This introduces some new functions to handle the specpdl. The basic

idea is that when a thread loses the interpreter lock, it will unbind
the bindings it has put in place.  Then when a thread acquires the
lock, it will restore its bindings.

This code reuses an existing empty slot in struct specbinding to store
the current value when the thread is "swapped out".

This approach performs worse than my previously planned approach.
However, it was one I could implement with minimal time and
brainpower.  I hope that perhaps someone else could improve the code
once it is in.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/thread.h b/src/thread.h
index 020346b9af2..def05fdaec9 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -83,6 +83,12 @@ struct thread_state
   struct specbinding *m_specpdl_ptr;
 #define specpdl_ptr (current_thread->m_specpdl_ptr)
 
+  /* Pointer to the first "saved" element in specpdl.  When this
+     thread is swapped out, the current values of all specpdl bindings
+     are pushed onto the specpdl; then these are popped again when
+     switching back to this thread.  */
+  struct specbinding *m_saved_specpdl_ptr;
+
   /* Depth in Lisp evaluations and function calls.  */
   EMACS_INT m_lisp_eval_depth;
 #define lisp_eval_depth (current_thread->m_lisp_eval_depth)