> If you could also share the two preprocessed versions of thread.c would
 > be of great help.  You should be able to do it adding to your original
 > GCC invokation like "-E -o thread.i".

What I ended up doing was:

cd src
make thread.o -W thread.c CFLAGS='-g3 -O2 -gdwarf-2 
-Wno-error=implicit-function-declaration -E -o thread.i'

I hope this is what you were after. The addition of
'-Wno-error=implicit-function-declaration' comes from issue #70889
btw. I've also edited the "E:/Git/emacs-debug-gcc14/emacs" paths in
the thread.i files by hand so they don't show up in the diff; they
were originally compiled in two separate folders. The resulting files
are almost exactly the same:


diff -ubBw thread.i thread-attr-no-optimize-sibling-calls.i
--- thread.i    2024-05-15 20:34:08 +0000
+++ thread-attr-no-optimize-sibling-calls.i     2024-05-15 20:33:44 +0000
@@ -152355,12 +152355,13 @@

  }

+__attribute__((optimize("no-optimize-sibling-calls")))
  void
  mark_threads (void)
  {
    flush_stack_call_func (mark_threads_callback,
-# 1181 "thread.c" 3 4
+# 1182 "thread.c" 3 4
                                                 ((void *)0)
-# 1181 "thread.c"
+# 1182 "thread.c"
                                                     );
  }