[EH] Make abort() work with EH

We used to implement `abort()` with throwing a JS exception. emscripten-core#9730
changed it to `RuntimeError`, because it is the class a trap becomes
once it hits a JS frame.

But this turned out to be not working, because Wasm EH currently does
not assume all `RuntimeError`s are from traps; it decides whether a
`RuntimeError` is from a trap or not based on a hidden field within the
object. Relevant past discussion:
WebAssembly/exception-handling#89 (comment)

So at the moment we don't have a way of throwing a trap from JS, which
is inconvenient. I think we eventually want to make JS API for this, but
for the moment, this PR resorts to a wasm function that traps. This at
least makes calling `std::terminate` work. So far calling it exhausted
the call stack and aborted.

Fixes emscripten-core#16407.

emcc.py

@@ -2566,6 +2566,9 @@ def get_full_import_name(name):
     if settings.SUPPORT_LONGJMP == 'wasm':
       settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.append('__c_longjmp')
 
+  if settings.EXCEPTION_HANDLING:
+    settings.REQUIRED_EXPORTS += ['__trap']
+
   return target, wasm_target
 
 

src/preamble.js

@@ -605,12 +605,20 @@ function abort(what) {
   // Use a wasm runtime error, because a JS error might be seen as a foreign
   // exception, which means we'd run destructors on it. We need the error to
   // simply make the program stop.
+  // FIXME This is not working now, because Wasm EH currently does not assume
+  // all RuntimeErrors are from traps; it decides whether a RuntimeError is from
+  // a trap or not based on a hidden field within the object. So at the moment
+  // we don't have a way of throwing a trap from JS. TODO Make a JS API that
+  // allows this.
 
   // Suppress closure compiler warning here. Closure compiler's builtin extern
   // defintion for WebAssembly.RuntimeError claims it takes no arguments even
   // though it can.
   // TODO(https://github.com/google/closure-compiler/pull/3913): Remove if/when upstream closure gets fixed.
-
+#if EXCEPTION_HANDLING == 1
+  // In the meantime, we resort to wasm code for trapping.
+  ___trap();
+#else
   /** @suppress {checkTypes} */
   var e = new WebAssembly.RuntimeError(what);
 
@@ -621,6 +629,7 @@ function abort(what) {
   // in code paths apart from instantiation where an exception is expected
   // to be thrown when abort is called.
   throw e;
+#endif
 }
 
 // {{MEM_INITIALIZER}}

system/lib/compiler-rt/exception_builtins.S

@@ -0,0 +1,6 @@
+.globl __trap
+
+__trap:
+  .functype __trap() -> ()
+  unreachable
+  end_function

tests/test_core.py

@@ -1639,6 +1639,20 @@ class Polymorphic {virtual void member(){}};
 }
 ''', 'exception caught: std::bad_typeid')
 
+  @with_both_eh_sjlj
+  def test_terminate_abort(self):
+    # std::terminate eventually calls abort(). We used ti implement abort() with
+    # throwing a JS exception, but this can be again caught by std::terminate's
+    # cleanup and cause an infinite loop. When Wasm EH is enabled, abort() is
+    # implemented by a trap now.
+    err = self.do_run(r'''
+#include <exception>
+int main() {
+  std::terminate();
+}
+''', assert_returncode=NON_ZERO)
+    self.assertNotContained('Maximum call stack size exceeded', err)
+
   def test_iostream_ctors(self):
     # iostream stuff must be globally constructed before user global
     # constructors, so iostream works in global constructors

tools/system_libs.py

@@ -708,7 +708,8 @@ class libcompiler_rt(MTLibrary, SjLjLibrary):
         'stack_ops.S',
         'stack_limits.S',
         'emscripten_setjmp.c',
-        'emscripten_exception_builtins.c'
+        'emscripten_exception_builtins.c',
+        'exception_builtins.S'
       ])