_FUNC_VIS void *
-__cxa_begin_catch(void *exceptionObject) NOTHROW;
+__cxa_begin_catch(void *exceptionObject) _NOTHROW;
 extern _LIBCXXABI_FUNC_VIS void __cxa_end_catch();
 #if defined(_LIBCXXABI_ARM_EHABI)
 extern _LIBCXXABI_FUNC_VIS bool
@@ -155,17 +159,17 @@ extern _LIBCXXABI_FUNC_VIS char *__cxa_demangle(const char *mangled_name,
 
 // Apple additions to support C++ 0x exception_ptr class
 // These are primitives to wrap a smart pointer around an exception object
-extern _LIBCXXABI_FUNC_VIS void *__cxa_current_primary_exception() NOTHROW;
+extern _LIBCXXABI_FUNC_VIS void *__cxa_current_primary_exception() _NOTHROW;
 extern _LIBCXXABI_FUNC_VIS void
 __cxa_rethrow_primary_exception(void *primary_exception);
 extern _LIBCXXABI_FUNC_VIS void
-__cxa_increment_exception_refcount(void *primary_exception) NOTHROW;
+__cxa_increment_exception_refcount(void *primary_exception) _NOTHROW;
 extern _LIBCXXABI_FUNC_VIS void
-__cxa_decrement_exception_refcount(void *primary_exception) NOTHROW;
+__cxa_decrement_exception_refcount(void *primary_exception) _NOTHROW;
 
 // Apple extension to support std::uncaught_exception()
-extern _LIBCXXABI_FUNC_VIS bool __cxa_uncaught_exception() NOTHROW;
-extern _LIBCXXABI_FUNC_VIS unsigned int __cxa_uncaught_exceptions() NOTHROW;
+extern _LIBCXXABI_FUNC_VIS bool __cxa_uncaught_exception() _NOTHROW;
+extern _LIBCXXABI_FUNC_VIS unsigned int __cxa_uncaught_exceptions() _NOTHROW;
 
 #if defined(__linux__) || defined(__Fuchsia__)
 // Linux and Fuchsia TLS support. Not yet an official part of the Itanium ABI.

system/lib/libcxxabi/readme.txt

@@ -16,6 +16,6 @@ Local modifications are marked with the comment: 'XXX EMSCRIPTEN'
 1. Add __cxa_can_catch and __cxa_is_pointer_type to private_typeinfo.cpp.
 
 2. Duplicate __isOurExceptionClass in cxa_handlers.cpp since we don't compile
-   cxa_exception.cpp.
+   cxa_exception.cpp in Emscripten EH mode.
 
-3. Define and use NOTHROW macro in cxxabi.h
+3. Define and use _NOTHROW macro in cxxabi.h

system/lib/libcxxabi/src/cxa_exception.cpp

@@ -181,7 +181,7 @@ extern "C" {
 //  object. Zero-fill the object. If memory can't be allocated, call
 //  std::terminate. Return a pointer to the memory to be used for the
 //  user's exception object.
-void *__cxa_allocate_exception(size_t thrown_size) throw() {
+void *__cxa_allocate_exception(size_t thrown_size) _NOTHROW {
     size_t actual_size = cxa_exception_size_from_exception_thrown_size(thrown_size);
 
     // Allocate extra space before the __cxa_exception header to ensure the
@@ -199,7 +199,7 @@ void *__cxa_allocate_exception(size_t thrown_size) throw() {
 
 
 //  Free a __cxa_exception object allocated with __cxa_allocate_exception.
-void __cxa_free_exception(void *thrown_object) throw() {
+void __cxa_free_exception(void *thrown_object) _NOTHROW {
     // Compute the size of the padding before the header.
     size_t header_offset = get_cxa_exception_offset();
     char *raw_buffer =
@@ -255,7 +255,12 @@ will call terminate, assuming that there was no handler for the
 exception.
 */
 void
+#ifdef __USING_WASM_EXCEPTIONS__
+// In wasm, destructors return their argument
+__cxa_throw(void *thrown_object, std::type_info *tinfo, void *(*dest)(void *)) {
+#else
 __cxa_throw(void *thrown_object, std::type_info *tinfo, void (*dest)(void *)) {
+#endif
     __cxa_eh_globals *globals = __cxa_get_globals();
     __cxa_exception* exception_header = cxa_exception_from_thrown_object(thrown_object);
 
@@ -293,7 +298,7 @@ The adjusted pointer is computed by the personality routine during phase 1
 
   Requires:  exception is native
 */
-void *__cxa_get_exception_ptr(void *unwind_exception) throw() {
+void *__cxa_get_exception_ptr(void *unwind_exception) _NOTHROW {
 #if defined(_LIBCXXABI_ARM_EHABI)
     return reinterpret_cast<void*>(
         static_cast<_Unwind_Control_Block*>(unwind_exception)->barrier_cache.bitpattern[0]);
@@ -308,7 +313,7 @@ void *__cxa_get_exception_ptr(void *unwind_exception) throw() {
 The routine to be called before the cleanup.  This will save __cxa_exception in
 __cxa_eh_globals, so that __cxa_end_cleanup() can recover later.
 */
-bool __cxa_begin_cleanup(void *unwind_arg) throw() {
+bool __cxa_begin_cleanup(void *unwind_arg) _NOTHROW {
     _Unwind_Exception* unwind_exception = static_cast<_Unwind_Exception*>(unwind_arg);
     __cxa_eh_globals* globals = __cxa_get_globals();
     __cxa_exception* exception_header =
@@ -419,7 +424,7 @@ to terminate or unexpected during unwinding.
   _Unwind_Exception and return a pointer to that.
 */
 void*
-__cxa_begin_catch(void* unwind_arg) throw()
+__cxa_begin_catch(void* unwind_arg) _NOTHROW
 {
     _Unwind_Exception* unwind_exception = static_cast<_Unwind_Exception*>(unwind_arg);
     bool native_exception = __isOurExceptionClass(unwind_exception);
@@ -627,7 +632,7 @@ void __cxa_rethrow() {
     Requires:  If thrown_object is not NULL, it is a native exception.
 */
 void
-__cxa_increment_exception_refcount(void *thrown_object) throw() {
+__cxa_increment_exception_refcount(void *thrown_object) _NOTHROW {
     if (thrown_object != NULL )
     {
         __cxa_exception* exception_header = cxa_exception_from_thrown_object(thrown_object);
@@ -644,7 +649,7 @@ __cxa_increment_exception_refcount(void *thrown_object) throw() {
     Requires:  If thrown_object is not NULL, it is a native exception.
 */
 _LIBCXXABI_NO_CFI
-void __cxa_decrement_exception_refcount(void *thrown_object) throw() {
+void __cxa_decrement_exception_refcount(void *thrown_object) _NOTHROW {
     if (thrown_object != NULL )
     {
         __cxa_exception* exception_header = cxa_exception_from_thrown_object(thrown_object);
@@ -667,7 +672,7 @@ void __cxa_decrement_exception_refcount(void *thrown_object) throw() {
     been no exceptions thrown, ever, on this thread, we can return NULL without 
     the need to allocate the exception-handling globals.
 */
-void *__cxa_current_primary_exception() throw() {
+void *__cxa_current_primary_exception() _NOTHROW {
 //  get the current exception
     __cxa_eh_globals* globals = __cxa_get_globals_fast();
     if (NULL == globals)
@@ -739,10 +744,10 @@ __cxa_rethrow_primary_exception(void* thrown_object)
 }
 
 bool
-__cxa_uncaught_exception() throw() { return __cxa_uncaught_exceptions() != 0; }
+__cxa_uncaught_exception() _NOTHROW { return __cxa_uncaught_exceptions() != 0; }
 
 unsigned int
-__cxa_uncaught_exceptions() throw()
+__cxa_uncaught_exceptions() _NOTHROW
 {
     // This does not report foreign exceptions in flight
     __cxa_eh_globals* globals = __cxa_get_globals_fast();

system/lib/libcxxabi/src/cxa_exception.hpp

@@ -38,7 +38,12 @@ struct _LIBCXXABI_HIDDEN __cxa_exception {
 
     //  Manage the exception object itself.
     std::type_info *exceptionType;
+#ifdef __USING_WASM_EXCEPTIONS__
+    // In wasm, destructors return their argument
+    void *(*exceptionDestructor)(void *);
+#else
     void (*exceptionDestructor)(void *);
+#endif
     std::unexpected_handler unexpectedHandler;
     std::terminate_handler  terminateHandler;
 

system/lib/libcxxabi/src/cxa_handlers.cpp

@@ -21,10 +21,11 @@
 
 namespace __cxxabiv1 {
 
-// XXX EMSCRIPTEN: Copied from cxa_exception.cpp since we don't compile that file.
-//                 Note that in no-exceptions builds we include cxa_noexception
-//                 which provides stubs of those anyhow.
-#if defined(__EMSCRIPTEN__) && !defined(_LIBCXXABI_NO_EXCEPTIONS)
+#ifdef __USING_EMSCRIPTEN_EXCEPTIONS__
+// XXX EMSCRIPTEN: Copied from cxa_exception.cpp since we don't compile that
+// file in Emscripten EH mode. Note that in no-exceptions builds we include
+// cxa_noexception.cpp which provides stubs of those anyhow.
+
 //  Is it one of ours?
 uint64_t __getExceptionClass(const _Unwind_Exception* unwind_exception) {
 //	On x86 and some ARM unwinders, unwind_exception->exception_class is