Make JS exception catching code conditional. NFC (#22380)

.circleci/config.yml

@@ -543,6 +543,7 @@ jobs:
           test_targets: "
             lto2.test_dylink_syslibs_all
             lto2.test_float_builtins
+            lto2.test_emscripten_lazy_load_code_unconditional
             lto0.test_exceptions_allowed_uncaught
             lto0.test_longjmp_standalone_standalone
             lto0.test_embind_i64_val

src/library_exceptions.js

@@ -144,6 +144,7 @@ var LibraryExceptions = {
 
   llvm_eh_typeid_for: (type) => type,
 
+#if !DISABLE_EXCEPTION_CATCHING
   __cxa_begin_catch__deps: ['$exceptionCaught', '__cxa_increment_exception_refcount',
                             '$uncaughtExceptionCount'],
   __cxa_begin_catch: (ptr) => {
@@ -283,7 +284,17 @@ var LibraryExceptions = {
     {{{ makeThrow('exceptionLast') }}}
   },
 
-#endif
+  $incrementExceptionRefcount__deps: ['__cxa_increment_exception_refcount'],
+  $incrementExceptionRefcount: (ptr) => ___cxa_increment_exception_refcount(ptr),
+
+  $decrementExceptionRefcount__deps: ['__cxa_decrement_exception_refcount'],
+  $decrementExceptionRefcount: (ptr) => ___cxa_decrement_exception_refcount(ptr),
+
+  $getExceptionMessage__deps: ['$getExceptionMessageCommon'],
+  $getExceptionMessage: (ptr) => getExceptionMessageCommon(ptr),
+#endif // !DISABLE_EXCEPTION_CATCHING
+#endif // !WASM_EXCEPTIONS
+
 #if WASM_EXCEPTIONS || !DISABLE_EXCEPTION_CATCHING
   $getExceptionMessageCommon__deps: ['__get_exception_message', 'free', '$stackSave', '$stackRestore', '$stackAlloc'],
   $getExceptionMessageCommon: (ptr) => {
@@ -358,19 +369,23 @@ var LibraryExceptions = {
     var ptr = getCppExceptionThrownObjectFromWebAssemblyException(ex);
     return getExceptionMessageCommon(ptr);
   },
+#endif // WASM_EXCEPTIONS
+};
 
-#elif !DISABLE_EXCEPTION_CATCHING
-  $incrementExceptionRefcount__deps: ['__cxa_increment_exception_refcount'],
-  $incrementExceptionRefcount: (ptr) => ___cxa_increment_exception_refcount(ptr),
-
-  $decrementExceptionRefcount__deps: ['__cxa_decrement_exception_refcount'],
-  $decrementExceptionRefcount: (ptr) => ___cxa_decrement_exception_refcount(ptr),
-
-  $getExceptionMessage__deps: ['$getExceptionMessageCommon'],
-  $getExceptionMessage: (ptr) => getExceptionMessageCommon(ptr),
+function addStub(name) {
+  LibraryManager.library[name] = function() { abort(); };
+  LibraryManager.library[`${name}__sig`] = '';
+#if !INCLUDE_FULL_LIBRARY
+  LibraryManager.library[`${name}__deps`] = [function() {
+    error(`DISABLE_EXCEPTION_CATCHING was set, which means no C++ exception catching support code is linked in, but such support is required by symbol '${name}'. Either set DISABLE_EXCEPTION_CATCHING=0 (if you do want exception catching) or compile all source files with DISABLE_EXCEPTION_CATCHING=1.`);
+  }];
+#endif
+}
 
+#if DISABLE_EXCEPTION_CATCHING
+addStub('__cxa_begin_catch');
+addStub('__cxa_end_catch');
 #endif
-};
 
 #if !WASM_EXCEPTIONS
 // In LLVM, exceptions generate a set of functions of form
@@ -379,6 +394,9 @@ var LibraryExceptions = {
 // a single function '__cxa_find_matching_catch' that variadically processes all
 // of these functions using JS 'arguments' object.
 addCxaCatch = (n) => {
+#if DISABLE_EXCEPTION_CATCHING
+  addStub(`__cxa_find_matching_catch_${n}`);
+#else
   const args = [];
   // Confusingly, the actual number of asrgument is n - 2. According to the llvm
   // code in WebAssemblyLowerEmscriptenEHSjLj.cpp:
@@ -395,6 +413,7 @@ addCxaCatch = (n) => {
   LibraryManager.library[`__cxa_find_matching_catch_${n}__sig`] = sig;
   LibraryManager.library[`__cxa_find_matching_catch_${n}__deps`] = ['$findMatchingCatch'];
   LibraryManager.library[`__cxa_find_matching_catch_${n}`] = eval(`(${args}) => findMatchingCatch([${argString}])`);
+#endif
 };
 
 // Add the first 2-5 catch handlers preemptively.  Others get added on demand in

test/test_other.py

@@ -11577,12 +11577,12 @@ def test_fignore_exceptions(self):
     # enabling exceptions at link and compile works
     'on': (['-fexceptions'], ['-fexceptions'], True),
     # just compile isn't enough as the JS runtime lacks support
-    'compile_only': (['-fexceptions'], [], False),
+    'compile_only': (['-fexceptions'], [], False, True),
     # just link isn't enough as codegen didn't emit exceptions support
     'link_only': ([], ['-fexceptions'], False),
     'standalone': (['-fexceptions'], ['-fexceptions', '-sSTANDALONE_WASM', '-sWASM_BIGINT'], True),
   })
-  def test_f_exception(self, compile_flags, link_flags, expect_caught):
+  def test_f_exception(self, compile_flags, link_flags, expect_caught, expect_link_failure=False):
     create_file('src.cpp', r'''
       #include <stdio.h>
       int main () {
@@ -11595,6 +11595,11 @@ def test_f_exception(self, compile_flags, link_flags, expect_caught):
       }
     ''')
     self.run_process([EMXX, 'src.cpp', '-c', '-o', 'src.o'] + compile_flags)
+    if expect_link_failure:
+      err = self.expect_fail([EMXX, 'src.o'] + link_flags)
+      self.assertContained("error: DISABLE_EXCEPTION_CATCHING was set, which means no C++ exception catching support code is linked in, but such support is required by symbol '__cxa_begin_catch'", err)
+      return
+
     self.run_process([EMXX, 'src.o'] + link_flags)
     result = self.run_js('a.out.js', assert_returncode=0 if expect_caught else NON_ZERO)
     if not expect_caught:

tools/maint/gen_sig_info.py

@@ -295,6 +295,7 @@ def extract_sig_info(sig_info, extra_settings=None, extra_cflags=None, cxx=False
     # Enable as many settings as we can here to ensure the maximum number
     # of JS symbols are included.
     'STACK_OVERFLOW_CHECK': 1,
+    'DISABLE_EXCEPTION_CATCHING': 0,
     'USE_SDL': 1,
     'USE_GLFW': 0,
     'FETCH': 1,