Auto-export setThrew for emscripten exceptions (#14153)

Normally if exceptions are enabled, we have exceptions thrown and caught
etc., and they will depend on setThrew. And then we will link it in. However,
if the code only contains invokes, but no exception catching, then there is
no library support that tells us to include it. To fix that, always include the
function when emscripten exceptions are enabled.

I don't think this can regress code size, as setThrew will almost certainly
be used anyhow, unless there are also no invokes at all. And in that case,
metadce can remove it.

Fixes #14078

Author: kripken

emcc.py

@@ -2168,10 +2168,18 @@ def check_memory_setting(setting):
     settings.EXPORTED_FUNCTIONS += ['_malloc', '_free']
 
   if not settings.DISABLE_EXCEPTION_CATCHING:
-    # If not for LTO builds, we could handle these by adding deps_info.py
-    # entries for __cxa_find_matching_catch_* functions.  However, under
-    # LTO these symbols don't exist prior the linking.
-    settings.EXPORTED_FUNCTIONS += ['___cxa_is_pointer_type', '___cxa_can_catch']
+    settings.EXPORTED_FUNCTIONS += [
+      # If not for LTO builds, we could handle these by adding deps_info.py
+      # entries for __cxa_find_matching_catch_* functions.  However, under
+      # LTO these symbols don't exist prior the linking.
+      '___cxa_is_pointer_type',
+      '___cxa_can_catch',
+
+      # Emscripten exception handling can generate invoke calls, and they call
+      # setThrew(). We cannot handle this using deps_info as the invokes are not
+      # emitted because of library function usage, but by codegen itself.
+      '_setThrew',
+    ]
 
   if settings.ASYNCIFY:
     if not settings.ASYNCIFY_IGNORE_INDIRECT:

tests/test_other.py

@@ -9242,6 +9242,30 @@ def test_f_exception(self, compile_flags, link_flags, expect_caught):
     result = self.run_js('a.out.js', assert_returncode=0 if expect_caught else NON_ZERO)
     self.assertContainedIf('CAUGHT', result, expect_caught)
 
+  def test_exceptions_with_closure_and_without_catching(self):
+    # using invokes will require setThrew(), and closure will error if it is not
+    # defined. this test checks that we define it even without catching any
+    # exceptions (if we did catch exceptions, that would include library code
+    # that would use setThrew() anyhow)
+    create_file('src.cpp', r'''
+      #include <stdio.h>
+      #include <emscripten.h>
+
+      struct A {
+        ~A() {
+          puts("~A");
+        }
+      };
+
+      int main() {
+        // Construct an instance of a class with a destructor, which will cause the
+        // use of invokes to ensure its destructor runs.
+        A a;
+        throw 5;
+      }
+      ''')
+    self.run_process([EMCC, 'src.cpp', '-fexceptions', '--closure=1'])
+
   def test_assertions_on_internal_api_changes(self):
     create_file('src.c', r'''
       #include <emscripten.h>