* __scm.h (SCM_ASYNC_TICK): Removed thread switching code.

(SCM_ALLOW_INTS): Added thread switching code before interrupts
get re-enabled.  The important effect of this is that interrupts
are blocked during thread switching so that thread data structures
don't risk getting messed up by an unfortunate signal.
(SCM_REDEFER_INTS, SCM_REALLOW_INTS): It turned out that gcc-2.8.0
seems to do more aggressive optimization which actually move
instructions around in these macros in a fatal way.  Therefore:
Introduce Anthony's SCM_FENCE macro!  (And I who thought he was
just superstitious...)
(SCM_TICK): Maybe do a context switch and take care of asyncs.
This macro should be used instead of SCM_ASYNC_TICK since the
latter doesn't do context switches any more.

libguile/__scm.h

@@ -252,23 +252,13 @@ typedef long SCM_STACKITEM;
 #endif
 
 extern unsigned int scm_async_clock;
-#if 0
+
 #define SCM_ASYNC_TICK \
 { \
   if (0 == --scm_async_clock) \
     scm_async_click (); \
 } \
 
-#else
-#define SCM_ASYNC_TICK \
-{ \
-  if (0 == --scm_async_clock) \
-    scm_async_click (); \
-  SCM_THREAD_SWITCHING_CODE; \
-} \
-
-#endif
-
 #ifdef SCM_CAREFUL_INTS
 #define SCM_CHECK_NOT_DISABLED \
   if (scm_ints_disabled) \
@@ -326,6 +316,8 @@ extern unsigned int scm_async_clock;
 { \
   SCM_FENCE; \
   SCM_CHECK_NOT_ENABLED; \
+  SCM_THREAD_SWITCHING_CODE; \
+  SCM_FENCE; \
   scm_ints_disabled = 0; \
   SCM_FENCE; \
   SCM_THREAD_ALLOW; \
@@ -336,23 +328,34 @@ extern unsigned int scm_async_clock;
 
 #define SCM_REDEFER_INTS  \
 { \
+  SCM_FENCE; \
   SCM_THREAD_REDEFER; \
   ++scm_ints_disabled; \
+  SCM_FENCE; \
 } \
 
 
 #define SCM_REALLOW_INTS \
 { \
+  SCM_FENCE; \
   SCM_THREAD_REALLOW_1; \
+  SCM_THREAD_SWITCHING_CODE; \
+  SCM_FENCE; \
   --scm_ints_disabled; \
   if (!scm_ints_disabled) \
     { \
       SCM_THREAD_REALLOW_2; \
       SCM_ASYNC_TICK; \
     } \
+  SCM_FENCE; \
 } \
 
 
+#define SCM_TICK \
+{ \
+  SCM_DEFER_INTS; \
+  SCM_ALLOW_INTS; \
+} \