[pypy-commit] stmgc default: ensure that the write of an stm_read() happens before the actual reading by making the C compiler believe that the write may alias the read (prevents some optimizations)

[Raemi]

Author: Remi Meier <remi.meier at inf.ethz.ch>
Branch: 
Changeset: r1366:e0c1255cc369
Date: 2014-09-08 14:18 +0200
http://bitbucket.org/pypy/stmgc/changeset/e0c1255cc369/

Log:	ensure that the write of an stm_read() happens before the actual
	reading by making the C compiler believe that the write may alias
	the read (prevents some optimizations)

diff --git a/c8/stm/core.c b/c8/stm/core.c
--- a/c8/stm/core.c
+++ b/c8/stm/core.c
@@ -262,7 +262,7 @@
         OPT_ASSERT((uintptr_t)STM_PSEGMENT->last_commit_log_entry->next == -1);
 
         to = &(STM_PSEGMENT->last_commit_log_entry->next);
-        bool yes = __sync_bool_compare_and_swap(to, -1, new);
+        bool yes = __sync_bool_compare_and_swap(to, (void*)-1, new);
         OPT_ASSERT(yes);
         return;
     }
diff --git a/c8/stm/misc.c b/c8/stm/misc.c
--- a/c8/stm/misc.c
+++ b/c8/stm/misc.c
@@ -31,8 +31,7 @@
 
 bool _stm_was_read(object_t *obj)
 {
-    uint8_t rm = ((struct stm_read_marker_s *)
-                  (STM_SEGMENT->segment_base + (((uintptr_t)obj) >> 4)))->rm;
+    uint8_t rm = *((char *)(STM_SEGMENT->segment_base + (((uintptr_t)obj) >> 4)));
     assert(rm <= STM_SEGMENT->transaction_read_version);
     return rm == STM_SEGMENT->transaction_read_version;
 }
diff --git a/c8/stmgc.h b/c8/stmgc.h
--- a/c8/stmgc.h
+++ b/c8/stmgc.h
@@ -24,19 +24,8 @@
 
 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 char stm_char;
 
-struct stm_read_marker_s {
-    /* In every segment, every object has a corresponding read marker.
-       We assume that objects are at least 16 bytes long, and use
-       their address divided by 16.  The read marker is equal to
-       'STM_SEGMENT->transaction_read_version' if and only if the
-       object was read in the current transaction.  The nurseries
-       also have corresponding read markers, but they are never used. */
-    uint8_t rm;
-};
-
 struct stm_segment_info_s {
     uint8_t transaction_read_version;
     int segment_num;
@@ -126,8 +115,7 @@
 __attribute__((always_inline))
 static inline void stm_read(object_t *obj)
 {
-    ((stm_read_marker_t *)(((uintptr_t)obj) >> 4))->rm =
-        STM_SEGMENT->transaction_read_version;
+    *((stm_char *)(((uintptr_t)obj) >> 4)) = STM_SEGMENT->transaction_read_version;
 }
 
 __attribute__((always_inline))