[EH] Don't export unnecessary functions for EH when not used (emscripten-core#26493)

When we enabled (Wasm / Emscripten) EH and didn't use it, we still
pulled in a lot of JS library functions + libc++abi library functions
that were not DCE'ed. The reason was we mandatorily exported many
functions whenever some EH options were enabled, regardless of whether
they were used or not.

1. We used to enable `EXPORT_EXCEPTION_HANDLING_HELPERS`, which exports
`getExceptionMessage` and `in/decrementExceptionRefcount`, whenever
`EXCEPTION_STACK_TRACES` was true. And `EXCEPTION_STACK_TRACES` is true
whenever `ASSERTIONS` is true, which is the default at `-O0`. And those
exported functions can pull in many libc++abi functions. As a result, at
`-O0`, we pulled in a lot of functions even when we were not using any
exceptions.

This PR removes that `EXCEPTION_STACK_TRACES` ->
`EXPORT_EXCEPTION_HANDLING_HELPERS` link. Without this link, we can
still see stack traces with exception messages with no problem, because
`__cxa_throw` -> `__throw_exception_with_stack_trace` ->
`getExceptionMessage` dependencies:
https://github.com/emscripten-core/emscripten/blob/6ad2f5e03021a39377428e9d476985fc967014d4/system/lib/libcxxabi/src/cxa_exception.cpp#L302-L304
https://github.com/emscripten-core/emscripten/blob/6ad2f5e03021a39377428e9d476985fc967014d4/src/lib/libexceptions.js#L311

2. If we do 1, Emscripten EH's `getExceptionMessage` does not work,
because unlike Wasm EH, its `getExceptionMessage` dependency is within
`runtime_exceptions.js`, where we can't attach `__deps`, so we can't
track it:
https://github.com/emscripten-core/emscripten/blob/6ad2f5e03021a39377428e9d476985fc967014d4/src/runtime_exceptions.js#L20
So, this adds `getExceptionMessage` as a dependency of
`libexception.js`'s `__cxa_throw` directly, when
`EXCEPTION_STACK_TRACES` is enabled.

3. This removes several functions from `REQUIRED_EXPORTS` when
Emscripten EH is used. Previously, the comment said, in LTO mode,
`_cxa_find_matching_catch_*` -> `__cxa_can_catch` dependency was not
tracked. But now, in Emscripten EH, we preemptively add several
`__cxa_find_matching_catch_n`s, and it depends on `findMatchingCatch`,
which depends on `__cxa_end_catch`:
https://github.com/emscripten-core/emscripten/blob/78403050f355085104175499224ad0f6bccb5fb1/src/lib/libexceptions.js#L371-L405
https://github.com/emscripten-core/emscripten/blob/78403050f355085104175499224ad0f6bccb5fb1/src/lib/libexceptions.js#L217
So we don't need to require exporting `__cxa_end_catch` anymore.

Also, other required exports (`__cxa_in/decrement_exception_count` and
`__cxa_free_exceptions`) are dependencies that can be naturally referred
within libc++abi. I think these were added here in emscripten-core#16627 when we were
using `deps_info.py`, which we don't use anymore.

After this commit, when you compile an empty `int main { return 0; }`
with `-O0` and `-fexceptions`/`-fwasm-exceptions`, the size decreases
to:
- `-fexceptions`: 113212 -> 1168
- `-fwasm-exceptions`: 109177 -> 1106

Author: aheejin

src/lib/libexceptions.js

@@ -85,6 +85,12 @@ var LibraryExceptions = {
   __cxa_throw__deps: ['$ExceptionInfo', '$exceptionLast', '$uncaughtExceptionCount',
 #if !DISABLE_EXCEPTION_CATCHING
     '__cxa_increment_exception_refcount',
+#endif
+#if EXCEPTION_STACK_TRACES
+    // When EXCEPTION_STACK_TRACES is enabled, storeException contains a call to
+    // 'new CppException', whose constructor calls getExceptionMessage. We can't
+    // track the dependency there, so we track it here.
+    '$getExceptionMessage',
 #endif
   ],
   __cxa_throw: (ptr, type, destructor) => {

test/codesize/test_codesize_cxx_except.json

@@ -1,10 +1,10 @@
 {
   "a.out.js": 22875,
   "a.out.js.gz": 8956,
-  "a.out.nodebug.wasm": 172540,
-  "a.out.nodebug.wasm.gz": 57452,
-  "total": 195415,
-  "total_gz": 66408,
+  "a.out.nodebug.wasm": 172516,
+  "a.out.nodebug.wasm.gz": 57438,
+  "total": 195391,
+  "total_gz": 66394,
   "sent": [
     "__cxa_begin_catch",
     "__cxa_end_catch",
@@ -87,7 +87,6 @@
   "exports": [
     "__cxa_can_catch",
     "__cxa_decrement_exception_refcount",
-    "__cxa_free_exception",
     "__cxa_get_exception_ptr",
     "__cxa_increment_exception_refcount",
     "__indirect_function_table",

test/codesize/test_codesize_cxx_mangle.json

@@ -1,10 +1,10 @@
 {
   "a.out.js": 22925,
   "a.out.js.gz": 8978,
-  "a.out.nodebug.wasm": 238981,
-  "a.out.nodebug.wasm.gz": 79855,
-  "total": 261906,
-  "total_gz": 88833,
+  "a.out.nodebug.wasm": 238957,
+  "a.out.nodebug.wasm.gz": 79847,
+  "total": 261882,
+  "total_gz": 88825,
   "sent": [
     "__cxa_begin_catch",
     "__cxa_end_catch",
@@ -88,7 +88,6 @@
     "__cxa_can_catch",
     "__cxa_decrement_exception_refcount",
     "__cxa_demangle",
-    "__cxa_free_exception",
     "__cxa_get_exception_ptr",
     "__cxa_increment_exception_refcount",
     "__indirect_function_table",

test/test_other.py

@@ -8692,6 +8692,20 @@ def test_multi_inheritance_exception_message(self):
     self.set_setting('ASSERTIONS')
     self.do_runf('src.cpp', 'ERROR\n')
 
+  @with_all_eh_sjlj
+  def test_unused_eh_dce(self):
+    # Programs that don't use exceptions should not pull in libc++abi or
+    # exception formatting helpers when compiled with -fexceptions /
+    # -fwasm-exceptions. We verify this by ensuring the output Wasm binary is
+    # small.
+    create_file('main.cpp', 'int main() { return 0; }')
+    self.do_runf('main.cpp', '', cflags=['-O0'])
+
+    self.assertExists('main.wasm')
+    size = os.path.getsize('main.wasm')
+    # 3KB is a very generous upper bound for an empty program without libc++abi
+    self.assertLess(size, 3000)
+
   @requires_node
   def test_jsrun(self):
     engine = self.get_current_js_engine()

tools/link.py

@@ -1703,27 +1703,11 @@ def limit_incoming_module_api():
     settings.REQUIRED_EXPORTS += ['realloc']
     options.post_js.append(utils.path_from_root('src/build_as_worker.js'))
 
+  # 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.
   if not settings.DISABLE_EXCEPTION_CATCHING:
-    settings.REQUIRED_EXPORTS += [
-      # For normal builds the entries in deps_info.py are enough to include
-      # these symbols whenever __cxa_find_matching_catch_* functions are
-      # found.  However, under LTO these symbols don't exist prior to linking
-      # so we include then unconditionally when exceptions are enabled.
-      '__cxa_can_catch',
-
-      # __cxa_begin_catch depends on this but we can't use deps info in this
-      # case because that only works for user-level code, and __cxa_begin_catch
-      # can be used by the standard library.
-      '__cxa_increment_exception_refcount',
-      # Same for __cxa_end_catch
-      '__cxa_decrement_exception_refcount',
-
-      # 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',
-      '__cxa_free_exception',
-    ]
+    settings.REQUIRED_EXPORTS += ['setThrew']
 
   if settings.ASYNCIFY:
     if not settings.ASYNCIFY_IGNORE_INDIRECT:
@@ -1812,8 +1796,6 @@ def get_full_import_name(name):
     # errors out.
     if settings.DISABLE_EXCEPTION_CATCHING and not settings.WASM_EXCEPTIONS:
       exit_with_error('EXCEPTION_STACK_TRACES requires either of -fexceptions or -fwasm-exceptions')
-    # EXCEPTION_STACK_TRACES implies EXPORT_EXCEPTION_HANDLING_HELPERS
-    settings.EXPORT_EXCEPTION_HANDLING_HELPERS = True
 
   # Make `getExceptionMessage` and other necessary functions available for use.
   if settings.EXPORT_EXCEPTION_HANDLING_HELPERS: