[pypy-commit] stmgc c7-refactor: Two days to convince myself that this version of stm_write() gives

[arigo]

Author: Armin Rigo <arigo at tunes.org>
Branch: c7-refactor
Changeset: r727:3ddbe9c6d224
Date: 2014-02-13 13:48 +0100
http://bitbucket.org/pypy/stmgc/changeset/3ddbe9c6d224/

Log:	Two days to convince myself that this version of stm_write() gives
	the best trade-offs

diff --git a/c7/stm/core.h b/c7/stm/core.h
--- a/c7/stm/core.h
+++ b/c7/stm/core.h
@@ -24,20 +24,20 @@
 
 
 enum {
-    /* set if the write-barrier slowpath needs to trigger. set on all
-       old objects if there was no write-barrier on it in the same
-       transaction and no collection inbetween. */
-    GCFLAG_WRITE_BARRIER = _STM_GCFLAG_WRITE_BARRIER,
-    /* set on objects which are in pages visible to others (SHARED
-       or PRIVATE), but not committed yet. So only visible from
-       this transaction. */
-    //GCFLAG_NOT_COMMITTED = _STM_GCFLAG_WRITE_BARRIER << 1,
+    /* this flag is not set on most objects.  when stm_write() is called
+       on an object that is not from the current transaction, then
+       _stm_write_slowpath() is called, and then the flag is set to
+       say "called once already, no need to call again". */
+    GCFLAG_WRITE_BARRIER_CALLED = _STM_GCFLAG_WRITE_BARRIER_CALLED,
+    /* set if the object can be seen by all threads.  If unset, we know
+       it is only visible from the current thread. */
+    //GCFLAG_ALL_THREADS = 0x04,
     /* only used during collections to mark an obj as moved out of the
        generation it was in */
-    //GCFLAG_MOVED = _STM_GCFLAG_WRITE_BARRIER << 2,
+    //GCFLAG_MOVED = 0x01,
     /* objects smaller than one page and even smaller than
        LARGE_OBJECT_WORDS * 8 bytes */
-    //GCFLAG_SMALL = _STM_GCFLAG_WRITE_BARRIER << 3,
+    //GCFLAG_SMALL = 0x02,
 };
 
 
diff --git a/c7/stmgc.h b/c7/stmgc.h
--- a/c7/stmgc.h
+++ b/c7/stmgc.h
@@ -33,6 +33,7 @@
 typedef TLPREFIX struct object_s object_t;
 typedef TLPREFIX struct stm_segment_info_s stm_segment_info_t;
 typedef TLPREFIX struct stm_read_marker_s stm_read_marker_t;
+typedef TLPREFIX struct stm_current_transaction_s stm_current_transaction_t;
 typedef TLPREFIX char stm_char;
 typedef void* stm_jmpbuf_t[5];  /* for use with __builtin_setjmp() */
 
@@ -40,6 +41,10 @@
     uint8_t rm;
 };
 
+struct stm_current_transaction_s {
+    uint8_t ct;
+};
+
 struct stm_segment_info_s {
     uint8_t transaction_read_version;
     int segment_num;
@@ -77,8 +82,8 @@
 void _stm_large_dump(void);
 #endif
 
-#define _STM_GCFLAG_WRITE_BARRIER  0x01
-#define STM_FLAGS_PREBUILT         _STM_GCFLAG_WRITE_BARRIER
+#define _STM_GCFLAG_WRITE_BARRIER_CALLED  0x80
+#define STM_FLAGS_PREBUILT                0
 
 
 /* ==================== HELPERS ==================== */
@@ -118,7 +123,12 @@
 
 static inline void stm_write(object_t *obj)
 {
-    if (UNLIKELY(obj->stm_flags & _STM_GCFLAG_WRITE_BARRIER))
+    /* this is:
+           'if (ct == 0 && (stm_flags & WRITE_BARRIER_CALLED) == 0)'
+         assuming that 'ct' is either 0 (no, not current transaction)
+           or 0xff (yes) */
+    if (UNLIKELY(!(((stm_current_transaction_t *)(((uintptr_t)obj) >> 8)->ct |
+                    obj->stm_flags) & _STM_GCFLAG_WRITE_BARRIER_CALLED)))
         _stm_write_slowpath(obj);
 }