Allow unused destructors to be completely elided (#20519)

Depends on llvm/llvm-project#68758

Fixes: #19993

Author: sbc100

test/other/test_unused_destructor.c

@@ -0,0 +1,12 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+__attribute((destructor)) void dtor() {
+  printf("hello from dtor");
+  abort();
+}
+
+int main() {
+  printf("in main\n");
+  return 0;
+}

test/test_other.py

@@ -14051,3 +14051,8 @@ def test_emmalloc_in_addition(self, args):
     emmalloc = path_from_root('system', 'lib', 'emmalloc.c')
     self.run_process([EMCC, test_file('other/test_emmalloc_in_addition.c'), emmalloc] + args)
     self.assertContained('success', self.run_js('a.out.js'))
+
+  def test_unused_destructor(self):
+    self.do_runf(test_file('other/test_unused_destructor.c'), emcc_args=['-flto', '-O2'])
+    # Verify that the string constant in the destructor is not included in the binary
+    self.assertNotIn(b'hello from dtor', read_binary('test_unused_destructor.wasm'))

tools/building.py

@@ -178,6 +178,15 @@ def lld_flags_for_executable(external_symbols):
   if settings.LINKABLE:
     cmd.append('--export-dynamic')
 
+  if settings.LTO and not settings.EXIT_RUNTIME:
+    # The WebAssembly backend can generate new references to `__cxa_atexit` at
+    # LTO time.  This `-u` flag forces the `__cxa_atexit` symbol to be
+    # included at LTO time.  For other such symbols we exclude them from LTO
+    # and always build them as normal object files, but that would inhibit the
+    # LowerGlobalDtors optimization which allows destructors to be completely
+    # removed when __cxa_atexit is a no-op.
+    cmd.append('-u__cxa_atexit')
+
   c_exports = [e for e in settings.EXPORTED_FUNCTIONS if is_c_symbol(e)]
   # Strip the leading underscores
   c_exports = [demangle_c_symbol_name(e) for e in c_exports]

tools/system_libs.py

@@ -890,9 +890,6 @@ class libcompiler_rt(MTLibrary, SjLjLibrary):
 
 class libnoexit(Library):
   name = 'libnoexit'
-  # __cxa_atexit calls can be generated during LTO the implemenation cannot
-  # itself be LTO.  See `get_libcall_files` below for more details.
-  force_object_files = True
   src_dir = 'system/lib/libc'
   src_files = ['atexit_dummy.c']