Allow -fexceptions to work in standalone mode (emscripten-core#19237)

Fixes: emscripten-core#19237

Author: sbc100

embuilder.py

@@ -61,7 +61,7 @@
     'libsockets',
     'libstubs',
     'libstubs-debug',
-    'libstandalonewasm',
+    'libstandalonewasm-nocatch',
     'crt1',
     'crt1_proxy_main',
     'libunwind-except',

system/lib/standalone/standalone.c

@@ -157,19 +157,13 @@ double emscripten_get_now(void) {
 
 // C++ ABI
 
-// Emscripten disables exception catching by default, but not throwing. That
-// allows users to see a clear error if a throw happens, and 99% of the
-// overhead is in the catching, so this is a reasonable tradeoff.
-// For now, in a standalone build just terminate. TODO nice error message
-//
-// Define these symbols as weak so that when we build with exceptions
-// enabled (using wasm-eh) we get the real versions of these functions
-// as defined in libc++abi.
-
-__attribute__((__weak__))
+#if EMSCRIPTEN_NOCATCH
+// When exception catching is disabled, we stub out calls to `__cxa_throw`.
+// Otherwise, `__cxa_throw` will be imported from the host.
 void __cxa_throw(void* ptr, void* type, void* destructor) {
   abort();
 }
+#endif
 
 // WasmFS integration. We stub out file preloading and such, that are not
 // expected to work anyhow.

test/test_other.py

@@ -10693,6 +10693,7 @@ def test_fignore_exceptions(self):
     'compile_only': (['-fexceptions'], [], False),
     # 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):
     create_file('src.cpp', r'''

tools/system_libs.py

@@ -1959,10 +1959,13 @@ class libstandalonewasm(MuslInternalLibrary):
 
   def __init__(self, **kwargs):
     self.is_mem_grow = kwargs.pop('is_mem_grow')
+    self.nocatch = kwargs.pop('nocatch')
     super().__init__(**kwargs)
 
   def get_base_name(self):
     name = super().get_base_name()
+    if self.nocatch:
+      name += '-nocatch'
     if self.is_mem_grow:
       name += '-memgrow'
     return name
@@ -1972,16 +1975,19 @@ def get_cflags(self):
     cflags += ['-DNDEBUG', '-DEMSCRIPTEN_STANDALONE_WASM']
     if self.is_mem_grow:
       cflags += ['-DEMSCRIPTEN_MEMORY_GROWTH']
+    if self.nocatch:
+      cflags.append('-DEMSCRIPTEN_NOCATCH')
     return cflags
 
   @classmethod
   def vary_on(cls):
-    return super().vary_on() + ['is_mem_grow']
+    return super().vary_on() + ['is_mem_grow', 'nocatch']
 
   @classmethod
   def get_default_variation(cls, **kwargs):
     return super().get_default_variation(
       is_mem_grow=settings.ALLOW_MEMORY_GROWTH,
+      nocatch=settings.DISABLE_EXCEPTION_CATCHING and not settings.WASM_EXCEPTIONS,
       **kwargs
     )